JournalTOCs

Physics of Fluids
Journal Prestige (SJR): 1.19

Citation Impact (citeScore): 3
Number of Followers: 59

Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1070-6631 - ISSN (Online) 1089-7666
Published by AIP

[28 journals]

A sharp interface immersed edge-based smoothed finite element method with
extended fictitious domain scheme
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  This paper proposes a versatile and robust immersed edge-based smoothed finite element method with the mass conservation algorithm (IESFEM/Mass) to solve partitioned fluid–structure interaction (FSI). A gradient smoothing technique was used to solve the system governing equations, which can improve the calculated capability of the linear triangular elements in two phases. Based on the quadratic sharp interface representation of immersed boundary, an extended fictitious domain constructed by a least squares method approximately corrected the residual flux error. The compatibility for boundary conditions on moving interfaces was satisfied, thus eliminating spurious oscillations. The results from all numerical examples were consistent with those from the existing experiments and published numerical solutions. Furthermore, the present divergence-free vector field had a faster-converged rate in the flow velocity, pressure, and FSI force. Even if in distorted meshes, the proposed algorithm maintained a stable accuracy improvement. The aerodynamics of one- and two-winged flapping motions in insect flight has been investigated through the IESFEM/Mass. It can be seen that the wing–wake interaction mechanism is a vital factor affecting the lift. The applicability of the present method in the biological FSI scenario was also well-demonstrated.
  Citation: Physics of Fluids
  PubDate: 2023-04-17T12:28:54Z
  DOI: 10.1063/5.0141727
Enhanced and reduced solute transport and flow strength in salt finger
convection in porous media
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  We report a pore-scale numerical study of salt finger convection in porous media, with a focus on the influence of the porosity in the non-Darcy regime, which has received little attention in previous research. The numerical model is based on the lattice Boltzmann method with a multiple-relaxation-time scheme and employs an immersed boundary method to describe the fluid–solid interaction. The simulations are conducted in a two-dimensional, horizontally periodic domain with an aspect ratio of 4, and the porosity [math] is varied from 0.7 to 1, while the solute Rayleigh number [math] ranges from [math] to [math]. Our results show that, for all explored [math], solute transport first enhances unexpectedly with decreasing [math] and then decreases when [math] is smaller than a [math]-dependent value. On the other hand, while the flow strength decreases significantly as [math] decreases at low [math], it varies weakly with decreasing [math] at high [math] and even increases counterintuitively for some porosities at moderate [math]. Detailed analysis of the salinity and velocity fields reveals that the fingered structures are blocked by the porous structure and can even be destroyed when their widths are larger than the pore scale, but become more ordered and coherent with the presence of porous media. This combination of opposing effects explains the complex porosity dependencies of solute transport and flow strength. The influence of porous structure arrangement is also examined, with stronger effects observed for smaller [math] and higher [math]. These findings have important implications for passive control of mass/solute transport in engineering applications.
  Citation: Physics of Fluids
  PubDate: 2023-04-17T12:28:50Z
  DOI: 10.1063/5.0141977
On the instability of the magnetohydrodynamic pipe flow subject to a
transverse magnetic field
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  Authors: Y. Velizhanina, B. Knaepen
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The linear stability of a fully developed liquid–metal magnetohydrodynamic pipe flow subject to a transverse magnetic field is studied numerically. Because of the lack of axial symmetry in the mean velocity profile, we need to perform a BiGlobal stability analysis. For that purpose, we develop a two-dimensional complex eigenvalue solver relying on a Chebyshev–Fourier collocation method in physical space. By performing an extensive parametric study, we show that in contrast to the Hagen–Poiseuille flow known to be linearly stable for all Reynolds numbers, the magnetohydrodynamic pipe flow with transverse magnetic field is unstable to three-dimensional disturbances at sufficiently high values of the Hartmann number and wall conductance ratio. The instability observed in this regime is attributed to the presence of velocity overspeed in the so-called Roberts layers and the corresponding inflection points in the mean velocity profile. The nature and characteristics of the most unstable modes are investigated, and we show that they vary significantly depending on the wall conductance ratio. A major result of this paper is that the global critical Reynolds number for the magnetohydrodynamic pipe flow with transverse magnetic field is Re = 45 230, and it occurs for a perfectly conducting pipe wall and the Hartmann number Ha = 19.7.
  Citation: Physics of Fluids
  PubDate: 2023-04-17T12:28:48Z
  DOI: 10.1063/5.0149639
The turbulence development at its initial stage: A scenario based on the
idea of vortices decay
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  Authors: S. V. Talalov
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  In this paper, a model of the development of a quantum turbulence in its initial stage is proposed. The origin of the turbulence in the suggested model is the decay of vortex loops with an internal structure. We consider the initial stage of this process, before an equilibrium state is established. As result of our study, the density matrix of developing turbulent flow is calculated. The quantization scheme of the classical vortex rings system is based on the approach proposed by the author earlier.
  Citation: Physics of Fluids
  PubDate: 2023-04-17T12:28:37Z
  DOI: 10.1063/5.0145537
Interstage difference and deterministic decomposition of internal unsteady
flow in a five-stage centrifugal pump as turbine
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  A five-stage centrifugal pump is utilized to investigate the interstage flow characteristics of the multistage centrifugal pump as turbine (PAT). The simulation results of performance are verified by comparing with the experimental results. Owing to the distinct structural attributes, significant differences in flow occur between the first stage and the other stages of the multistage PAT. To enhance the understanding of these disparities and explore their repercussions, this study focuses on analyzing the flow within the impellers in the first and second stages by a deterministic analysis. The main conclusions are as follows: The discrepancies in the inflow conditions are the major reason for the dissimilarities in the flow of impellers between stages. The impact loss generated by the misalignment between the positive guide vane outlet angle and the impeller inlet angle leads to flow deviation between impeller passages and affects the internal flow pattern. The unsteadiness under low flow rates is mostly produced by the spatial gradient of the blade-to-blade nonuniformities, which is relevant to the relative position between blades and the positive guide vanes. At high flow rates, especially in the second-stage impeller, the pure unsteady term is the primary cause of flow unsteadiness as a result of the flow separation induced by interactions between the blades and the positive guide vanes. This study can provide some references for the practical operation and performance optimization of the multistage PATs in the future.
  Citation: Physics of Fluids
  PubDate: 2023-04-17T12:28:29Z
  DOI: 10.1063/5.0150300
Effect of gravity on phase transition for liquid–gas simulations
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  Authors: Luiz Eduardo Czelusniak, Luben Cabezas-Gómez, Alexander J. Wagner
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Direct simulations of phase-change and phase-ordering phenomena are becoming more common. Recently, qualitative simulations of boiling phenomena have been undertaken by a large number of research groups. One seldom discussed limitation is that large values of gravitational forcing are required to simulate the detachment and rise of bubbles formed at the bottom surface. The forces are typically so large that neglecting the effects of varying pressure in the system becomes questionable. In this paper, we examine the effect of large pressure variations induced by gravity using pseudopotential lattice Boltzmann simulations. These pressure variations lead to height dependent conditions for phase coexistence and nucleation of either gas or liquid domains. Because these effects have not previously been studied in the context of these simulation methods, we focus here on the phase stability in a one-dimensional system, rather than the additional complexity of bubble or droplet dynamics. Even in this simple case, we find that the different forms of gravitational forces employed in the literature lead to qualitatively different phenomena, leading to the conclusion that the effects of gravity induced pressure variations on phase-change phenomena should be very carefully considered when trying to advance boiling and cavitation as well as liquefaction simulations to become quantitative tools.
  Citation: Physics of Fluids
  PubDate: 2023-04-17T12:28:26Z
  DOI: 10.1063/5.0144470
Entrapment and mobilization dynamics during the flow of viscoelastic
fluids in natural porous media: A micro-scale experimental investigation
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  Authors: Abdelhalim I. A. Mohamed, Mahdi Khishvand, Mohammad Piri
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Capillary desaturation process was investigated as a function of wetting phase rheological signatures during the injection of Newtonian and non-Newtonian fluids. Two sets of two-phase imbibition flow experiments were conducted on a water-wet sandstone core sample using brine and viscoelastic polymer solutions. During the experiments, a high-resolution micro-computed tomography scanner was employed to directly map pore-level fluid occupancies within the pore space. The results of the experiments revealed that at a given capillary number, the viscoelastic polymer was more efficient than the brine in recovering the non-wetting oil phase. At low capillary numbers, this is attributed to the improved accessibility of the viscoelastic polymer solution to the entrance of pore elements, which suppressed snap-off events and allowed more piston-like and cooperative pore-body filling events to contribute to oil displacement. For intermediate capillary numbers, the onset of elastic turbulence caused substantial desaturation, while at high capillary numbers, the superimposed effects of higher viscous and elastic forces further improved the mobilization of the trapped oil ganglia by the viscoelastic polymer. In the waterflood, however, the mobilization of oil globules was the governing recovery mechanism, and the desaturation process commenced only when the capillary number reached a threshold value. These observations were corroborated with the pore-level fluid occupancy maps produced for the brine and viscoelastic polymer solutions during the experiments. Furthermore, at the intermediate and high capillary numbers, the force balance and pore-fluid occupancies suggested different flow regimes for the non-Newtonian viscoelastic polymer. These regions are categorized in this study as elastic-capillary- and viscoelastic-dominated flow regimes, different from viscous-capillary flow conditions that are dominant during the flow of Newtonian fluids. Moreover, we have identified novel previously unreported pore-scale displacement events that take place during the flow of viscoelastic fluids in a natural heterogeneous porous medium. These events, including coalescence, fragmentation, and re-entrapment of oil ganglia, occurred before the threshold of oil mobilization was reached under the elastic-capillary-dominated flow regime. In addition, we present evidence for lubrication effects at the pore level due to the elastic properties of the polymer solution. Furthermore, a comparison of capillary desaturation curves generated for the Newtonian brine and non-Newtonian viscoelastic polymer revealed that the desaturation process was more significant for the viscoelastic polymer than for the brine. Finally, the analysis of trapped oil clusters showed that the ganglion size distribution depends on both the capillary number and the rheological properties of fluids.
  Citation: Physics of Fluids
  PubDate: 2023-04-17T12:28:24Z
  DOI: 10.1063/5.0139401
Impact of wettability on interface deformation and droplet breakup in
microcapillaries
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  Authors: P. Giefer, A. Kyrloglou, U. Fritsching
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The objective of this research paper is to relate the influence of dynamic wetting in a liquid/liquid/solid system to the breakup of emulsion droplets in capillaries. Therefore, modeling and simulation of liquid/liquid flow through a capillary constriction have been performed with varying dynamic contact angles from highly hydrophilic to highly hydrophobic. Advanced advection schemes with geometric interface reconstruction (isoAdvector) are incorporated for high interface advection accuracy. A sharp surface tension force model is used to reduce spurious currents originating from the numerical treatment and geometric reconstruction of the surface curvature at the interface. Stress singularities from the boundary condition at the three-phase contact line are removed by applying a Navier-slip boundary condition. The simulation results illustrate the strong dependency of the wettability and the contact line and interface deformation.
  Citation: Physics of Fluids
  PubDate: 2023-04-17T12:28:23Z
  DOI: 10.1063/5.0135101
Drag increase and turbulence augmentation in two-way coupled
particle-laden wall-bounded flows
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  Authors: F. Battista, P. Gualtieri, J.-P. Mollicone, F. Salvadore, C. M. Casciola
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The exact regularized point particle method is used to characterize the turbulence modulation in two-way momentum-coupled direct numerical simulations of a turbulent pipe flow. The turbulence modification is parametrized by the particle Stokes number, the mass loading, and the particle-to-fluid density ratio. The data show that in the wide region of the parameter space addressed in the present paper, the overall friction drag is either increased or unaltered by the particles with respect to the uncoupled case. In the cases where the wall friction is enhanced, the fluid velocity fluctuations show a substantial modification in the viscous sub-layer and in the buffer layer. These effects are associated with a modified turbulent momentum flux toward the wall. The particles suppress the turbulent fluctuations in the buffer region and concurrently provide extra stress in the viscous sub-layer. The sum of the turbulent stress and the extra stress is larger than the Newtonian turbulent stress, thus explaining the drag increase. The non-trivial turbulence/particles interaction turns out in a clear alteration of the near-wall flow structures. The streamwise velocity streaks lose their spatial coherence when two-way coupling effects are predominant. This is associated with a shift of the streamwise vortices toward the center of the pipe and with the concurrent presence of small-scale and relatively more intense vortical structures near the wall.
  Citation: Physics of Fluids
  PubDate: 2023-04-17T12:28:20Z
  DOI: 10.1063/5.0141964
Partial and complete wetting of thin films with dynamic contact angle
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  Authors: Dirk Peschka
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The wetting of thin films depends critically on the sign of the spreading coefficient [math]. We discuss the cases S 0 for transient models with contact line dissipation and find that the use of a dynamic contact angle solves problems for S > 0 that models might otherwise have. For initial data with a non-zero slope and S > 0, we show that there exists a finite time [math] at which the contact angle of the thin film goes to zero. Then, a molecular precursor emerges from the thin film and moves outward at a constant velocity.
  Citation: Physics of Fluids
  PubDate: 2023-04-17T12:28:18Z
  DOI: 10.1063/5.0146538
The effects of pulsed blowing jets on power gain of vortex-induced
vibrations of a circular cylinder
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  Authors: Yujie Guo, Zhengui Huang, Chun Zheng, Zhihua Chen
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  To enhance the power gain of vortex-induced vibration of a circular cylinder, the active control method of pulsed blowing jets located at θ = 90° is utilized to intensify its oscillation with the two-dimensional simulation of Reynolds-averaged Navier–Stokes at 2.0 × 104 ≤ Re ≤ 9.6 × 104. Different from traditional continuous jets, the blowing jets used in this paper start once the cylinder moves to the upper limited position and last for a certain duration. Based on the combination of nine momentum coefficients and four pulse durations of the jets, the oscillation responses of the cylinder at a series of reduced velocities are calculated and distinct responses are observed in three branches. In the initial branch (U* ≤ 4.27), no matter what the values of Cμ and n are, the vortex patterns keep 2S accompanied by the amplitude ratios vibrating around the benchmarks. In the fore part of the upper branch (4.27 < U* ≤ 6.17), as Cμ ≤ 0.1005, the control effect is similar to that at U* ≤ 4.27; as Cμ> 0.1005, both slight enhancement and suppression in amplitude ratios are observed, as well as the small values of power gain ratios. In the rear part of the upper branch and lower branch (U*> 6.17), the enlarged disturbance of the jets to wake results in enhanced amplitude ratios for most cases. Galloping is observed at n = 1/4 and 1/2 with a maximum amplitude ratio 13 times the benchmark, except for some suppressed cases at Cμ> 0.1005, n = 1/16, and 1/8. Though large amplitude ratios are achieved, considering more energy consumed as Cμ increases, the better control strategy with η ranging from 5.45% to 19.78% falls in U*> 6.17 and Cμ < 0.1005.
  Citation: Physics of Fluids
  PubDate: 2023-04-17T12:28:16Z
  DOI: 10.1063/5.0146352
Wall-distance free transition model based on the laminar kinetic energy
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  Authors: D. Bulgarini, A. Ghidoni, G. Noventa
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The next fluid dynamics solvers will be based on innovative numerical schemes and models to increase the fidelity and decrease the computational cost. The higher accuracy and geometrical flexibility guaranteed by discontinuous Galerkin spatial discretization methods in solving Reynolds-Averaged Navier–Stokes equations could represent an appealing solution in comparison with finite volume solvers for real-life simulations. In this context, numerical models able to accurately predict transitional flows are mandatory to overcome the limits of turbulence models and the costs of high-fidelity approaches, e.g., Direct Numerical Simulations and Large Eddy Simulations, for the efficient design of many industrial applications, e.g., aerospace, turbomachinery, maritime, automotive, and cooling applications. Among the transition models proposed in the literature, the local and phenomenological formulation seems to guarantee better robustness, fidelity, and easiness of implementation in all the solvers. All the transition models are based also on the wall-distance, to define some local terms or parameters and model the transition phenomenon. The calculation of the wall-distance can be critical in the discontinuous Galerkin framework for the high-order representation of the boundaries, which can become very expensive and high-memory consuming. To alleviate this problem, a wall-distance free version of a transition model based on the laminar kinetic energy is proposed and implemented in a high-order discontinuous Galerkin solver, and the robustness and fidelity are assessed by computing flows with bypass and separation-induced transition and different Reynolds number, turbulent intensity, and pressure gradient on flat plates. The wall-distance free formulation proves robustness and fidelity in all the cases, in comparison with the original formulation and an ad hoc modified formulation for the separation-induced transition cases.
  Citation: Physics of Fluids
  PubDate: 2023-04-17T01:04:44Z
  DOI: 10.1063/5.0144792
Experimental study of extreme waves based on nonlinear Schrödinger
equation under background of a random sea
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The Peregrine breather (PB) solution of the nonlinear Schrödinger equation is used to model ocean extreme waves in a water wave flume. Triangular spectral features of wave elevations are observed over the nonlinear evolution of the extreme waves, which can be applied for early detection of the formation of extreme waves. To model a more realistic sea state, a background random wave is superposed to the PB in this study. We examine the spectral features of the nonlinear wave evolution with random background waves by the spectral analysis. It is found that the wave elevations show similar triangular spectral features for extreme waves with a relatively mild background wave. Moreover, we find that the second harmonic elevations of the extreme waves also show triangular spectral features, suggesting the potential use of the second harmonic elevations (in addition to the first) for detection of the formation of extreme waves.
  Citation: Physics of Fluids
  PubDate: 2023-04-14T11:18:33Z
  DOI: 10.1063/5.0142180
Ring-bouncing induced by the head-on impact of two nanodroplets on
superhydrophobic surfaces
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Efficient droplet shedding from surfaces is fundamentally interesting and important due to its promising potential in numerous applications, such as anti-erosion, anti-icing, and self-cleaning. In this work, the bouncing dynamics of the head-on impact of two nanodroplets on superhydrophobic surfaces are investigated through molecular dynamics simulations. Three bouncing patterns, including regular-coalescence-bouncing, coalescence-hole-bouncing, and ring-bouncing, are identified at a wide range of impacting Weber numbers. For three bouncing patterns, the time evolutions of the spreading factors and the vertical velocity components are employed to analyze the particular dynamic behaviors and elucidate the underlying physics. As a counter-intuitive bouncing pattern, the ring-bouncing that two impact nanodroplets coalesce, spread, and then leave the surface in a ring shape without retracting exhibits a remarkable reduction in contact time by up to 60%. Considering four typical states for the ring-bouncing pattern, the comparison of the velocity distribution within the droplet clearly reveals that the ring-shaped droplet reshapes interfaces, which leads to a special hydrodynamics distribution. As a result, the internal flows at the inner and outer edges along the opposite direction collide with each other, leading to a sudden increase in the upward velocity. Combining the largely decreased contact area between solid and liquid with the small surface adhesion, the ring-shaped droplet rapidly bounces off the surface at the maximum spreading state. Finally, it is significantly highlighted that the ring-bouncing pattern offers a new avenue to break the contact time limit for efficient droplet shedding.
  Citation: Physics of Fluids
  PubDate: 2023-04-14T11:18:30Z
  DOI: 10.1063/5.0142401
Thermally driven dynamics of interacting droplet-pairs in micro-confined
shear flow: Beyond the realm of droplet coalescence
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  Authors: Sayan Das, Somnath Santra, Suman Chakraborty
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Pattern formation and dynamics of interacting droplets in confined passages are ubiquitous in a variety of natural, physical, and chemical processes and appears to be contrasting as compared to single droplet dynamics. However, while the dynamical evolution of single droplets under various forces, including their thermally driven motion, has been explored extensively, the concerned physical facets cannot be trivially extended for addressing the motion of multiple droplets. By considering temperature-gradient-driven interfacial transport, here, we unveil four different modes of thermally activated migration of a droplet-pair in microchannels. These include pure reversing motion, sliding-over motion, follow-up motion, and direct coalescence. The presence of follow-up motion, because of the imposed temperature gradient, has not been investigated before. We further put forward the possibility of conversion of one pattern to another by modulating different tuning parameters, such as the wall temperature, channel dimension, and the relative initial positioning of the droplets. These results may turn out to be of profound importance in a wide variety of applications ranging from materials processing to micro-reactor technology.
  Citation: Physics of Fluids
  PubDate: 2023-04-14T11:18:27Z
  DOI: 10.1063/5.0146224
Research on non-Newtonian characteristics of crude oil flow at micro-nano
scale
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  Authors: Fuquan Song, Heying Ding, Lintao Huang, Yong Wang, Yeheng Sun
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The characteristic scale of flow in micro–nanochannels is generally in the range of 0.01 μm∼1 μm. When crude oil passes through micro-nano channels and tight reservoirs, it shows obvious nonlinear seepage characteristics, which does not conform to the continuity assumption of fluid. Therefore, a non-Newtonian model of crude oil flowing in micro-nano channels and tight reservoirs under the action of shear stress is established, and the relationship between flow rate and apparent viscosity and shear rate is analyzed. The experiment of crude oil flow in micro-nano channels and tight oil reservoir cores shows that the model can be used to describe the nonlinear seepage law of liquid through the nonlinear fitting. The power law index of the oil-phase power-law non-Newtonian fluid is greater than 1 at the micro-nano scale, which conforms to the flow characteristics of the expansive fluid, thus verifying the effectiveness of the non-Newtonian model. In addition, the study of apparent viscosity and shear rate of non-Newtonian fluid shows that the increasing and decreasing trends of flow rate and shear rate and the changing trends of flow rate and pressure gradient are consistent, and shear rate can be used to describe the characteristics of fluid instead of the pressure gradient.
  Citation: Physics of Fluids
  PubDate: 2023-04-14T11:18:26Z
  DOI: 10.1063/5.0145727
Theory and simulations of linear and nonlinear two-dimensional
Rayleigh–Taylor dynamics with variable acceleration
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  Authors: Suhas S. Jain, Annie Naveh, Snezhana I. Abarzhi
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Interfacial Rayleigh–Taylor mixing is crucial to describing important natural and engineering processes, such as exploding supernovae, laser micromachining, hot spots in inertial confinement fusion, and optical telecommunications. These require the characterization of the time dependence of the driving acceleration. We compare our theoretical formulation based on group theory foundations with interface-capturing numerical simulations for linear and nonlinear two-dimensional Rayleigh–Taylor instabilities in a finite-sized domain with time-varying acceleration over broad ranges of Atwood numbers and acceleration exponents. Detailed corroboration between theory and simulations is provided for this foundational case. Both demonstrate the strong interfacial nature of Rayleigh–Taylor instabilities, which suggests that practical flow fields can be reconstructed from the derived fluid potential using the proposed theory. A robust agreement is also obtained for the early and late-time evolution of the amplitudes of the bubble and spike, which demonstrate that the Rayleigh–Taylor flow can transition to the mixing regime even for a single-mode initial perturbation. Corroboration with experiments of high energy density plasmas motivated by studies of supernovae is also achieved. In addition, a long-standing puzzle in Rayleigh–Taylor dynamics on the interplay between the acceleration, the shear, and the interface morphology in the theory and simulations is resolved by accounting for finite viscosity of the fluids. The characterization of Rayleigh–Taylor instabilities as a highly interfacial phenomenon provides valuable insight into its multiscale nature, which enhances the design and understanding of numerous processes of practical interest.
  Citation: Physics of Fluids
  PubDate: 2023-04-14T11:18:22Z
  DOI: 10.1063/5.0137462
Breakup-based preparation of ultra-thin solid-in-water-in-oil conformal
droplets in a microchannel
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Droplets encapsulating solid microparticles with a thin coating layer have extensive applications in the fields of biochemical, advanced materials, and inertial confinement fusion. In this work, the droplet break-up technique is employed to prepare solid–water–oil (S/W/O) conformal droplets with an ultra-thin coating layer. A microfluidic chip, consisting of a T-junction and a Y-junction, was designed and constructed for the controlled preparation of ultra-thin S/W/O conformal droplets by generating–splitting integration. The flow pattern, regime, and dynamic mechanisms of the S/W/O droplet break-up were also experimentally investigated. The results show that there are three break-up regimes: breakup, non-breakup, and transition. Two different modes are observed in the break-up regime: without solid core stagnation and with solid core stagnation. In the case of the solid core without stagnation, the neck goes through three stages: squeezing, transition, and pinch-off. When the solid core stagnates, the neck goes through one more solid core stagnation stage after squeezing. The stagnation percentage decreases as the dispersed phase capillary number increases and increases as the continuous phase capillary number increases. The coating thickness of the S/W/O droplet increases and then decreases as the continuous phase flow rate increases. The coating thickness of the daughter S/W/O droplet was significantly reduced and was less affected by the continuous phase flow rate.
  Citation: Physics of Fluids
  PubDate: 2023-04-14T11:18:19Z
  DOI: 10.1063/5.0146977
Influence of surface nanostructures on the catalytic recombination of
hyperthermal non-equilibrium flow
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  One of the key challenges for accurate prediction of hypersonic aerodynamic heating is the exothermic uncertainty due to the complex surface catalytic recombination effect, which is caused by the strong interactions between highly non-equilibrium dissociated gas and the thermal protection material surface. Employing engineered surface morphology to improve thermal protection effects has been proposed, but its effects on surface catalytic recombination remain unclear. To address this problem, this work employs the reactive molecular dynamics method to investigate the surface adsorption and recombination characteristics of continuous impingement of atomic oxygen upon eight different nano-structured silica surfaces. A parametric study of the influences of the gas incident angles and the surface structural parameters, i.e., roughness factor and surface fraction, is conducted. The results show that the surface catalytic recombination performance is very sensitive to the incident angle of the incoming gas, and the presence of nanostructures increases the recombination rate. The influence of surface morphology shows a complicated feature, where nanostructures with moderated fin height and high surface fraction are beneficial for the inhibition of surface recombination effects, leading to reduced exothermic heat release. Such microscopic revelation of the surface morphology effect is helpful for accurate prediction of aerodynamic heat and provides guidance for the surface engineering of optimized morphology to achieve improved thermal protection effect.
  Citation: Physics of Fluids
  PubDate: 2023-04-14T11:18:14Z
  DOI: 10.1063/5.0145963
Time-domain motion of a floating or obliquely submerged non-uniform
elastic plate
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  Authors: Mansi Singh, Michael H. Meylan, R. Gayen
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  We consider the motion of a thin elastic plate with non-uniform thickness. The plate is either submerged and has some inclination with the vertical or is floating on the upper surface of the water. Green's function arising from the fourth-order boundary condition for the non-uniform plate (which we refer to as plate Green's function) is determined using two different methods in terms of the vibrating modes of the plate. These, in turn, are derived from the modes of a plate with constant thickness. The problem is finally reduced to a boundary integral equation involving the plate Green's function and the fundamental Green's function. This equation is hypersingular in the case of a submerged plate. A numerical solution to the integral equation is used to find results for elastic plates with variable thicknesses. The results are validated by comparing them with those of an elastic plate with uniform thickness. We also present simulations of the time-domain motion when the plate–fluid system is subject to an incident wave pulse using Fourier transform.
  Citation: Physics of Fluids
  PubDate: 2023-04-14T11:18:12Z
  DOI: 10.1063/5.0143362
Joule heating and Soret effects on an electro-osmotic viscoelastic fluid
flow considering the generalized Phan-Thien–Tanner model
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  Authors: A. Hernández, A. Mora, J. C. Arcos, O. Bautista
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  This work analyzes the non-isothermal electro-osmotic fluid flow in a microchannel considering the Soret effect and temperature-dependent properties. The constitutive equation that models the fluid rheology corresponds to the generalized Phan-Thien–Tanner (gPTT) model. Temperature and pressure gradients are induced due to the interaction between an ionized fluid and the electrical field imposed at the microchannel's ends, resulting in Joule heating. The temperature-dependent physical properties of the fluid modify the ionic distribution in the electric double layer and its thickness change along the microchannel walls. The generalized Phan-Thien–Tanner (gPTT) model is used as a constitutive equation that describes the fluid rheology, where the trace-stress tensor is based on the Mittag–Leffler function, which represents the destruction of physical junctions and entanglements in the Lodge–Yamamoto network of viscoelastic fluids, through the inclusion of two fitting parameters: α and β. The gPTT model allows better fitting and flexibility to experimental data and a wider range of variation in the description of rheological responses of complex fluids. The hydrodynamics and thermodiffusion obtained through the gPTT model are compared against that using the linear form of the Phan-Thien–Tanner model (lPTT).
  Citation: Physics of Fluids
  PubDate: 2023-04-14T11:18:11Z
  DOI: 10.1063/5.0146034
Direct numerical simulations of hypersonic boundary layer transition over
a hypersonic transition research vehicle model lifting body at different
angles of attack
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  This paper performs direct numerical simulations of hypersonic boundary layer transition over a Hypersonic Transition Research Vehicle (HyTRV) model lifting body designed by the China Aerodynamic Research and Development Center. Transitions are simulated at four angles of attack: 0°, 3°, 5°, and 7°. The free-stream Mach number is 6, and the unit Reynolds number is 107 m−1. Four distinct transitional regions are identified: the shoulder cross-flow and vortex region and the shoulder vortex region on the leeward side, the windward vortex region and the windward cross-flow region on the windward side. As the angle of attack increases, the transition locations on the leeward side generally move forward and the transition ranges expand, while the transition locations generally move backward and the transition ranges decrease on the windward side. Moreover, the shoulder vortex region moves toward the centerline of the leeward side. At large angles of attack (5° and 7°), the streamwise vortex on the shoulder cross-flow and vortex region will enable the transition region to be divided into the cross-flow instability region on both sides and the streamwise vortex instability region in the middle. In addition, the streamwise vortex also leads to a significant increase in cross-flow instability in their upper region, which can generate a new streamwise vortex instability region between the two transition regions on the leeward side. Furthermore, since the decrease in the intensity and the range for the cross-flow on the windward side, the windward cross-flow region tends to become narrow and ultimately disappears.
  Citation: Physics of Fluids
  PubDate: 2023-04-14T11:18:09Z
  DOI: 10.1063/5.0146651
Chemotaxis of two chiral squirmers
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  Authors: Ruma Maity, P. S. Burada
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  External gradients can strongly influence the collective behavior of microswimmers. In this paper, under an external linear chemical gradient, we study the behavior of two hydrodynamically interacting self-propelled chiral swimmers in the low-Reynolds number regime. We use the generalized squirmer model called the chiral squirmer, a spherically shaped body with an asymmetric surface slip velocity, to represent the swimmer. We find that the external gradient favors the attraction between the swimmers and, in some situations, leads to a bounded state in which the swimmers move in a highly synchronous manner. Furthermore, due to this cooperative motion, these swimmers reach the chemical target faster than individual swimmers. This study may help in understanding the collective behavior of chiral swimmers and in designing synthetic microswimmers for targeted drug delivery.
  Citation: Physics of Fluids
  PubDate: 2023-04-13T02:16:38Z
  DOI: 10.1063/5.0139016
Pulsating pressurization of two-phase fluid in a pipe filled with water
and a little gas
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Although two-phase flows containing gas and water have received extensive attention, the pulsating pressurization effect of a two-phase fluid in a pipe is unclear and the influence of the gas-phase content has not been revealed. This paper discusses the pulsating pressurization of such a two-phase fluid. First, the two-phase Navier–Stokes equations are derived and an algorithm is developed based on MacCormack's method. The reliability of the algorithm is examined and validated using Poiseuille's theory and existing experimental two-phase flow data. Finally, the influence of several key factors is discussed, including the gas-phase fraction and pipe slenderness. Our results show that a significant pulsating supercharging phenomenon occurs when the gas-phase fraction is less than 10−3. When the gas-phase fraction is greater than this critical value, the pulsating supercharging effect decreases significantly with the increasing gas-phase fraction. The equivalent elastic modulus of the two-phase fluid rapidly decreases as the gas-phase fraction increases, and the pressure disturbance is absorbed by the gas bubbles, causing an apparent weakening of the pulsating supercharging effect. Thus, decreasing the gas-phase content can enhance the pulsating supercharging effect. The pipe slenderness has a very limited influence on the pulsating pressurization process, and the maximum reduction is only 7.3% for slenderness ratios of up to 2000. Moreover, we derive and propose a new mathematical expression for the inlet boundary that is applicable to gas–liquid two-phase flows. To our knowledge, this paper extends the pulsating pressurization range from the single-phase to two-phase fluid for the first time and reports different physical phenomena and regularity. The present research clarifies the pulsating pressurization phenomenon in two-phase flows, providing a valuable reference for pulsating pressurization design.
  Citation: Physics of Fluids
  PubDate: 2023-04-13T02:16:38Z
  DOI: 10.1063/5.0147273
Coupled aero-hydro-mooring dynamic analysis of floating offshore wind
turbine under blade pitch motion
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Accurate prediction of the dynamic responses of the floating offshore wind turbine (FOWT) under the blade pitch motion is quite challenging because of the strong nonlinear effects. In this study, a fully coupled and highly elaborated model was established based on the computational fluid dynamics, with the dynamic fluid body interaction method. The multi-stage movements consisting of the six degrees of freedom motions of the platform, the rotation of the rotor, and the blade pitch motion were defined by the superposition motion technologies. The blade pitch control module was created through the user-defined function to regulate the blade pitch motion. Then, several coupled dynamic simulations of the full-configuration DeepCwind floating wind turbine system were performed in power production, shutdown, and startup cases. The simulation results in the power production case indicate that the blade pitch motion decreases the generated aerodynamic loads and amplifies the response amplitude of the platform as negative damping is introduced in the FOWT system. The simulation results in the shutdown and startup cases indicate that the extreme motion responses are enlarged, and the mooring line tension oscillates dramatically when it is in high-tension states. In addition, the nonlinear interference effects in the unsteady flow fields, such as the shedding vortices broken by the blade pitch motion, are visualized and investigated in detail.
  Citation: Physics of Fluids
  PubDate: 2023-04-13T02:16:36Z
  DOI: 10.1063/5.0141237
Transition in steady streaming and pumping caused by a sphere oscillating
in a viscous incompressible fluid
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  Authors: B. U. Felderhof
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The steady streaming flow pattern caused by a no-slip sphere oscillating in an unbounded viscous incompressible fluid is calculated exactly to second order in the amplitude. The pattern depends on a dimensionless scale number, determined by sphere radius, frequency of oscillation, and kinematic viscosity of the fluid. At a particular value of the scale number, there is a transition with a reversal of flow. The analytical solution of the flow equations is based on a set of antenna theorems. The flow pattern consists of a boundary layer and an adjacent far field of long range, falling off with the inverse square distance from the center of the sphere. The boundary layer becomes thin in the limit where inertia dominates over viscosity. The system acts as a pump operating in two directions, depending on the scale number. The efficiency of the pump is estimated from a comparison of the rate of flow with the rate of dissipation.
  Citation: Physics of Fluids
  PubDate: 2023-04-13T02:16:33Z
  DOI: 10.1063/5.0143377
Transitional criterion and hysteresis of multiple shock–shock
interference
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  In this study, oblique-shock/bow-shock interference is theoretically and numerically studied with two incident shock waves. The transition criteria between the two modes of multiple shock–shock interference, i.e., the concomitant-jet (CJ) and dual-jet (DJ) modes, are given. The oblique shock relationship and shock polar analysis are utilized to obtain the analytical solution of the transition condition. The theoretical results indicate the existence of a dual solution interval (DSI) that widens with increasing Mach number and narrows with increasing deflection angle induced by the first incident shock wave. The DSI obtained by numerical simulation is considerably narrower than that theoretically predicted due to the advanced CJ→DJ and DJ→CJ transitions. The analysis reveals that the transitions are advanced due to the downstream disturbance and secondary waves in the flow field.
  Citation: Physics of Fluids
  PubDate: 2023-04-13T02:16:31Z
  DOI: 10.1063/5.0146200
Smoothed particle hydrodynamics simulations for wave induced ice floe
melting
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  Authors: Thien Tran-Duc, Michael H. Meylan, Ngamta Thamwattana
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  In this paper, ice melting under the impacts of water waves was studied numerically via smoothed particle hydrodynamics simulations. Effects due to the ice elasticity were also included. Accordingly, the melting of an ice plate, modeled as an elastic object and interacting with transitional water waves with wave height and wave steepness up to 0.32 m and 0.093, respectively, was simulated and analyzed. The simulations showed that water waves' effects on the ice melting are seen via overflow over the top surface and local fluid circulations in the submerged region due to water–ice interactions and wave motions. Those effects result in a melting amount of the ice plate up to 1.78 times higher than the ice in still water. The overflow contributes up to 25% of the total amount of the melted ice. In comparison, fluid convection in the submerged region also leads to an increase in about 43% in the ice-melting amount over the submerged region. The melting rate is seen highest at the early stage of the simulation period and then is constantly reducing. The melting rate of the ice is seen linearly varying with the initial water temperature.
  Citation: Physics of Fluids
  PubDate: 2023-04-13T02:16:31Z
  DOI: 10.1063/5.0138858
An investigation of the effects of wall materials on flame dynamics inside
a H2-air micro-combustor
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  Authors: Debjit Kundu, Arijit Bhattacharya, Sourav Sarkar, Sandip Sarkar, Achintya Mukhopadhyay
  Abstract: Physics of Fluids, Volume HFDP2022, Issue 1, April 2023.
  Micro-combustors, which are emerging as portable power sources, have serious flame stabilization issues due to enhanced heat losses. Hydrogen, an eco-friendly alternative to conventional fossil fuels, can be a potential fuel for micro-combustors because of its high calorific value, leading to high energy density. In the present work, numerical simulations of premixed lean (equivalence ratio = 0.5) hydrogen-air flames in a 2 mm wide channel with three different wall materials (glass, steel, and aluminum) were performed. The effects of the wall material on the dynamics of the flames were extensively studied. The walls of the combustor play an important role by conducting heat upstream and facilitating ignition and stabilization of the flame. For different values of wall thermal diffusivity, periodically oscillating flames of varying frequencies ([math]) and intermittent bursting flames were observed. Time series analysis and modal decomposition of temperature fields were utilized to quantify the flame dynamics and to identify the dominant structures of the flames. A recurrence analysis using the temperature time series data revealed significant differences in flame dynamics, including period-2 oscillations and intermittency, for different wall materials. The underlying physics behind the periodic oscillations and intermittent bursting has been explained.
  Citation: Physics of Fluids
  PubDate: 2023-04-13T02:16:30Z
  DOI: 10.1063/5.0144679@phf.2023.HFDP2022.issue-1
A note on pressure and pressure-correction-based fractional-step
approaches for low Reynolds number incompressible flows
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  Authors: Mandeep Deka
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  A comparative study on pressure and pressure-correction-based fractional-step methods for incompressible flows is presented. Implicit fractional-step methods are shown to give rise to a splitting error due to the implicit temporal discretization of the intermediate momentum equations. Through mathematical reasoning and a numerical example, a relative advantage of pressure-correction-based approaches over pressure-based approach for the computation of low Reynolds number flows using implicit fractional-step methods is demonstrated.
  Citation: Physics of Fluids
  PubDate: 2023-04-13T02:16:30Z
  DOI: 10.1063/5.0150098
An investigation of the effects of wall materials on flame dynamics inside
a H2-air micro-combustor
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  Authors: Debjit Kundu, Arijit Bhattacharya, Sourav Sarkar, Sandip Sarkar, Achintya Mukhopadhyay
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Micro-combustors, which are emerging as portable power sources, have serious flame stabilization issues due to enhanced heat losses. Hydrogen, an eco-friendly alternative to conventional fossil fuels, can be a potential fuel for micro-combustors because of its high calorific value, leading to high energy density. In the present work, numerical simulations of premixed lean (equivalence ratio = 0.5) hydrogen-air flames in a 2 mm wide channel with three different wall materials (glass, steel, and aluminum) were performed. The effects of the wall material on the dynamics of the flames were extensively studied. The walls of the combustor play an important role by conducting heat upstream and facilitating ignition and stabilization of the flame. For different values of wall thermal diffusivity, periodically oscillating flames of varying frequencies ([math]) and intermittent bursting flames were observed. Time series analysis and modal decomposition of temperature fields were utilized to quantify the flame dynamics and to identify the dominant structures of the flames. A recurrence analysis using the temperature time series data revealed significant differences in flame dynamics, including period-2 oscillations and intermittency, for different wall materials. The underlying physics behind the periodic oscillations and intermittent bursting has been explained.
  Citation: Physics of Fluids
  PubDate: 2023-04-13T02:16:30Z
  DOI: 10.1063/5.0144679
Propulsion of a combined heaving and trailing-edge morphing foil for
bio-inspired applications
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  Authors: Ishan Neogi, Vardhan Niral Shah, Pragalbh Dev Singh, Vaibhav Joshi
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Locomotion of aquatic animals involves flapping of their body to generate lift and thrust. Through evolution, they have mastered their ability to move through complex environments in an energy-efficient manner. A crucial component of this movement is the ability to actively bend their bodies to generate maximum thrust. This motion is widely termed as morphing. A simplification of this motion is implemented for a foil in this study to realize a thrust-generating bio-inspired device. The propulsive performance of the heaving foil undergoing a prescribed trailing-edge morphing is numerically studied by a stabilized finite element moving mesh formulation. The effects of the morph position and amplitude on the flow dynamics and propulsion of the foil are investigated in the present work. The position of trailing-edge morphing varies from the leading edge to half of the foil's chord, whereas the morph amplitude varies from [math] to [math] at the trailing edge. The instantaneous thrust is analyzed with vorticity plots and surface pressure diagrams. Within the parametric space, it is found that the foil is highly efficient in generating propulsive forces at high morph amplitudes and low morph positions. The interplay between the thrust-generating leading-edge vortex (LEV) and the drag-inducing trailing-edge vortex (TEV), which governs the thrust cycle of a morphing–heaving foil, is elucidated. It is observed that the LEV-induced thrust is higher at low morph positions, while the TEV-induced drag is dominant at high morph amplitudes. An ideal balance of these opposing effects of LEV and TEV occurs at the lowest morph position and intermediate morph amplitudes, emphasizing the optimal flexibility for the maximum propulsive performance of the foil.
  Citation: Physics of Fluids
  PubDate: 2023-04-13T02:16:29Z
  DOI: 10.1063/5.0145443
Active control of vortex shedding past finite cylinders under the effect
of a free surface
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  Authors: I. A. Carvalho, G. R. S. Assi
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  This paper presents the analysis of the active flow control promoted by low-aspect-ratio cylinders under the effect of a free surface at a low Froude number, modeled as a slip-allowing plane. To advance the literature in this merit, that is scarce compared with infinitely long and surface-mounted bodies, we carry out Detached-eddy simulations at Reynolds number of 1000 to investigate the active control provided by eight spinning rods surrounding a larger body. One of the ends of this system was immersed in the free stream, while the other was in contact with a free water surface. Our results reveal that when the rods spun with sufficiently large angular velocities, the (non-Kármán) vortex street was progressively organized and the part of the wake associated with the mechanism of vortex formation described by Gerrard [“The mechanics of the formation region of vortices behind bluff bodies,” J. Fluid Mech. 25, 401–413 (1966)] was eliminated. Nevertheless, tip-vortices prevailed throughout the examined range of spinning velocities. We also contrasted drag mitigation with power loss due to viscous traction and found that to reduce the mean drag on the system to a lower value than that of the bare cylinder necessarily required power expenditure. Steady reduction of mean drag and less significant mitigation of root mean square of lift and mean side force were verified to occur for the entire system and for the central body. However, the side force proved less affected by the wake-control mechanism. We demonstrate this to be associated with a novel ring-like vortex that prevailed throughout the simulations. Vortex dynamics and formation of these turbulent structures are explored.
  Citation: Physics of Fluids
  PubDate: 2023-04-13T02:16:27Z
  DOI: 10.1063/5.0147760
Investigating the dynamics of point helical vortices on a rotating sphere
to model tropical cyclones
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  Authors: Sergey G. Chefranov, Igor I. Mokhov, Alexander G. Chefranov
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  A general exact weak solution to the non-linear equation of the conservation of the absolute vorticity in a thin layer of an incompressible medium on a rotating sphere is proposed. It takes into account the helicity of the point vortices and the non-uniformity of the depth of the layer. This is used to develop a model of the observed interactions of spiral atmospheric vortices. The fusion of two-point helical vortices (HVs) on the rotating sphere is considered. We also analyze the prognostic applicability of the dynamics of the HVs for modeling the abrupt changes observed in the trajectories of tropical cyclones and their landfall in comparison with the traditional approach. The analytical condition for chiral symmetry violation related to the direction of the movement of the center of a helical cyclone is obtained.
  Citation: Physics of Fluids
  PubDate: 2023-04-12T04:25:54Z
  DOI: 10.1063/5.0143023
Role of dual breakwaters and trenches on efficiency of an oscillating
water column
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  Authors: Nikita Naik, Harekrushna Behera
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  In this paper, the effects of double-submerged breakwaters and trenches on the hydrodynamic performance of an oscillating water column (OWC) are investigated. The multi-domain boundary element method is used to tackle the physical problem of wave scattering and radiation from the device. The role of the height of the breakwaters, depth of the trenches, width of the breakwaters and trenches, spacing between the structures, length of the OWC chamber, and other wave and structural parameters is investigated on the efficiency of OWC. The study reveals that there is an oscillating pattern of the efficiency curve in the presence of single or double breakwater/trenches; this pattern is absent when the bottom is flat. Moreover, compared to single or no breakwaters/trenches, the occurrence of full OWC efficiency is higher in the presence of double breakwaters/trenches. Furthermore, the amplitude of the oscillating pattern in the efficiency curve increases with an increase in the height and depth of the breakwaters and trenches, respectively. For some particular wave and structural parameters, zero OWC efficiency occurs nearly [math] within [math] (k0 wave number and h water depth). This zero efficiency moves toward small wave numbers as the spacing between OWC and rigid breakwater/trench increases. The radiation conductance of OWC decreases with an increase in the barrier height. The findings outline the structural criteria that can be employed to build and deploy an effective OWC device.
  Citation: Physics of Fluids
  PubDate: 2023-04-12T03:48:24Z
  DOI: 10.1063/5.0146004
Near-surface cloud dispersion and detonation propagation law
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Fuel/air mixture clouds have important research value in the process industry and military applications. Different from condensed explosions, blast height has a direct impact on the fuel cloud field and the detonation power field. In this paper, we establish numerical models of the detonation process of propylene-oxide clouds generated by the dispersion of 2 kg fuel/air explosives at different blast heights. The process of fuel dispersion, detonation propagation, and the distribution of the near-surface detonation power field are explored. Through theoretical analysis, we establish optimization models of the fuel/air explosive dispersion under different blast heights. The relationship between the proportional blast height, proportional distance, and power field peaks is quantitatively revealed. The results show that the effect of cloud detonation on the ground power field is obvious. The optimal proportional blast height exists. When the cloud mass is 2 kg, the optimum proportional blast height is 0.8 m/kg1/3. At the optimum blast height, the overpressure effect of cloud detonation is the strongest (the peak overpressure is 2.19 MPa, and the action time is 1.77 ms), and the temperature range of cloud detonation is the largest (the peak temperature is 1462.16 K, and the action time is 2.34 ms). Under the condition that the proportional blast height is less than or equal to the optimal proportional blast height, the power field peaks show N-shaped trends with the increase in the proportional distance. When the proportional blast height> proportional ignition radius is > 0.8 m/kg1/3, the peaks decrease with the increase in the proportional distance.
  Citation: Physics of Fluids
  PubDate: 2023-04-12T02:34:05Z
  DOI: 10.1063/5.0141578
Kinetic modeling of immersed boundary layer for accurate evaluation of
local surface stresses and hydrodynamic forces with diffuse interface
immersed boundary method
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The motivation of this paper is to examine the evaluation of local surface stresses and hydrodynamic forces acting on a stationary or moving body using a diffuse interface immersed boundary method (IBM). This task is not trivial for the diffuse IBM because it uses a smoothed regularized delta function in the transfer steps between Lagrangian and Eulerian locations. In our earlier work [D. Xu et al., Phys. Rev. E 105, 035306 (2022)], a particle distribution function (PDF) discontinuity-based kinetic immersed boundary method (KIBM) was proposed based on the Boltzmann equation. This paper is a continuation of our work on the improvement of the KIBM in the framework of the diffuse interface IBM. In the present study, the concept of the immersed boundary layer (IBL) is brought forward, and the dynamic effects of particle advection and collision in the IBL are coupled and evaluated within a numerical time step scale in a kinetic manner. Consequently, the PDFs on both sides of the IBL are reconstructed, and the general immersed boundary force density can be obtained accurately and efficiently. Meantime, the local surface stress distribution acting on the body wall from the actual fluid can be conveniently and accurately calculated by the moment of the PDFs. Finally, some commonly used problems involving incompressible fluid flows in the continuum flow regime with stationary and moving boundaries are simulated by the present KIBM, and the results show that the present KIBM can significantly accelerate the rate of convergence and has a good agreement with other numerical and experimental results.
  Citation: Physics of Fluids
  PubDate: 2023-04-12T01:46:14Z
  DOI: 10.1063/5.0145096
Unsteady electrorotation of a viscous drop in a uniform electric field
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  Authors: Amalendu Sau
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  A dielectric drop suspended in an immiscible dielectric fluid of higher conductivity can spontaneously generate the so-called Quincke rotation (a rotating activity that a weakly conducting drop/solid particle displays in an electric field) subjected to sufficiently strong electric field strength. The steady tilt has been extensively studied and is well elucidated now. However, the unsteady electrorotation of drop remains a largely unclear, complex issue. Motivated by this, we examine the unsteady drop electrorotation in this work with the required integrated convective bulk charge transport effect. First, for the steady rotation, the transient evolution to a steady droplet tilt from the symmetric Taylor state is analyzed in-depth. Here we discover several new phenomena, including the evolving equatorial charge jets. For unsteady rotation, based on a drop's interfacial charge variation, deformation, and tilt angle, the study reports the growth of three distinct rotating patterns in the viscosity ratio range [math] and electric field strength [math] at a fixed conductivity ratio Q ( = [math]) = 0.026 and permittivity ratio S (=[math]) = 0.566. A low-viscosity drop ([math]) exhibits only the periodic rotation. For the viscosity ratio [math], the increased electric intensity creates two new unsteady rotation modes: the pseudo-periodic tumbling and the irregular one. For [math], the periodic mode remains absent; instead, the drop displays the electric intensity-dependent tumbling and irregular rotation patterns. Our study shows that the rotation reduces a drop's transitory interfacial charge. At this stage, the drop rotation behavior is controlled by competing charge convection due to fluid flow and charge supply by conduction. The resulting varying electric Reynolds number [math] (the time ratio of charge relaxation and charge convection) explains the created different rotation mechanisms. For [math], owing to lacking enough interfacial charge to sustain rotation, the drop's transition to a temporary non-rotating Taylor state occurs until the interface recharges. The resultant mechanism supports the periodic batch-type rotation for a low-viscosity drop and the irregular rotation for a high-viscosity drop in a higher electric field. In contrast, for [math], the drop timely acquires sufficient charge to support continuous tumbling electrorotation.
  Citation: Physics of Fluids
  PubDate: 2023-04-12T01:46:12Z
  DOI: 10.1063/5.0140845
Turbulent separations around a slanted-back Ahmed body with square and
rounded leading edge
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  Authors: Amir Sagharichi, Seyed Sobhan Aleyasin, Mark Francis Tachie
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  An experimental study was conducted to study the effects of rounded (RL) and squared leading edge (SL) on the time-averaged and temporal characteristics around a slanted-back Ahmed body. Measurements were conducted at two Reynolds numbers of [math] = 1.70 × 104 and 3.60 × 104. The results showed that sharpening the leading edge induces a larger recirculation region near the leading edge of the body, but slightly reduces the recirculation region in the wake region. In both leading and near wake of bodies, the recirculation length for SL cases was independent of ReH, but for the RL body, it decreases in the leading edge and increases in the wake region as ReH increases. The analysis of turbulent structures showed that the extent of the region of elevated integral timescale around the body is larger in the SL case than RL one. Statistical analysis showed that sharpening the leading edge suppresses downwash flow, which in turn reduces the shear layer interaction behind the body and decreases the dominant shedding frequency. The dominant frequencies obtained using velocity fluctuations, reverse flow area, and the coefficient of the first proper orthogonal decomposition confirmed that the dominant frequency near the leading edge and the wake region of the RL body increases with [math], while it is insensitive to [math] for SL case. The analysis performed in the spanwise plane also revealed that a region with higher streamwise mean velocity forms in the wake region of the RL body, which originates from the higher flow deviation near the trailing edge of the body.
  Citation: Physics of Fluids
  PubDate: 2023-04-12T01:46:10Z
  DOI: 10.1063/5.0143457
Investigation of motion characteristics of coarse particles in hydraulic
collection
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The solid–fluid two-phase flow with coarse particles is an important research object in the two-phase transportation field, such as deep-sea mining. This paper adopts the resolved computational fluid dynamics-discrete element method to investigate the motion and mechanical characteristics of the coarse particles during the hydraulic collection. First, the rising process of coarse particles by combining the particle trajectory with the qualitative force analysis is analyzed during the hydraulic collection. The spiral phenomenon of the particle is found through the particle trajectory in numerical results, and the centripetal force is the reason for the spiral phenomenon of the particle. Second, the variations of the normalized fluid drag force and the rise time of particles are investigated at different fluid velocities and particle sizes. The results show that the rise of particles during hydraulic collection results from the rising and settling effects characterized by the fluid drag force and the relative gravity, respectively. Finally, appropriate particle size is recommended to save energy and improve the efficiency of hydraulic collection. In addition, the influence of the horizontal distance between coarse particles and the inlet of the suction pipe on particle rise is discussed.
  Citation: Physics of Fluids
  PubDate: 2023-04-12T01:46:08Z
  DOI: 10.1063/5.0142221
Proof that all dissipation rates are only functions of time for
transported joint-normal distributions
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  Authors: Andrew P. Wandel
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  It has previously been proven that the conditional dissipation rate to transport a Gaussian distribution is equal to the mean dissipation rate throughout the variables' space and that only a Gaussian distribution can have a conditional dissipation rate that is only a function of time. This article extends both proofs to a joint-normal distribution for any number of dimensions.
  Citation: Physics of Fluids
  PubDate: 2023-04-12T01:46:07Z
  DOI: 10.1063/5.0142876
Accurate modeling of blood flow in a micro-channel as a non-homogeneous
mixture using continuum approach-based diffusive flux model
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  Authors: Shivji Prasad Yadav, Atul Sharma, Amit Agrawal
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  This paper presents a continuum approach for the blood flow simulation, inside the micro-channel of the few micrometers characteristics dimension, within the context of the finite volume method on unstructured grids. The velocity and pressure fields, for the blood flow, are obtained here by solving the Navier–Stokes equations. A particle transport equation, based on the diffusive flux model, provides the hematocrit distribution (i.e., the red blood cells volume-fraction). The momentum conservation equation for a non-Newtonian fluid model is coupled with the particle transport equation through the constitutive blood viscosity model, and this blood viscosity is dependent on hematocrit and shear rate. The continuum approach for blood flow inside the micro-channel of the length scale of a few micrometers to a few hundred micrometers is expected to break down. Interestingly, the present approach provides meaningful insights into biophysics with less computational cost and shows a good match with the experiments and mesoscale simulation with a maximum average deviation of 11% even at the characteristic dimensions of 10–300 μm. A correlation is proposed for additional-local shear rate in terms of the hematocrit and the ratio of red blood cells diameter to the channel diameter, which helps us to demonstrate an increase in the accuracy and also eliminates the issues of unphysical hematocrit reported in the earlier studies available in the literature. The study is extended to provide new results inside a square and rectangular cross section micro-channels, under a range of inlet parameters.
  Citation: Physics of Fluids
  PubDate: 2023-04-12T01:46:04Z
  DOI: 10.1063/5.0144794
Experimental study of a dental airotor cooling spray system
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  Authors: Binita Pathak, Saurabh Yadav
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  In this paper, we have characterized a spray system used in dental airotors. Experimental data with respect to droplet size and velocity are generated at different locations in the spray. The impact dynamics of the spray upon substrates are also analyzed. The breakup modes have been identified in the system, and appropriate physical insights into the dynamics are provided. The impact of the spray upon both the hard substrates results in highly rebounding daughter droplets, which can contribute to bio-aerosols. The risks of cross-contamination due to aerosol can thus be prevented with appropriate modifications of the spray nozzles.
  Citation: Physics of Fluids
  PubDate: 2023-04-12T01:46:04Z
  DOI: 10.1063/5.0143781
A hybrid boundary element method based model for wave interaction with
submerged viscoelastic plates with an arbitrary bottom profile in
frequency and time domain
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  Authors: Kottala Panduranga, Santanu Koley, Michael H. Meylan
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  This study investigates the scattering of surface ocean waves by a submerged viscoelastic plate placed over the variable bottom topography under the assumptions of linear theory. The solution is derived using the hybrid boundary element method. The boundary element method is faster and easier to use when compared to the most widely used analytical method, such as the eigenfunction expansion method. Moreover, the application of analytical methods is restricted to structures with regular geometries and flat sea bottom. However, the present solution technique works for the plate at any angle placed over the variable bottom topography. Furthermore, as a particular case, the scattering problem is analyzed when a rigid wall is placed downstream. Energy balance relations are derived to check the accuracy of the computed numerical results. The effect of sinusoidally varying bottom topography, damping parameter, and plate edge conditions on the Bragg resonance phenomenon is analyzed. Initially, the solutions are presented in the frequency domain using the hybrid boundary element method and then extended to the time domain using the Fourier transform. It is observed that when the edges of the submerged plate are fixed, the Bragg resonance occurs at lower values of the frequency parameter. However, the Bragg resonance occurs around the primary Bragg value when the plate has free edges. For certain incident wave frequencies, the viscoelastic plate that completely covers the undulating bed dissipates a greater amount of wave energy than when the plate only partially covers the seabed.
  Citation: Physics of Fluids
  PubDate: 2023-04-11T11:39:05Z
  DOI: 10.1063/5.0143412
Effect of pressure gradient on flow instability in the
subsonic–supersonic mixing layer
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  In accordance with high-speed schlieren results, the flow instabilities in the subsonic–supersonic mixing layer with a convective Mach number of 0.19 are investigated in detail. In the incipient stage of the mixing layer, wave structures caused by the pressure gradient affect the evolution of the Kelvin–Helmholtz vortexes. The dynamic mode decomposition (DMD) analysis reveals that the pressure gradient from the subsonic side to the supersonic side promotes flow instability. At this time, the Kelvin–Helmholtz vortexes mode is found to be dominant. A high temporal resolution is proven to play an important role in the DMD analysis to capture high-frequency modes.
  Citation: Physics of Fluids
  PubDate: 2023-04-11T11:39:04Z
  DOI: 10.1063/5.0147675
Study on flow noise characteristic of transonic deep buffeting over an
airfoil
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Transonic buffeting can induce strong noise and reduce aircraft lifespan. In view of the complexity of the transonic buffeting flow, this study combines the highly accurate Delayed-Detached Eddy Simulation and Discrete Frequency Response method to analyze the flow field and sound propagation law in different buffeting states and also investigates its noise-generating characteristics by Dynamic Mode Decomposition and Pearson correlation. It is found that the low-frequency and small-amplitude shock oscillation of the light buffeting state is insufficient to trigger large separated flow. Besides, the reattachment phenomenon occurs in the trailing edge, which is the second mode of boundary layer separation, corresponding to the lower Sound Pressure Levels (SPL). In the deep buffeting state, however, the shock oscillates with high frequency and large amplitude, producing large separated bubbles without the reattachment phenomenon, which is the first mode of boundary layer separation. Moreover, there is a large-scale vortex structure with high energy content in the recirculation zone, which develops toward the trailing edge under the action of convection and produces strong Upstream Traveling Waves (UTWs). The collision occurs between UTWs and the shock wave oscillation. In this process, they promote each other, which increases the shock wave oscillation frequency and SPL. This state is not the superposition effect of buffeting and stall. And its main sound sources are shock oscillation and the von Kármán mode.
  Citation: Physics of Fluids
  PubDate: 2023-04-11T11:39:02Z
  DOI: 10.1063/5.0138636
Modeling geysers triggered by an air pocket migrating with running water
in a pipeline
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Storm sewer systems may experience storm geysers, raising concerns about public safety. A thorough understanding of the influential factors of the geysers is essential yet insufficiently investigated in the literature. A transient three-dimensional (3D) computational fluid dynamics model incorporating the volume of fluid method is used to investigate the geyser formation mechanism and hydrodynamics. An air pocket in a pressurized pipe travels with water past a vertical shaft, producing an air-releasing geyser and, subsequently, a rapid-filling geyser. If the air pocket in the pipe is too small or if it moves too quickly, a hybrid geyser might be set off when the air-releasing and rapid-filling geysers overlap. A hybrid geyser has unique properties since it combines an air-releasing geyser and a rapid-filling geyser. The presence of hybrid geysers lowers the height of air-releasing and rapid-filling geysers. Equations are proposed for predicting the heights of the geysers with errors of about 15%. The height of the air-releasing geyser increases with the water level in the shaft. As the pressure difference between the two ends of the pipe reduces, the height of the rapid-filling geyser increases. The vertical shaft diameter has little influence on rapid-filling geysers, while a small diameter often results in high air-releasing geysers. The effect on the height of both kinds of geysers is negligible when the air pocket volume is large enough. The findings can be used for designing storm geyser mitigation measures.
  Citation: Physics of Fluids
  PubDate: 2023-04-11T02:45:03Z
  DOI: 10.1063/5.0138342
Coherent organizational states in turbulent pipe flow at moderate Reynolds
numbers
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  Authors: R. Jäckel, B. Magacho, B. E. Owolabi, L. Moriconi, D. J. C. Dennis, J. B. R. Loureiro
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Turbulent pipe flow is still an essentially open area of research, boosted in the last two decades by considerable progress achieved on both the experimental and numerical frontiers, mainly related to the identification and characterization of coherent structures as basic building blocks of turbulence. It has been a challenging task, however, to detect and visualize these coherent states. We address, by means of stereoscopic particle image velocimetry, that issue with the help of a large diameter (6 in.) pipe loop, which allowed us to probe for coherent states at various moderate Reynolds numbers (5300
  Citation: Physics of Fluids
  PubDate: 2023-04-10T02:05:38Z
  DOI: 10.1063/5.0143815
Direct numerical simulations of the Taylor–Green vortex interacting with
a hydrogen diffusion flame: Reynolds number and non-unity-Lewis number
effects
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  Abstract: Physics of Fluids, Volume HFDP2022, Issue 1, April 2023.
  Understanding the interactions between hydrogen flame and turbulent vortices is important for developing the next-generation carbon neutral combustion systems. In the present work, we perform several direct numerical simulation cases to study the dynamics of a hydrogen diffusion flame embedded in the Taylor–Green Vortex (TGV). The evolution of flame and vortex is investigated for a range of initial Reynolds numbers up to 3200 with different mass diffusion models. We show that the vortices dissipate rapidly in cases at low Reynolds numbers, while the consistent stretching, splitting, and twisting of vortex tubes are observed in cases with evident turbulence transition at high Reynolds numbers. Regarding the interactions between the flame and vortex, it is demonstrated that the heat release generated by the flame has suppression effects on the turbulence intensity and its development of the TGV. Meanwhile, the intense turbulence provides abundant kinetic energy, accelerating the mixing of the diffusion flame with a contribution to a higher strain rate and larger curvatures of the flame. Considering the effects of the non-unity-Lewis number, it is revealed that the flame strength is more intense in the cases with the mixture-averaged model. However, this effect is relatively suppressed under the impacts of the intense turbulence.
  Citation: Physics of Fluids
  PubDate: 2023-04-10T02:05:37Z
  DOI: 10.1063/5.0144764@phf.2023.HFDP2022.issue-1
Direct numerical simulations of the Taylor–Green vortex interacting with
a hydrogen diffusion flame: Reynolds number and non-unity-Lewis number
effects
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Understanding the interactions between hydrogen flame and turbulent vortices is important for developing the next-generation carbon neutral combustion systems. In the present work, we perform several direct numerical simulation cases to study the dynamics of a hydrogen diffusion flame embedded in the Taylor–Green Vortex (TGV). The evolution of flame and vortex is investigated for a range of initial Reynolds numbers up to 3200 with different mass diffusion models. We show that the vortices dissipate rapidly in cases at low Reynolds numbers, while the consistent stretching, splitting, and twisting of vortex tubes are observed in cases with evident turbulence transition at high Reynolds numbers. Regarding the interactions between the flame and vortex, it is demonstrated that the heat release generated by the flame has suppression effects on the turbulence intensity and its development of the TGV. Meanwhile, the intense turbulence provides abundant kinetic energy, accelerating the mixing of the diffusion flame with a contribution to a higher strain rate and larger curvatures of the flame. Considering the effects of the non-unity-Lewis number, it is revealed that the flame strength is more intense in the cases with the mixture-averaged model. However, this effect is relatively suppressed under the impacts of the intense turbulence.
  Citation: Physics of Fluids
  PubDate: 2023-04-10T02:05:37Z
  DOI: 10.1063/5.0144764
Transverse mixing zone under dispersion in porous media: Effects of medium
heterogeneity and fluid rheology
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The addition of an environmental remediation modifier—polymer solution—to a Newtonian fluid expands the distribution of remediation agents injected in situ into saturated aquifers (affecting plume velocity and deformation), enhancing remediation efficiency. However, the effect of the flow properties of the polymer solution on the macroscopic transverse dispersion remains poorly understood. In this work, a transparent thin-layer two-dimensional sandbox was constructed to simulate the aquifer, and the transverse distribution range of colored solute—permanganate solution and viscous shear-thinning fluid (permanganate solution + xanthan gum)—was captured in real-time by a camera device during transport in porous media. The boundary dispersion coefficient was obtained by fitting a breakthrough curve of the boundary concentration, while the overall plume dispersion coefficient was determined via image moment analysis. The effects of fluid rheology and heterogeneity on the transverse mixing of the plume were analyzed, and the mechanism of viscoelasticity-induced transverse dispersion and mixing enhancement was summarized. The results indicated that the anisotropic stress generated by polymer fluid deformation at high water velocity increased the fluctuation and transverse distribution of the plume, while higher-viscosity polymers increased the initial extrusion swelling and additional compressive stress, covering a larger area. Xanthan gum enhanced the transverse distribution of the plume mainly through initial injection-extrusion expansion effect, viscoelastic stability of the post-injection part, and streamline crossing attributed to elastic turbulence. This study also verified that the shear-thinning fluid enhanced the effect of transverse dispersion and mixing under heterogeneous conditions, providing insights applicable to groundwater remediation.
  Citation: Physics of Fluids
  PubDate: 2023-04-10T02:05:36Z
  DOI: 10.1063/5.0141837
Efficiency of energy and enstrophy transfers in periodical flows
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  Authors: A. De Leo, A. Stocchino
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  We apply a coarse-graining technique to understand the efficiency of scale-to-scale transport of energy and enstrophy in a quasi-two-dimensional weakly turbulent periodic flow. The investigated periodic flow resembles the propagation of a monochromatic tide in a tidal channel, connected to open sea through an inlet. The interaction of the periodic flow with the inlet mouth generates vortical structures in a wide spectrum of scales, and recently, how the corresponding energy and enstrophy fluxes change their signs depending on the tidal phase has been shown. In the present study, we are interested to extend the analysis to the efficiency of the nonlinear transfer rates by analyzing the geometric alignment between the turbulent stresses and the strain rates for the energy, and the vorticity stress and large-scale vorticity gradient for the enstrophy. Our results suggest that, depending on the phase of the period, energy is efficiently transferred to larger scales (inverse cascade) in a finite range of scales, whereas the observed direct energy cascade for very small and very large scales is much less efficient. Enstrophy shows similar behaviors in terms of transitions between direct and inverse cascading; however, all transfers seem to be relatively inefficient.
  Citation: Physics of Fluids
  PubDate: 2023-04-10T02:05:34Z
  DOI: 10.1063/5.0142848
Experiments to understand microlayer and dry patch dynamics under
subcooled nucleate flow boiling in a vertically oriented rectangular
channel
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  Authors: Mohd Moiz, Sai Raja Gopal Vadlamudi, Atul Srivastava
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Nucleate flow boiling offers high heat transfer rates and is considered an effective mode of heat transfer in many systems involving high heat loads. The phenomenon is characterized by the inception of vapor bubble(s) and its growth, followed by its departure in a periodic manner. The evolution of the nucleating bubble's footprint—microlayer and dry patch dynamics—is important in understanding the heat transfer capability and limiting heat flux values. However, efforts toward developing a fundamental understanding of this phenomenon during the nucleate flow boiling regime under subcooled bulk conditions are scarce in the open literature. Toward bridging this gap, we report flow boiling experiments on a hydrophilic surface for investigating the plausible influence of subcooling and minimize the influence of the hydrodynamic movement of contact lines on the dry patch dynamics. Experiments have been conducted in a vertically oriented rectangular channel with water as the working fluid for a Reynolds number of Re = 2400. Real-time microlayer dynamics have been mapped using thin-film interferometry, while the bubble evolution has been captured using one of the gradients-based imaging approaches employed from the side view. Experiments revealed a noticeable influence of subcooling on dry patch and microlayer dynamics. The size of the dry patch and the radial spread of the microlayer showed a decreasing trend with increasing subcooling level. Experimental conclusions are also supported with theoretical considerations.
  Citation: Physics of Fluids
  PubDate: 2023-04-10T02:05:31Z
  DOI: 10.1063/5.0142177
Influences of semi-circular, square, and triangular grooves on mixing
behavior of an axisymmetric supersonic jet
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  Authors: Amit Krishnat Mali, Tamal Jana, Mrinal Kaushik, Debi Prasad Mishra
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The length of the supersonic jet ejected from the military aircraft must be reduced in order to decrease its heat signature and aeroacoustic noise and thereby to enhance its stealth capability. The reduction or manipulation of the supersonic core can be achieved through various passive control techniques. Considering this, the present study explores the mixing characteristics of supersonic jets with and without passive controls. Passive controls in the form of grooves configured at the exit of a Mach 1.73 convergent–divergent nozzle are investigated computationally. Particularly, the supersonic jet decay characteristics and flow development for a plain nozzle and a nozzle with semi-circular, square, and triangular grooves are presented. In addition, the study explores different turbulence models, namely, Spalart–Allmaras, realizable k-ε, std k-ω, shear stress transport (SST) k-ω, and SST transition. The realizable k-ε turbulence model is found to be the most effective one in capturing the supersonic jet structure. It is observed that the grooves produce large distortions in the jet structure, accompanied by significant mass entrainment and lateral spread. Interestingly, semi-circular grooves are proven to be most effective in all cases of expansion level than square and triangular grooves. For the semi-circular grooves, a maximum of 48.5% reduction in the supersonic core length of the correctly expanded jet at nozzle pressure ratio (NPR) of 5 is achieved. The reduction in the supersonic core length for semi-circular grooves is 31% for the overexpanded jet at NPR 4 and 29% for the underexpanded jet at NPR 7.
  Citation: Physics of Fluids
  PubDate: 2023-04-10T02:05:30Z
  DOI: 10.1063/5.0146672
Electro-osmotic flow in different phosphorus nanochannels
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  Authors: S. M. Kazem Manzoorolajdad, Hossein Hamzehpour, Jalal Sarabadani
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The electrokinetic transport in a neutral system consists of an aqueous NaCl solution confined in a nanochannel with two similar parallel phosphorene walls and is investigated for different black, blue, red, and green phosphorene allotropes in the presence of an external electric field in the directions x (parallel to the walls roughness axis) and y (perpendicular to the walls roughness axis). The results show that irrespective of the electric field direction, the thickness of the Stern layer increases with the increase in the magnitude of the negative electric surface charge density (ESCD) on the nanochannel walls, and it also increases with the increase in the roughness ratio for different allotropes. Moreover, three different regimes of Debye–Hückel (DH), intermediate, and flow reversal appear as the absolute value of the negative ESCD on the walls grows. With the increase in the absolute value of the negative ESCD, in the DH regime, the flow velocity grows, then in the intermediate regime, it decreases, and finally, at sufficiently high ESCD, the flow reversal occurs. When the external electric field is applied in the y direction, the dynamics of the system are slower than that of the x direction; therefore, the flow reversal occurs at the smaller absolute values of the negative ESCD.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T12:57:24Z
  DOI: 10.1063/5.0142011
Analytical and numerical investigation on the energy of free and locked
tsunami waves generated by a submarine landslide
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Submarine landslides are capable of causing locally catastrophic tsunamis. Landslide parameters, particularly those related to the landslide motion, are highly uncertain in a real landslide tsunami event. To date, a practical method for effectively and efficiently modeling the landslide tsunami generation process is still lacking. To gain insight into the landslide tsunami generation mechanism, we employed a combination of analytical derivation and numerical computation. From the wave energy perspective, we found the locked wave component of a landslide tsunami to be as important as the free wave component. Thus, the locked wave component cannot be neglected. We showed that for a geophysically relevant submarine landslide speed, the locked wave component has a deceivingly small wave amplitude with large flow velocities. Thus, careful attention must be paid to flow velocities when modeling landslide tsunamis. For a submarine landslide forcing water waves at a constant speed, we found that the total wave energy first evolves from zero to a peak value, before decreasing to an asymptotic value. These two distinct energy values and the corresponding wave generation times may serve as conservative estimates in predictive studies, in which precise information on the landslide dynamics is impossible to obtain. Finally, we used the 1998 Papua New Guinea landslide tsunami as an example to demonstrate how the findings in this study aid in the modeling effort for a real event.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T10:21:44Z
  DOI: 10.1063/5.0144533
Publisher’s Note: “Investigation of the load and flow characteristics
of variable mass forced sloshing” [Phys. Fluids 35, 033325 (2023)]
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  
  Citation: Physics of Fluids
  PubDate: 2023-04-07T02:23:56Z
  DOI: 10.1063/5.0151632
Reducing the contact time of off-center impacts
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  When a droplet off-center impacts a macro-ridge, the contact time increases with off-center distance [math], which are closely related to two mechanisms, i.e., the redistribution of liquid volume and the asymmetry of the liquid film. Therefore, changing the asymmetry of the liquid film may provide fundamental inspiration for the efficient control of the contact time. Using lattice Boltzmann method simulations, the dynamics of a droplet off-center impacting a ridge on a superhydrophobic surface are explored to demonstrate the feasibility of reducing contact time by changing the asymmetry of the liquid film, which is changed by manipulating the inclination of the ridge. For positive off-center impact [math], the contact time decreases with the increase in the inclined angle as increasing the inclination can decrease the asymmetry of the liquid film. For negative off-center impact [math], tilting the ridge can further reduce the asymmetry of the liquid film to a limit, and its influence can be ignored at [math], leading to the contact time decreasing more significantly compared with that for [math]. On this basis, a quantitative relationship of contact time for a droplet off-center impacting an inclined ridge is established. This work provides fundamental and practical inspiration for the efficient reduction of contact time for off-center impacts.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T02:23:25Z
  DOI: 10.1063/5.0146943
Weighted compact nonlinear hybrid scheme based on a family of mapping
functions for aeroacoustics problem
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The study of differential schemes with high accuracy and low dispersion is of great significance for the numerical simulation of aeroacoustics over complex geometries. The midpoint-and-node-to-node explicit finite difference is employed to solve the flux derivatives of the linear Euler equations for improving the robustness. To obtain the numerical flux at the center of the grid element, we adopted a hybrid interpolation method of center and upwind interpolations, combined with a symmetrical conservative metric method, to achieve high-resolution discretization of the acoustic field variables and geometric variables of the structural grid. To suppress spurious oscillations and improve the resolution of discontinuous regions, a family of mapping functions is developed to establish different smoothness indicators and applied to the sixth-order weighted compact nonlinear hybrid scheme (WCNHS), forming a new WCNHS scheme based on piecewise exponential mapping functions (WCNHS-Pe). The approximate dispersion relation shows that the dispersion error and numerical dissipation of WCNHS-Pe are smaller than those of WCNHS with a simple mapping function to the original weights in Jiang and Shu and other mapping function-weighted WCNHS schemes. We have applied various WCNHS schemes to the numerical simulation of the Shu–Osher problem, propagation of Gaussian impulses on two-dimensional wavy grids, sound transmission at discontinuous interfaces, propagation of Gaussian impulses on three-dimensional wavy grids, etc. Numerical results indicate that the WCNHS-Pe scheme has better discontinuity capture capability, higher resolution, and lower dispersion under the same differential stencil, and is suitable for computational aeroacoustics of complex geometries.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T02:19:54Z
  DOI: 10.1063/5.0144741
Direct numerical simulation of detonation–turbulence interaction in
hydrogen/oxygen/argon mixtures with a detailed chemistry
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  Authors: Kazuya Iwata, Sou Suzuki, Reo Kai, Ryoichi Kurose
  Abstract: Physics of Fluids, Volume HFDP2022, Issue 1, April 2023.
  Direct numerical simulation is conducted to address the detonation–turbulence interaction in a stoichiometric hydrogen/oxygen/argon mixture. The argon dilution rate is varied so that the mixture composition is 2H2 + O2 + 7Ar and 2H2 + O2 + Ar to discuss the effects of cell regularity on the sensitivity to turbulence. Turbulent Reynolds number and turbulent Mach number are taken to be common for both mixtures. The results show that the shock and flame of detonation in both mixtures are significantly deformed into corrugated ones in the turbulent flow, producing many small unburned gas pockets. However, one-dimensional time-averaged profiles reveal the different sensitivity of the mixtures: in the highly diluted mixture (2H2 + O2 + 7Ar), the reaction progress is not much influenced by turbulence, whereas in the less-diluted mixture (2H2 + O2 + Ar), the reaction takes place more rapidly with turbulence. Analysis of the properties of turbulence and turbulent fluctuations in the detonations clarifies that the direct contribution of turbulence to the flame front is weaker; there is no clear correlation between the heat release and the curvature of the flame. On the other hand, a broader Mach number distribution just upstream of the shock front creates more hot spots in the less-diluted mixture, which results in a shorter induction length. These results indicate that the main contribution of turbulence is creation of different shock strength, which could lead to different reaction rates depending on the cell regularity.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T02:14:24Z
  DOI: 10.1063/5.0144624@phf.2023.HFDP2022.issue-1
Direct numerical simulation of detonation–turbulence interaction in
hydrogen/oxygen/argon mixtures with a detailed chemistry
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  Authors: Kazuya Iwata, Sou Suzuki, Reo Kai, Ryoichi Kurose
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Direct numerical simulation is conducted to address the detonation–turbulence interaction in a stoichiometric hydrogen/oxygen/argon mixture. The argon dilution rate is varied so that the mixture composition is 2H2 + O2 + 7Ar and 2H2 + O2 + Ar to discuss the effects of cell regularity on the sensitivity to turbulence. Turbulent Reynolds number and turbulent Mach number are taken to be common for both mixtures. The results show that the shock and flame of detonation in both mixtures are significantly deformed into corrugated ones in the turbulent flow, producing many small unburned gas pockets. However, one-dimensional time-averaged profiles reveal the different sensitivity of the mixtures: in the highly diluted mixture (2H2 + O2 + 7Ar), the reaction progress is not much influenced by turbulence, whereas in the less-diluted mixture (2H2 + O2 + Ar), the reaction takes place more rapidly with turbulence. Analysis of the properties of turbulence and turbulent fluctuations in the detonations clarifies that the direct contribution of turbulence to the flame front is weaker; there is no clear correlation between the heat release and the curvature of the flame. On the other hand, a broader Mach number distribution just upstream of the shock front creates more hot spots in the less-diluted mixture, which results in a shorter induction length. These results indicate that the main contribution of turbulence is creation of different shock strength, which could lead to different reaction rates depending on the cell regularity.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T02:14:24Z
  DOI: 10.1063/5.0144624
Large eddy simulation of shock wave/turbulent boundary layer interaction
under incipient and fully separated conditions
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Large eddy simulations of shock wave/turbulent boundary layer interaction on a compression ramp at the Mach number [math] and Reynolds number [math] are performed to investigate the impact of the incipient and fully separated conditions on the development of the flow field. The quasi-dynamic subgrid-scale kinetic energy equation model, which combines the benefits of the gradient model with the eddy-viscosity model, has been applied. Compared with the previous experimental and numerical results, the simulation was validated. The flow structures, turbulence properties, vortex structures, and low-frequency unsteadiness are all investigated. The flow field of the incipient separation is attached and rarely impacted by shock. An evident separation bubble and localized high wall temperatures in fully separated flow are caused by the separation shock's significant reverse pressure gradient. The Reynolds stress components exhibit significant amplification in both cases, and the peak outward shifts from the near-wall region to the center of the free shear layer. Turbulent kinetic energy terms were analyzed, and the two scenarios show a significant difference. The power spectral density of the wall pressure fluctuations shows that the low-frequency motion of the incipient separation is not apparent relative to the fully separated flow.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T02:12:04Z
  DOI: 10.1063/5.0147829
Numerical simulation of ice shedding motion characteristic on airfoil
surface
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  In order to accurately predict the motion trajectory of ice shedding and ensure the safe flight of aircraft, the motion characteristics of ice shedding under two-dimensional (2D) and three-dimensional (3D) conditions are simulated and analyzed. Considering the influence of any possible shape of shedding ice and its rotation and the magnitude and direction of acceleration with time under aerodynamic force, a six degree-of-freedom analysis method is introduced in this paper. This paper proposes a theoretical model, which can be used to calculate the 3D trajectory of ice shedding with arbitrary shape. The dynamic analysis of real 3D shedding ice is carried out, and the motion behavior of shedding ice with different positions and shapes is calculated. The results show that the movement mode of the shedding ice after leaving the aircraft is translation and rotation. The shape of the low-speed region on the leeward side of the shedding ice will first increase, then decrease, and then increase with the rotation of the ice body. The influence of ice shape on ice shedding trajectory is mainly that the shedding ice continues to flip during the downstream movement of the flow field, and the projected area of the effective windward area in the lift and the drag direction changes with time. The average deviation of the shedding ice at position 5 along the spanwise is only 22.9% of that at position 1. Finally, the closer the initial position of ice shedding is to the airfoil root, the greater the probability of ice shedding hitting the aircraft fuselage. In this paper, the probability of ice shedding hitting the aircraft fuselage is 8%, which all occurred in the case with position 1 as the initial position of ice shedding.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T02:08:14Z
  DOI: 10.1063/5.0143751
Experimental and numerical investigations of effects of ship
superstructures on wind-induced loads for benchmarking
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  Authors: Ould el Moctar, Udo Lantermann, Vladimir Shigunov, Thomas E. Schellin
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  For a representative large modern containership, the effects of a deck container arrangement on the wind-induced loads were systematically investigated using physical model tests and numerical computations. Numerical simulations based on various turbulence models were performed to validate our predictions against comparative wind tunnel measurements. Not only standard two-equation turbulence models of the unsteady Reynolds-Averaged Navier–Stokes (URANS) equations solver but also the improved delayed detached eddy simulation (IDDES) and large eddy simulation (LES) turbulence models were used to determine their limits in the prediction of aerodynamic loads. Systematic discretization studies ensured adequate discretization independent predictions. With URANS, numerically predicted wind forces and moments in near-head and near-tail winds were compared favorably with the measured data. However, in oblique winds, URANS predictions deviated from measurements. In oblique winds, flow separations were pronounced; therefore, the flow was strongly transient. Consequently, a two-equation turbulence model was inappropriate. With IDDES, more accurate predictions were achieved, especially in oblique winds. With LES, although the computational effort was high, the agreement of the computed forces and moments with the measured values was superior. Flow details were also presented and discussed. The container arrangement on deck showed major effects on aerodynamic forces and moments. A tarpaulin covering the containers on deck reduced wind resistance by up to 70%.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T02:04:44Z
  DOI: 10.1063/5.0146778
Effect of micro-grooves on drag reduction in Taylor–Couette flow
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Taylor–Couette flow with micro-grooves on the rotating inner cylinder is investigated to reveal the effect of surface structures on drag reduction. The Reynolds number (Re) ranges from 160 to 18 700. On the one hand, in the regimes of wave vortex flow (WVF, 160 < Re < 1010) and modulated wavy vortices (MWV, 1010 < Re < 1380) flow, the micro-grooves always reduce the torque, indicating drag reduction. Increasing either the size of micro-groove or Re, drag reduction will be enhanced. On the other hand, when the flow regime enters turbulent Taylor vortices (TTV, Re> 1380), drag reduction will be suppressed as Re increases and eventually turns to drag increase. The bigger the groove size, the smaller the critical Re where it turns from drag reduction to drag increase. To reveal the underlying mechanism of the effect of micro-grooves on drag reduction, particle image velocimetry measurements are conducted to observe the vortex flow structures, which demonstrates two aspects affecting the drag of Taylor–Couette flow over micro-grooved wall. First, the weakening of the large-scale Taylor vortex will lead to drag reduction. Second, the roughness effect will result in drag increase. In WVF/MWV, the former plays a dominant role, while in TTV, the latter dominates. In addition, a relationship between the micro-groove size and the predictive critical Reynolds number (Rec) is developed, providing a method for controlling the wall drag.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T02:02:04Z
  DOI: 10.1063/5.0145900
The enhancement of flow induced vibration of a circular cylinder using a
rotating control rod
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  Authors: Erfan Taheri, Ming Zhao, Helen Wu
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The enhancement of flow induced vibration of a circular cylinder by a rotating control rod is investigated through two-dimensional numerical simulations. The Reynolds number, diameter ratio, and gap ratio are 150, 0.2, and 0.2, respectively. Simulations are conducted for two rod position angles of β = 90° and 135°, rotation rates ranging from 0 to 6, and reduced velocities ranging between 1 and 20. The response of the cylinder–rod system at the rotation rates 0 and 1 has a lock-in regime where the vibration amplitude is high and the vibration frequency stops increasing with the increase in reduced velocity linearly. For rotation rates exceeding 2, the response amplitude increases with the increase in reduced velocity and enters the lock-in regime at the lower boundary reduced velocity. It remains high until the largest studied reduced velocity of 20; as a result, the higher boundary reduced velocity of the lock-in regime cannot be determined. The vibration with large amplitudes and large rotation rates repeats cyclically after every two or more vibration periods. As a result, two combined wake modes are found: 2S/P + S and 2P/P + S. In a combined mode, the vibration changes from one mode to another within each cycle. The cylinder receives power from the fluid, and the rotating rod gives power to the fluid although the net power exchange between the whole system and the fluid is zero.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T01:59:04Z
  DOI: 10.1063/5.0146552
Slot coating flows with a Boussinesq–Scriven viscous interface
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  Authors: F. O. Silva, I. R. Siqueira, M. S. Carvalho, R. L. Thompson
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  We present a computational study of free surface flows with rheologically complex interfaces in the film formation region of a slot coater. The equations of motion for incompressible Newtonian liquids in the bulk flow are coupled with the Boussinesq–Scriven constitutive equation for viscous interfaces in the dynamic boundary condition at the liquid-air free surface and solved with a mixed finite element method. We show that the interfacial viscosity plays a major role in the flow dynamics and operating limits of slot coating. We find that the interfacial viscosity makes viscous interfaces generally stiffer than their simple counterparts, affecting both the normal and the tangential stress jumps across the free surface. As a result, the interfacial viscosity counteracts the meniscus retraction and slows down the film flow, increasing the development length over the substrate and changing the topology of the recirculation region in the coating bead. Remarkably, we also find that the interfacial viscosity can substantially broaden the operating boundaries of the coating window associated with the low-flow limit, suggesting that surface-active components can be suitably designed to allow for the stable production of thinner films at higher speeds by tuning interfacial material properties in slot coating applications.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T01:50:54Z
  DOI: 10.1063/5.0147030
Phase-locked measurements of linear and weakly nonlinear interfacial waves
in a stratified turbulent gas–liquid pipe flow
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  Authors: P. S. C. Farias, L. F. A. Azevedo, I. B. de Paula
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The present work reports an experimental characterization of linear and weakly nonlinear interfacial waves in a stratified air–water horizontal pipe flow. An oscillating paddle was employed to generate controlled waves at the liquid interface. The driving signal of the oscillating paddle was controlled and synchronized with image acquisitions, enabling phase-locked measurements and the application of ensemble averaging techniques. Velocity field measurements in the liquid and gas phases were performed simultaneously using an off-axis particle image velocimetry setup and shadowgraphy. The combined techniques allowed us to extract the coherent part of flow fluctuations related to the excited waves. This was done for a range of flow rates and wave frequencies. The selected conditions are close to the transition from stratified to slug/plug flow regimes. In the presence of linear waves, the coherent disturbances in both phases were weakly dependent on near-wall disturbances. Flow changes in the presence of weakly nonlinear waves were also investigated. In these cases, noticeable modifications in the mean flow and in turbulence distribution were observed near the interface, whereas close to the wall, the flow was weakly affected. This investigation follows the work of Farias et al. [“Characterization of interfacial waves in stratified turbulent gas-liquid pipe flow using Particle Image Velocimetry and controlled disturbances,” Int. J. Multiphhase Flow 161, 104381 (2023)], where the threshold for linear and weakly nonlinear waves was studied. Here, a clear comparison between wave-induced disturbances in linear and weakly nonlinear regimes is reported in the literature for the first time for stratified turbulent gas–liquid pipe flows. The methodology proposed is relatively simple and can contribute to describe wave-related phenomena in stratified pipe flows.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T01:47:44Z
  DOI: 10.1063/5.0143911
An analytical double-Gaussian wake model of ducted horizontal-axis tidal
turbine
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The wake development of a tidal turbine should be fully considered in the array arrangement. There are many studies on wake characteristics, mainly focusing on a conventional horizontal-axis turbine, while a ducted turbine has attracted little attention. This paper investigates the wake characteristic of a ducted turbine using flume experiments and large eddy simulations. An analytical wake model of the ducted turbine is proposed and verified by the wake profile under different inflow velocities and the downstream turbine performance under different tandem arrangements. The results show that a ducted turbine wake still maintains a high self-similarity, and the wake profile is approximately the double-Gaussian curve. Compared with a conventional tidal turbine, a ducted turbine has a faster wake recovery speed, but a larger radial influence range. Therefore, ducted turbine arrays should be configured with wider radial distances and shorter axial distances.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T01:44:24Z
  DOI: 10.1063/5.0146196
Comparing classical electrodynamic theories predicting deformation of a
water droplet in a tightly focused Gaussian beam
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  Authors: Cael Warner, Chun-Sheng Wang, Kenneth J. Chau
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Optical forces are used to accelerate and trap water droplets in applications such as remote spectroscopy and noninvasive surgery. However, the microscopic deformation of droplets is difficult to predict. In this work, the local electrodynamic impulse imparted by a focused laser beam to a water droplet is numerically modeled via a simulation that invokes intensive conservation of electrodynamic and kinetic momentum. Electrodynamic momentum is modeled locally using a D3Q7 electrodynamic lattice-Boltzmann method, and kinetic momentum is modeled locally using a multi-phase D3Q27 weighted-orthogonal lattice-Boltzmann method. Six different electrodynamic theories are implemented in the simulation domain predicting three unique types of droplet dynamics driven by differences in the direction and distribution of force density. The unique water droplet morphology affects the center-of-mass acceleration of the droplet. This study suggests that empirical measurement of the light-driven acceleration of a droplet may help to validate a single electrodynamic theory.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T01:40:04Z
  DOI: 10.1063/5.0139855
The stabilizing effect of grooves on Görtler instability-induced boundary
layer transition in hypersonic flow
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  Authors: Xi Chen, Jianqiang Chen, Xianxu Yuan, Guohua Tu
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Görtler vortex-induced hypersonic boundary layer transition controlled by grooves is investigated using direct numerical simulations and spatial bi-global stability analysis. In the simulations, Görtler vortices are excited by wall steady blowing and suction with spanwise wavelengths of 3 mm. It is found that when the wall is covered with grooves, the Görtler streaks keep more regular even at the end of the model. In addition, the skin friction coefficient is reduced efficiently. Furthermore, the wall-normal and spanwise velocity shear are both reduced, suppressing growths of secondary instabilities. In conclusion, grooves can delay Görtler vortex-induced transition by modifying the Görtler streaks structure and instability, which would shed light on hypersonic boundary layer transition control.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T01:34:35Z
  DOI: 10.1063/5.0146348
Equilibria of liquid drops on pre-stretched, nonlinear elastic membranes
through a variational approach
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  Authors: Vineet Nair, Ishan Sharma
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  We study the equilibrium of planar systems consisting of sessile and pendent drops on pre-stretched, nonlinear elastic membranes. The membrane experiences large deformations due to both capillary forces and the drop's weight. The membrane's surface energies are allowed to depend upon stretches in the membrane. We minimize the free energy of the system to obtain the governing equations. This recovers all equations found by force balance, in addition to an extra condition that must hold at the triple point. The latter closes the system's mathematical description and defines a unique equilibrium given the membrane's material and pre-stretch, and the properties of drop's fluid and its volume. The extra condition simplifies to continuity of stretches at the triple point when the surface energies are strain-independent. We then solve these coupled nonlinear equations to obtain the global equilibria of the drop–membrane system. We report the effects of drop's volume and membrane's pre-tension on the system's geometry and tension distribution in the membrane. Through this, we align the theory closely with experiments, which will then allow the use of the present system both as an elastocapillary tension probe and as a device to measure solid surface energies.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T01:26:49Z
  DOI: 10.1063/5.0140077
Multiphase flow characteristics and gas loss in the shear layer on a
ventilated supercavity wall
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The shear flow on the large-scale gas–water wall inside a ventilated supercavity exhibits gas entrainment mode and determines the change law of the supercavity's gas loss, significantly impacting the shape and dynamics of the supercavity. Therefore, to develop an accurate prediction model and a ventilation control method for a supercavity under complex motion conditions, it is required to systematically and quantitatively study the shear flow characteristics and rules. This study calculates and comparatively analyzes the shear layers on either side of the supercavity wall based on numerical simulations of ventilated supercavitating flows in an unbounded field using the gas–vapor–water multi-fluid model. It is shown that the external shear layer with a very irregular outer boundary is considerably thinner than the internal shear layer. We further analyze the flow and distribution characteristics of all the phases in the shear layers with and without the influence of gravity. Our analysis confirms that all the phases exhibit a similar velocity change rule along the supercavity radial direction in the shear layer, whereas gas and water phases exhibit opposite radial phase distribution trends. It was also seen when natural cavitation occurs that the vapor phase is mainly distributed in the head of the supercavity. Moreover, at the same radial position, it was seen that the vapor velocity was higher than the gas velocity and slightly lower than the water velocity. Using the shear flow and phase distribution characteristics, a shear-layer gas loss model is established and validated for ventilated supercavitating flows.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T01:23:44Z
  DOI: 10.1063/5.0141678
Oil–water two-phase flow-induced vibration of a cylindrical cyclone
with vortex finder
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Cylindrical cyclones play an important role in oil–water separation and sewage treatment in the petroleum industry. Here, we describe the characteristics of vibration induced by a two-phase rotational flow in a cylindrical cyclone. The cyclone operating parameters together with a dimensional analysis and multiphase flow numerical simulation were used to understand the flow field characteristics. The frequency and amplitude of pressure fluctuation were obtained by measuring pressure changes at points on the axis of the device. It shows that the pressure in a cylindrical cyclone varies periodically during separation and that fluctuation frequency and amplitude are related to the inlet velocity and flow split ratio. The effect of the overflow split ratio on the pressure fluctuation frequency is negligible, but increasing the overflow split ratio will cause greater fluctuation of the flow. For a cylindrical cyclone, the pressure fluctuation frequency can be calculated from the inlet velocity. Adjusting the inlet velocity and the overflow split ratio changes the mechanical response of the structure. The results of a modal analysis show that the structural vibration response is consistent with the response state of the lowest point of the internal central-vortex pressure and that both are in approximate circular motion. Furthermore, the frequency of pressure fluctuation induced by the flow is close to the intrinsic frequency of the structure with a single bottom constraint, which can cause unwanted resonance easily. Therefore, an appropriately added constraint on a cylindrical cyclone should be taken into consideration to avoid the resonance frequency.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T01:21:24Z
  DOI: 10.1063/5.0140066
A physics-preserving pure streamfunction formulation and high-order
compact solver with high-resolution for three-dimensional steady
incompressible flows
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  Authors: Xiaohu Guo
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  In this paper, a pure streamfunction high-order compact (HOC) difference solver is proposed for three-dimensional (3D) steady incompressible flows. A physics-preserving pure streamfunction formulation is first introduced for the steady 3D incompressible Navier–Stokes (NS) equations without in-flow and out-flow boundary conditions, where the divergence of streamfunction [math] is maintained in the convective and the vortex-stretching terms together in the nonlinear term of equations to reduce the physics-informed loss. Moreover, taking the streamfunction-vector components and their first-order partial derivatives as unknown variables, some fourth-order compact schemes are suggested for the partial derivatives that appear in the streamfunction formulation, and a high-resolution HOC scheme is introduced for approximating the pure third-order partial derivatives in the convective term. Meanwhile, a new HOC scheme is proposed for the first-type boundary conditions of the streamfunction. Finally, a fourth-order compact difference scheme and its algorithm are established for the 3D steady incompressible NS equations in the streamfunction form, subject to no in-flow and out-flow boundary conditions. Several numerical examples are carried out to validate and prove the accuracy, convergence, and efficiency of the proposed new method. Numerical results reveal that the proposed method not only can achieve fourth-order accuracy but also has excellent convergence, high-resolution, and low computational cost at higher Reynolds number.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T01:16:05Z
  DOI: 10.1063/5.0140054
Particle propulsion from attached acoustic cavitation bubble under strong
ultrasonic wave excitation
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Particle propulsion by an attached acoustic cavitation bubble under strong ultrasonic wave excitation occupies the core of many applications, including ultrasonic cleaning, ultrasonography, targeted therapy, and microbubble motors. However, the driving capacity and mode of bubbles in the field of ultrasonics are far from being well understood, which severely limits its applicability in a variety of fields. In this study, a fluid–structure interaction model based on the boundary integral method is proposed to simulate complex interactions between a suspended spherical particle and an attached cavitation bubble. A one-to-one comparison between the numerical results and experimental data demonstrates the distinct advantage of our model over conventional approaches. Thereafter, we systematically investigate the dependence of bubble–particle interactions on the governing parameters, including the amplitude and phase of the ultrasonic wave, particle density, and particle-to-bubble size ratio. We also document different types of bubble dynamic behaviors under various governing parameters. Finally, we obtain scaling laws for the maximum displacement of the particle with respect to the governing parameters.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T01:11:44Z
  DOI: 10.1063/5.0143762
Thermo-fluid-dynamics of inverse Leidenfrost levitation of small
liquid/solid spheres over liquid pools
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  Authors: Gaurav Shakya, Purbarun Dhar, Prasanta Kumar Das
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The present study provides a detailed theoretical investigation of the thermo-fluid-dynamics of the inverse Leidenfrost levitation phenomenon of a microscale droplet/solid on a liquid pool, and also the conditions essential for solid/liquid spherical objects to levitate. The theoretical model is developed for the floating characteristic of liquid/solid objects based on the thermo-fluid-dynamics of the vapor film during the inverse Leidenfrost effect. A very small thickness of the vapor layer, approximately of the order of micrometers, formed between the object and liquid pool during levitation, and its variation with the angular position and time history is considered in contrast to previous works. The actual magnitude of the overlapping contact angle is estimated and also incorporated in the present study. The effects of various influencing parameters, like nondimensionalized sphere radius, contact angle, and density ratio, on the levitation possibility and dynamics, are analyzed. The model is validated against experimental observations of the inverse Leidenfrost phenomenon for water drop levitating on a nitrogen liquid pool, and the effects of droplet parameters on total levitation time and dynamics are noted to provide accurate predictions. The approach presented is noted to provide a more accurate estimate of inverse Leidenfrost levitation compared to previous reports.
  Citation: Physics of Fluids
  PubDate: 2023-04-07T01:08:24Z
  DOI: 10.1063/5.0145922
Vibration and stability of a spinning functionally graded cylinder in a
liquid-filled concentric drum
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The vibration and stability of an axially functionally graded (AFG) cylinder with whirl motion in the annular liquid environment are investigated. The model of the performed system is given by the spinning Rayleigh beam assumptions with the rotary inertia and the gyroscopic effects. The fluid forces exerted on the cylinder, as a result of the external fluid, are calculated analytically. The coupled governing equation of motion for the system is developed via Hamilton's principle. The exact and approximate whirl frequency equations are presented for vibration and stability analysis of the AFG cylinder. The validity of the proposed model is confirmed by comparing it with the numerical solutions available in the literature. Detailed parameter discussions are conducted to evaluate the effects of the density ratio, outer-to-inner radius ratio, hollowness ratio, and slenderness ratio on the whirl characteristics and stability of the system. The results show that the whirl characteristics and instability of the AFG cylinder are strongly dependent on the external fluid.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T04:02:50Z
  DOI: 10.1063/5.0148437
Publisher's Note: “A unified theory for bubble dynamics” [Phys. Fluids
35, 033323 (2023)]
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:51:50Z
  DOI: 10.1063/5.0151631
Erratum: The ocean fine spray [Phys. Fluids 35, 023317 (2023)]
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  Authors: Alfonso M. Gañán-Calvo
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:50:29Z
  DOI: 10.1063/5.0149463
Effect of H2 addition on the local extinction, flame structure, and flow
field hydrodynamics in non-premixed bluff body stabilized flames
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  Authors: Kuppuraj Rajamanickam, Franck Lefebvre, Carole Gobin, Gilles Godard, Corine Lacour, Bertrand Lecordier, Armelle Cessou, David Honoré
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  We examined the effect of hydrogen (H2) enrichment on the primary fuel methane (CH4) in a canonical non-premixed bluff-body stabilized burner operating under typical central jet-dominated flame mode. In the chosen mode of operation, globally, the flow field and flame feature three important successive spatial zones: the recirculation zone, the neck zone, and the jet-like flame zone. The flame is exposed to a higher stretch rate in the neck zone in such a configuration and eventually undergoes local extinction. Such local extinction and subsequent re-ignition/reconnection of broken flame branches have substantial implications for the hydrodynamic instability of the coaxial annular air shear layer. It is well known that H2 addition increases the flame extinction strain rate ([math] and thus alters the local extinction phenomenon. To understand this, we performed experiments at 0%, 10%, 20%, 30%, 50%, 80%, and 100% hydrogen proportion in the H2-CH4 blend. High repetition rate (5 kHz) Particle Image Velocimetry and OH Planar Laser Induced Fluorescence (PLIF) measurements are simultaneously implemented to gain quantitative insight into the flow field and flame structure. A detailed analysis performed over the instantaneous OH–PLIF datasets reveals the absence of local extinctions in flames with H2 enrichment >30% due to an increased extinction strain rate ([math]. Furthermore, it is found that H2 enrichment plays a significant role in the reconnection/re-ignition of the broken flame branches formed during the local extinction. For instance, a high reconnection probability is observed in flames with an H2 addition of ≥20%. Consequently, variations in the mean reaction zone height are witnessed for different H2 enrichment levels. Further analysis of the influence of variation in reaction zone height on flow field hydrodynamics is explored using Proper Orthogonal Decomposition (POD) and Continuous Wavelet Transform (CWT). The results obtained from POD and CWT indicated the suppression of vortex shedding at the annular air shear layer for H2 addition greater than 20% and irregular wrinkling of flame fronts. Thus, they quantified the beneficial effect of H2 addition in turbulent flame stabilization.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:49:50Z
  DOI: 10.1063/5.0142921
A systematic investigation on flow characteristics of needle-ring-net
electrohydrodynamic gas pump
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  In this study, a two-dimensional numerical simulation is conducted to investigate the characteristics of gas flow induced by an electrohydrodynamic (EHD) pump with needle-ring-net electrodes. A needle electrode and a ring electrode are used as the high-voltage electrode, and a net electrode is used as the grounding one. The electric field distribution, space charge distribution, and flow field distribution behaviors were simulated and analyzed in detail. The simulation results were in good agreement with the experimentally measured data. The influence of key parameters, including applied voltage, electrode configurations, and channel diameter, on the flow characteristics and energy efficiency of an EHD pump was studied systematically. The results showed that the most pronounced electric field strength locates at the region around the needle tip and the edge of the ring electrode, while there is no obvious evidence showing more space charge located at the vicinity of the ring electrode. The airflow velocity at the net pores is higher than that at the central circular hole. Flow velocity and energy conversion efficiency of the pump monotonically increase with applied voltage. A combinational effect of tip-ring distance, ring inner diameter, and pump channel size should be considered to design the EHD pump to achieve maximum efficiency. The results also showed that an optimal energy conversion efficiency of 4.26% can be achieved, which is higher than most of the other EHD pumps (0.11–2.56%). The proposed model can serve as an efficient tool for the design and optimization of the needle-ring-net EHD gas pumps.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:46:22Z
  DOI: 10.1063/5.0140445
The transition of Riemann solutions for the drift-flux model with the
pressure law for the extended Chaplygin gas
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The complete Riemann solutions for the drift-flux model with the pressure law given both for the extended Chaplygin gas and also for the Chaplygin gas are solved in fully explicit forms. By the Chaplygin gas, we mean that the fluid obeys the pressure-density relation where the pressure is negative and also the inverse of the density, and further the extended Chaplygin gas is the extension of the Chaplygin gas by adding up the barotropic equation of state with higher orders. Furthermore, the transition of Riemann solutions for this model is analyzed carefully when the pressure law changes from the extended Chaplygin gas to the Chaplygin gas as all the perturbed parameters go to zero. The formation of delta shock solution from the Riemann solution consisting of 1-shock wave, 2-contact discontinuity, and 3-shock wave is identified and investigated in this limiting circumstance. In addition, the formation of the combination of three contact discontinuities from four different combinations of Riemann solutions is also inspected and studied in this limiting situation.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:43:50Z
  DOI: 10.1063/5.0146460
Skin-friction drag reduction by axial oscillations of the inner cylinder
in turbulent Taylor–Couette flows
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  Authors: Obaidullah Khawar, M. F. Baig, Sanjeev Sanghi
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Skin-friction drag reduction by axial oscillations of an inner cylinder is numerically investigated at radius ratio (η = 0.5) using direct numerical simulation. In the present study, at fixed optimal oscillating period, the effect of oscillating amplitude on skin-friction drag reduction is investigated in detail. Furthermore, the effect of Reynolds number (ranging from 1000 to 5000) is also investigated. Our results show that as we keep increasing the oscillating amplitude, the drag reduction first increases and then decreases after a critical threshold dependent on the considered Reynolds number. Crossing the threshold value leads to re-organization of flow into a patchy turbulent state with large presence of small-scale structures. With increasing oscillating amplitude, the near-wall high and low-speed streaks get skewed in the θ–z plane followed by break down of high-speed streaks. Spatial density of the vortical structure decreases till threshold amplitude while the quadrant analysis shows that the movement of high-speed fluid away from walls plays an important role in the attenuation of Reynolds shear stresses.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:40:29Z
  DOI: 10.1063/5.0142862
End effects in the wake of a hydrofoil working downstream of a propeller
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  Authors: A. Posa
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Large-eddy simulations are reported on a system consisting of a marine propeller and a downstream, semi-infinite hydrofoil, carried out on a cylindrical grid of about 3.8 × 109 points. The results are compared with those of an earlier study, considering a similar hydrofoil of infinite spanwise extent, to shed light on the influence of the end effects on the wake flow. The comparisons show good agreement between the two cases at conditions of no incidence of the hydrofoil. However, as its incidence angle grows, end effects become important. Accounting for the limited spanwise extent of the hydrofoil results in the generation of a couple of streamwise-oriented vortices from the port and starboard edges of its tip, a reduced spanwise elongation of the propeller wake, and lower turbulent stresses on the suction side of the hydrofoil, where the massive separation phenomena characterizing the infinite hydrofoil at large incidence angles are missing. In the wake of the overall system, the peak values of turbulent stresses are produced in the region of shear between the vortex shed from the pressure side edge of the tip of the hydrofoil and the tip vortices from the propeller. The latter vortices roll around the former, resulting in an intense interaction between them. In contrast, downstream of the infinite hydrofoil, the highest turbulent stresses are achieved within its wake, due to its shear with the elongated wake of the propeller.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:38:09Z
  DOI: 10.1063/5.0146297
Insight into the dynamic evolution behavior of subsonic streamers in water
and their voltage polarity effect
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Due to the complex interaction between liquid, gas, and plasma, the pre-breakdown process in water under quasi-static moderate electric fields, namely the development of subsonic streamers, was unclearly understood so far. In this paper, the dynamic evolution behavior of subsonic streamers and their voltage polarity effects were investigated. It was indicated that the whole streamer development process can be divided into two successive stages: bottom-up period characterized by root spherical expansion and OH (309 nm) emission line; top-down period characterized by head burst expansion and Hβ (486 nm), Hα (656 nm), and O (777 nm) emission lines. Further analysis revealed that the magnetic pinch effect on the internal plasma distribution determines the expansion mode of the streamer. The low capture energy of the solvated electron and local space charge accumulation make the positive streamer propagate faster at a low voltage level. However, the limited carrier resource and relatively divergent internal plasma distribution (weak magnetic pinch effect) hinder the propagation acceleration of the positive streamer with the applied voltage. Thus, the voltage polarity effect variation can be observed at high voltage levels. Finally, a novel framework model was proposed to depict the dynamic evolution behavior of subsonic streamers. Our results can provide a deeper insight into the electrohydrodynamics of dielectric fluid and promote the relevant industry applications.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:32:49Z
  DOI: 10.1063/5.0138397
Synchrotron radiography of Richtmyer–Meshkov instability driven by
exploding wire arrays
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  Authors: J. Strucka, B. Lukic, M. Koerner, J. W. D. Halliday, Y. Yao, K. Mughal, D. Maler, S. Efimov, J. Skidmore, A. Rack, Y. Krasik, J. Chittenden, S. N. Bland
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  We present a new technique for the investigation of shock-driven hydrodynamic phenomena in gases, liquids, and solids in arbitrary geometries. The technique consists of a pulsed power-driven resistive wire array explosion in combination with multi-MHz synchrotron radiography. Compared to commonly used techniques, it offers multiple advantages: (1) the shockwave geometry can be shaped to the requirements of the experiment, (2) the pressure (P > 300 MPa) generated by the exploding wires enables the use of liquid and solid hydrodynamic targets with well-characterized initial conditions (ICs), (3) the multi-MHz radiography enables data acquisition to occur within a single experiment, eliminating uncertainties regarding repeatability of the ICs and subsequent dynamics, and (4) the radiographic measurements enable estimation of compression ratios from the x-ray attenuation. In addition, the use of a synchrotron x-ray source allows the hydrodynamic samples to be volumetrically characterized at a high spatial resolution with synchrotron-based microtomography. This experimental technique is demonstrated by performing a planar Richtmyer–Meshkov instability (RMI) experiment on an aerogel–water interface characterized by Atwood number [math] and Mach number [math]. The qualitative and quantitative features of the experiment are discussed, including the energy deposition into the exploding wires, shockwave generation, compression of the interface, startup phase of the instability, and asymptotic growth consistent with Richtmyer's impulsive theory. Additional effects unique to liquids and solids—such as cavitation bubbles caused by rarefaction flows or initial jetting due to small perturbations—are observed. It is also demonstrated that the technique is not shape dependent by driving a cylindrically convergent RMI experiment.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:30:50Z
  DOI: 10.1063/5.0144839
Combined space–time reduced-order model with three-dimensional deep
convolution for extrapolating fluid dynamics
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  Authors: Indu Kant Deo, Rui Gao, Rajeev Jaiman
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  There is a critical need for efficient and reliable active flow control strategies to reduce drag and noise in aerospace and marine engineering applications. While traditional full-order models based on the Navier–Stokes equations are not feasible, advanced model reduction techniques can be inefficient for active control tasks, especially with strong non-linearity and convection-dominated phenomena. Using convolutional recurrent autoencoder network architectures, deep-learning-based reduced-order models have been recently shown to be effective while performing several orders of magnitude faster than full-order simulations. However, these models encounter significant challenges outside the training data, limiting their effectiveness for active control and optimization tasks. In this study, we aim to improve the extrapolation capability by modifying the network architecture and integrating coupled space–time physics as an implicit bias. Reduced-order models via deep learning generally employ decoupling in spatial and temporal dimensions, which can introduce modeling and approximation errors. To alleviate these errors, we propose a novel technique for learning coupled spatial–temporal correlation using a three-dimensional convolution network. We assess the proposed technique against a standard encoder–propagator–decoder model and demonstrate a superior extrapolation performance. To demonstrate the effectiveness of the three-dimensional convolution network, we consider a benchmark problem of the flow past a circular cylinder at laminar flow conditions and use the spatiotemporal snapshots from the full-order simulations. Our proposed three-dimensional convolution architecture accurately captures the velocity and pressure fields for varying Reynolds numbers. Compared to the standard encoder–propagator–decoder network, the spatiotemporal-based three-dimensional convolution network improves the prediction range of Reynolds numbers outside of the training data.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:22:09Z
  DOI: 10.1063/5.0145071
Volumetric measurement of a Newtonian fluid flow through three-dimensional
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  Authors: Maryam Bagheri, Parisa Mirbod
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  This work experimentally investigates the pressure-driven flow of a pure Newtonian fluid through three-dimensional (3D) porous media models. The porous media model consists of square arrays of rods that also could be interpreted as a periodic tandem rod arrangement. We employed a time-resolved three-dimensional particle tracking velocimetry (3D Shake-the-Box) technique for a range of Reynolds numbers [math] to observe flow structures and vortex formation between the rods in porous media structures with different porosities of [math] which corresponds to the spacing ratio of [math], where L is the distance between the centers of the rods, and D is the diameter of the rods. For all the examined cases, we further analyzed the effect of the Reynolds number and the spacing ratio on the instantaneous and averaged patterns of velocity, vorticity, and the other flow parameters after obtaining the two-dimensional velocity fields using the bin-averaging method. We observed both symmetrical and asymmetrical patterns of structure and recirculation regions between the rods depending on the Reynolds number and spacing ratio. Increasing the Reynolds number reduced the symmetrical patterns of flow structures with respect to the centerline of the gap region, while the spacing ratio was randomly affecting the symmetry degree. Vortex shedding was considerable for the two examined high Reynolds numbers of Re = 444 and Re = 890 behind the upstream rod as the porosity increased. The backward movement of the reattachment point has been observed by increasing the Reynolds number.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:19:30Z
  DOI: 10.1063/5.0141535
Fundamental understanding of open keyhole effect in plasma arc welding
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The keyhole arc welding technique has the advantage of improving welding efficiency by utilizing a stable keyhole mode. Accurate understanding of the keyhole effect is necessary to enhance the welding quality. Due to the high temperature and strong arc force involved, the complex gas–liquid–solid interactions in the complete keyhole process need to be explored. In order to fully demonstrate open keyhole mode welding, a three-tier sandwiched model based on multiphysics and multiphase effects was developed. The top layer of the model is filled with plasma arc, which gradually fuses and penetrates through the middle metal layer. Finally, it enters the third layer, resulting in an open keyhole mode. Multiphysics phenomena due to the plasma arc are fully included in the model, and the gas–liquid–solid interactions are calculated by combining the Volume of Fluid technique and the Enthalpy-porous technique. Arc ignition and dynamic open keyhole effect are demonstrated, and an arc discharge is shown from the open keyhole exit. The arc reflection phenomenon is observed as the arc is blocked by the weld pool frontier. The electric current path varies with the welding movement, and most of the current comes from the weld pool frontier. An experiment was conducted to obtain weld pool and keyhole images, which basically agree with the calculated results. Additionally, the calculated open keyhole time and electric potential drops also coincide well with experimental data.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:16:49Z
  DOI: 10.1063/5.0144148
Numerical investigation on the impact pressure induced by a cavitation
bubble collapsing near a solid wall
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Cavitation erosion often occurs on the surface of many underwater applications, which can cause severe damage to materials and reduce their performance. Since the cause of erosion is the impact pressure induced by the collapse of an individual cavitation bubble near the wall, to make a better prediction and prevent the damage potential, in this paper, we carry out systematic investigations on the impact characteristics by direct numerical simulation using a vapor bubble model. The volume of fluid (VOF) method is adopted to capture the interface between the two phases. The numerical results show that pressure wave and jet are two primary inducements of the impacts on the wall. The reason for the pressure wave impacts is the pressure wave emission after the collapse of the bubble's main part. And the reason for the jet impact is the stagnation pressure in front of the jet. After a parametric study of the two impacts with respect to the initial radius, driving pressure, and stand-off distance, the predicting equations for the pressure wave impact and jet impact are proposed at γ ≥ 1.74. When γ < 1.74, the impact pattern becomes complex due to the arrival time of the two impacts and the collapse of the vapor fragments right on the wall.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:14:29Z
  DOI: 10.1063/5.0145499
Eulerian multifluid simulations of proppant transport with different sizes
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Proppant transport is critical in hydraulic fractures and enhanced geothermal systems. Proppant transport is essentially a dense granular flow in narrow slots, and the Euler–Euler methods are commonly used to study the principle of proppant transport at the field scale. However, the simulated results cannot reproduce the laboratory observations well because some closure equations are not suitable for describing the quasi-static state of proppants after settlement, and only monodisperse granular flow is considered in simulations, which neglects the interaction between large and small particles. To improve the applicability of the numerical simulation of proppant transport in hydraulic fracturing treatment, binary-size proppant transport numerical simulations using the Eulerian multifluid method (EMM) are performed in this study. First, the motion characteristics of the suspended and settled proppants were analyzed using the kinetic theory of granular flow (KTGF) and the frictional theory of viscous particles. Thereafter, the solid–liquid momentum exchange considering the wall retardation effect and the solid–solid momentum exchange considering the endurable contact among the particles are discussed. Finally, the numerical results are qualitatively and quantitatively verified using proppant transport experiments and particle image velocimetry tests. The combination of traditional KTGF models and frictional models exhibits better performance than the modified KTGF models when considering the inertia flow regime in the proppant transport simulation, and the contribution of viscous-particle cohesion to friction must be considered. Notably, the simulated results are close to the experimental results for the development process of sand banks and the velocity distribution of particles. This verified method is efficient in computing and it will provide new insights into the pumping procedure design for hydraulic fracturing.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:11:09Z
  DOI: 10.1063/5.0141909
Numerical modeling and quantification of droplet mixing using
mechanowetting
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  Authors: Edwin De Jong, Mark L. Van Der Klok, Jaap M. J. Den Toonder, Patrick R. Onck
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Capillary forces are often found in nature to drive fluid flow, and methods have been developed aimed to exploiting these forces in microfluidic systems to move droplets or mix droplet contents. Mixing of small fluid volumes, however, is challenging due to the laminar nature of the flow. Here, we show that mechanowetting, i.e., the capillary interaction between droplets and deforming surfaces, can effectively mix droplet contents. By concentrically actuating the droplet, vortex-like flow patterns are generated that promote effective mixing. To quantify the degree of mixing, we introduce two strategies that are able to determine mixer performance independent of the initial solute distribution within a droplet, represented by single scalars derived from a matrix-based method. We compare these strategies to existing measures and demonstrate the full decoupling from the initial condition. Our results can be used to design efficient mixers, featuring mechanowetting as a new enabling technology for future droplet mixers.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:07:10Z
  DOI: 10.1063/5.0143208
Biomimetic corneas of individual profile-followed coating for encapsulated
droplet array
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  Abstract: Physics of Fluids, Volume EYES2022, Issue 1, April 2023.
  The ability to acquire a varifocal optical system with excellent reversibility and repeatability is crucial for many applications. However, current strategies for improving varifocal ability primarily focus on enlarging tunable focal length. The demonstration of an individually encapsulated microlens array is one of the key technological challenges in the varifocal microoptic industry. Inspired by corneas of natural eyes, we develop a self-assembly flow molding approach to fabricate a profile-followed coating onto the droplet surface for long-term encapsulation. Meanwhile, the coating has a supersmooth surface with roughness less than 1 nm within an area of 10 [math] 10 μm2, which unprecedentedly facilitates the high transparency and photosensitivity of a micro-optical system. Significantly, the elastic modulus of profile-followed coating is as low as 0.25 MPa, which satisfies the large varifocal capacity ranging from 773 to 1600 μm at a low voltage of 5 V. This work opens a new window for exploring the encapsulated fluid components with profile-followed coating in tunable optical systems.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:05:19Z
  DOI: 10.1063/5.0146387@phf.2023.EYES2022.issue-1
Biomimetic corneas of individual profile-followed coating for encapsulated
droplet array
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The ability to acquire a varifocal optical system with excellent reversibility and repeatability is crucial for many applications. However, current strategies for improving varifocal ability primarily focus on enlarging tunable focal length. The demonstration of an individually encapsulated microlens array is one of the key technological challenges in the varifocal microoptic industry. Inspired by corneas of natural eyes, we develop a self-assembly flow molding approach to fabricate a profile-followed coating onto the droplet surface for long-term encapsulation. Meanwhile, the coating has a supersmooth surface with roughness less than 1 nm within an area of 10 [math] 10 μm2, which unprecedentedly facilitates the high transparency and photosensitivity of a micro-optical system. Significantly, the elastic modulus of profile-followed coating is as low as 0.25 MPa, which satisfies the large varifocal capacity ranging from 773 to 1600 μm at a low voltage of 5 V. This work opens a new window for exploring the encapsulated fluid components with profile-followed coating in tunable optical systems.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:05:19Z
  DOI: 10.1063/5.0146387
Coupled pressure-driven flow and spontaneous imbibition in shale oil
reservoirs
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Coupled pressure-driven (viscous) flow and spontaneous imbibition are the main regimes during shale oil production. Revealing the unclear mechanisms of this coupled flow is a major concern for scholars and field engineers. In this work, the oil–water flow mechanisms within shale pore structures are investigated by pore-scale modeling methods in focused ion beam scanning electron microscopy digital rocks enhanced by applying super-resolution reconstruction (SRR). More small pores are identified with SRR, and the connectivity is improved. The enhanced pore size distribution is consistent with the nitrogen adsorption measurement; hence, more representative capillary pressure and relative permeability curves are obtained with essential experimental measurements. Then, an analytical solution of coupled pressure-driven (viscous) flow and spontaneous imbibition is derived, and a corresponding algorithm is proposed. Based on the pore-scale calculated relative permeability and capillary pressure curves, the analytical solution is applied to investigate the variations in water saturation profiles and conductance of the oil phase during the shale reservoir development. The results demonstrate that most of the shale oil is recovered by pressure dropdown-induced viscous flow and that imbibition is a minor factor. The overall oil-relative permeability decreases due to imbibition invasion. When the fracture spacing increases, the impairment of the overall oil-relative permeability decreases.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:02:49Z
  DOI: 10.1063/5.0146836
Entrance loss of capillary flow in narrow slit nanochannels
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The entrance loss of capillary flow at the nanoscale is crucial but often overlooked. This study investigates the entrance loss of capillary flow in narrow slit nanochannels using molecular dynamics simulations. The results show that the early stage of capillary flow is determined by entrance loss. During this period, capillary length increases linearly, while the capillary velocity remains constant. The effect of length-dependent friction loss becomes more apparent in the subsequent stages, causing the capillary length to deviate from linear and the capillary velocity to decrease. Roscoe's equation, which describes the flow through an infinitely thin slit, is used to model the entrance loss. Finite element simulations of flow through slits of varying height and length demonstrate the validity of Roscoe's equation in the continuum theory framework. Based on this, a capillary flow model is proposed that can accurately depict the hydrodynamic behavior of a capillary flow. Additionally, an approximate model ignoring the friction loss is proposed that predicts the linear increase in capillary length at the early stage. Theoretical analysis shows that the effect of entrance loss on capillary velocity is limited to the early stage, while the effect on capillary length can be extended to a large scale. Overall, the results of this study and the proposed models provide important theoretical support for applications related to capillary flows in nanoslits. The study emphasizes the importance of considering entrance loss in the early stages of a capillary flow and demonstrates the applicability of Roscoe's equation in modeling capillary flows in nanochannels.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T02:00:30Z
  DOI: 10.1063/5.0144696
Data-driven modal decomposition methods as feature detection techniques
for flow problems: A critical assessment
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  Authors: B. Begiashvili, N. Groun, J. Garicano-Mena, S. Le Clainche, E. Valero
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Modal decomposition techniques are showing a fast growth in popularity for their wide range of applications and their various properties, especially as data-driven tools. There are many modal decomposition techniques, yet Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are the most widespread methods, especially in the field of fluid dynamics. Following their highly competent performance on various applications in several fields, numerous extensions of these techniques have been developed. In this work, we present an ambitious review comparing eight different modal decomposition techniques, including most established methods, i.e., POD, DMD, and Fast Fourier Transform; extensions of these classical methods: based either on time embedding systems, Spectral POD and Higher Order DMD, or based on scales separation, multi-scale POD (mPOD) and multi-resolution DMD (mrDMD); and also a method based on the properties of the resolvent operator, the data-driven Resolvent Analysis. The performance of all these techniques will be evaluated on four different test cases: the laminar wake around cylinder, a turbulent jet flow, the three-dimensional wake around a cylinder in transient regime, and a transient and turbulent wake around a cylinder. All these mentioned datasets are publicly available. First, we show a comparison between the performance of the eight modal decomposition techniques when the datasets are shortened. Next, all the results obtained will be explained in detail, showing both the conveniences and inconveniences of all the methods under investigation depending on the type of application and the final goal (reconstruction or identification of the flow physics). In this contribution, we aim at giving a—as fair as possible—comparison of all the techniques investigated. To the authors' knowledge, this is the first time a review paper gathering all these techniques have been produced, clarifying to the community what is the best technique to use for each application.
  Citation: Physics of Fluids
  PubDate: 2023-04-06T01:58:01Z
  DOI: 10.1063/5.0142102
Electrification in turbulent channel flows of liquid dielectrics
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  Authors: Mathieu Calero, Holger Grosshans, Miltiadis V. Papalexandris
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Electrification of wall-bounded flows of liquid dielectrics occurs via the transport of electric-charge carriers (ions) from the electrical double layer at a liquid–solid interface to the bulk of the flow. This phenomenon is currently not well understood, but it has been proposed that flow turbulence plays a major role on it. However, conclusive studies about the role of turbulence and the underpinning mechanisms of flow electrification are still lacking. In this paper, we report on direct numerical simulations (DNS) of electrification in turbulent channel flow of liquid dielectrics and for friction Reynolds numbers ranging from 150 to 210. Our simulations confirm that turbulence increases dramatically the amount of charge transported in the bulk of the flow. Also, the electrification rate increases with the turbulence intensity. Nonetheless, ionic diffusion does not influence the electrification process, due to the large value of the ionic Schmidt number. Our simulations further predict that, upon electrification, the charge-density profile consists of three zones. In the first one, adjacent to the wall, the dominant mechanism is ionic diffusion, whereas in the second one, the dominant mechanisms are convective and conductive currents. In the third zone, the bulk of the flow, the charge density remains almost constant. Also, according to the budget of the charge-density variance, molecular transport counterbalances molecular dissipation in the first zone, and production counterbalances turbulent transport in the second one. Finally, we provide a closed-form expression for the mean charge-density profile based on the gradient assumption, which agrees well with our DNS results.
  Citation: Physics of Fluids
  PubDate: 2023-04-05T11:31:22Z
  DOI: 10.1063/5.0138425
Experimental study on the auto-initiation of rotating detonation with
high-temperature hydrogen-rich gas
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  An experimental study on the auto-initiation process of rotating detonation waves (RDWs) was conducted with high-temperature hydrogen-rich gas as the fuel and air as the oxidant. Spontaneous combustion of high-temperature hydrogen-rich gas and air occurred after they were injected into a rotating detonation chamber (RDC), which resulted in hot spots in the RDC and induced the formation of a rotating deflagration flame. Then, an RDW formed through the deflagration-to-detonation transition process in the RDC. The auto-initiation process of high-temperature hydrogen-rich gas and the formation mechanism of RDWs were studied in detail through experiments. The influences of the equivalence ratio on the RDW propagation characteristics of high-temperature hydrogen-rich gas were analyzed. The results showed that with the increase in the equivalence ratio from 0.61 to 1.93, five RDW propagation modes appeared in the RDC: Failure, two counter rotating detonation wave (TCRDW), Mixed, intermittent single rotating demodulation wave, and single rotating detonation wave (SRDW) modes. The Mixed mode was the transition mode from the TCRDW mode to the SRDW mode. The highest RDW velocity was 1485.9 m/s when the equivalence ratio was 1.32, in which the propagation mode was the stable SRDW mode.
  Citation: Physics of Fluids
  PubDate: 2023-04-05T11:15:05Z
  DOI: 10.1063/5.0144322
Influence of Reynolds number on the natural transition of boundary layers
over underwater axisymmetric bodies
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The influence of the Reynolds number on the natural transition of boundary layers over underwater axisymmetric bodies is studied using numerical approaches. This is a fundamental problem in fluid mechanics and is of great significance in practical engineering problems. The transition locations are predicted over diameter Reynolds numbers ranging from 1.79 × 105 to 2.32 × 108 for eight different forebody shapes. The transition onsets are predicted using the semi-empirical eN method based on the linear stability theory (LST), and the wall pressure fluctuation spectra are estimated. The effects of the forebody shapes and the Reynolds numbers on the transition location are studied. At the same Reynolds number, the forebody shape has a great influence on transition. As the Reynolds number increases, the changes in the dimensionless transition location are qualitatively similar for different forebody shapes. The dimensionless transition location shifts closer to the leading edge as the Reynolds number increases and is more sensitive at lower Reynolds numbers. However, the quantitative changes in transition location for different forebody shapes are distinctly different. Consequently, the sequential order of the transition locations for the eight forebody shapes is not fixed but changes dramatically with increasing Reynolds number. This irregularity in the sequential order of the transition locations is called the “Reynolds number effect.” Finally, the fundamental causes of this effect are analyzed.
  Citation: Physics of Fluids
  PubDate: 2023-04-05T11:15:03Z
  DOI: 10.1063/5.0143497
Harmonic structure of the nonlinear force on a fixed ship-shaped floating
production, storage and offloading vessel under dispersive phase-focused
wave groups
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  Authors: Hao Chen, Ling Qian, Deping Cao
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  This paper presents a numerical investigation on the harmonic structure of hydrodynamic forces on a fixed and simplified representative floating production, storage and offloading (FPSO) vessel hull under dispersive phase-focused wave groups. The high-fidelity numerical model utilizes the two-phase flow solver in the open-source toolbox OpenFOAM. A series of cases were computed using the numerical model, where the effects of wave steepness, bow diameter, and length of the FPSO are investigated. It is found that given an FPSO under different wave steepness, the non-dimensional inline force exhibits remarkable similarity in terms of the temporal development. The harmonic structure of the inline force is only weakly dependent on the steepness of the incident wave group and the bow diameter, but strongly dependent on the FPSO length. When [math], where L is the length of the FPSO and kp is the wave number at peak frequency, the incident wave group is diffracted significantly by the FPSO. The entire wave–structure interaction process is largely linear, where transfer between different harmonics is rarely seen. However, when kpL is further reduced to 0.57, globally the disturbance of the FPSO on the far field incident wave group is reduced, but locally a strongly nonlinear flow occurs at the rear of the FPSO, where severe run-up occurs at the downstream stagnation point. Higher-order harmonics of inline forces are excited, and the interaction process becomes much more nonlinear.
  Citation: Physics of Fluids
  PubDate: 2023-04-05T11:15:02Z
  DOI: 10.1063/5.0141342
Investigation of Ultraviolet-C light-emitting diode for airborne
disinfection in air duct
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  Authors: Nitin Loganathan, Uvarajan M. Velayutham
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Given the current coronavirus (COVID-19) situation around the world, we may have to face a long-term battle with coronavirus. It is necessary to prepare and stay resilient with some other techniques to improve air quality in buildings, especially in clinics and hospitals. In this paper, we have developed Ultraviolet-C (UVC) light-emitting diode (LED) modules which can be implemented in air ducts in heating, ventilation, and air conditioning system for airborne disinfection. An LED module is designed with LED panels as the basic unit so that it is easy to scale up to accommodate for air ducts with different sizes. Both experiments and simulations are carried out to study its disinfection performance. The results show that more than 76% and 85% of the pathogen can be inactivated within 60 and 90 min, respectively, in a meeting room with a volume of 107 m3 by using one LED module. Simulations for two LED modules show that the disinfection efficacy is more than two times compared to that of one LED module. In addition to the pathogen used in the experiments, the disinfection performance of the LED module for inactivation of SARS-CoV-2 virus based on the literature is investigated numerically. It shows that more than 99.70% of pathogens receive UV dose larger than 4.47 J/m2, leading to an almost 89.10% disinfection rate for SARS-CoV-2 virus within one hour using the two LED modules in the same meeting room.
  Citation: Physics of Fluids
  PubDate: 2023-04-05T11:15:02Z
  DOI: 10.1063/5.0144729
Helical model based on artificial neural network for large eddy simulation
of compressible wall-bounded turbulent flows
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Similar to the kinetic energy cascade, a helicity cascade is also a basic and key process in the generation and evolution of the turbulent flows. Furthermore, the helicity flux (HF) plays a crucial role between two scales in the helicity cascade. In this study, we will supply a new helical model constrained by the helicity flux for the large eddy simulation of the compressible turbulent flows. Then, in order to obtain a more precise HF, the local coefficient of the modeled HF is determined by the artificial neural network (ANN) method. The new model combines merits of the high robustness and the correlation with the real turbulence. In the test case of the compressible turbulent channel flow, the new model can supply a more accurate mean velocity profile, turbulence intensities, Reynolds stress, etc. Then, for the test in the compressible flat-plate boundary layer, the new model can also precisely predict the onset and peak of the transition process, the skin-friction coefficient, the mean velocity in the turbulent region, etc. Moreover, the ANN here is a semi-implicit method, and the new model would be easier to be generalized to simulate other types of the compressible wall-bounded turbulent flows.
  Citation: Physics of Fluids
  PubDate: 2023-04-05T10:27:47Z
  DOI: 10.1063/5.0137607
Study on the quasi-isentropic model for aluminized explosive driving the
cylinder in the direction perpendicular to detonation wave propagation
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  A cylinder test was designed for the CL-20-based aluminized explosives to study the influence of aluminum (Al) powder properties on the explosives' metal driving performance in the direction perpendicular to detonation wave propagation. The research results showed that: in the direction perpendicular to detonation wave propagation, as the Al powder particle size became larger (in the range of 2–43 μm), the metal driving performance of the explosives grew stronger; the CL-20-based explosive containing 25% Al was much less capable of accelerating the metal than the formulation containing 15% Al. Considering the two-dimensional flow characteristics of the detonation products in the radial and axial directions as the aluminized explosive expands and drives the cylinder, a quasi-isentropic theoretical model for the aluminized explosive driving the cylinder was proposed. In the model, the calculation methods for the variations of the cylinder expansion velocity, Al reaction degree, and detonation product parameters with time, axial space, and radial space were developed. According to the experimental data of the cylinder test, the correctness of the proposed quasi-isentropic theoretical model was verified; the variation laws of the physical parameters, such as the pressure and temperature of the detonation products under different radial distributions in the cylinder with time and axial positions, were calculated. It was found that the pressure and temperature of the detonation products in the non-inner-wall place of the cylinder were significantly higher than those on the inner wall of the cylinder at the same axial position; the pressure and temperature of the detonation products on the inner wall decreased rapidly at the early timeframes; as the Al reaction proceeded, the pressure gradually turned to a constant value, and the temperature dropped slowly; for the CL-20-based explosives with 15% Al, the temperature of the detonation products in the non-inner-wall place rose slightly at first and then decreased slowly; for the formulation with 25% Al, the temperature of the detonation products in the non-inner-wall place kept rising at a small rate.
  Citation: Physics of Fluids
  PubDate: 2023-04-05T10:27:45Z
  DOI: 10.1063/5.0139386
Boundary-value-problem examination of the stability of a symmetrical rotor
partially filled with a viscous incompressible fluid
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Instabilities in a rotor system partially filled with a fluid can have an exponentially increasing amplitude, and this can cause catastrophic damage. Numerous theoretical models have been proposed, and numerous experiments have been conducted to investigate the mechanisms of this phenomenon. However, the explanation of the existence of the first unstable region induced by a viscous incompressible fluid is unclear, and only one solving method, a standard finite difference procedure, was proposed in 1991 for solving the instabilities in a system containing a symmetric rotor partially filled with a viscous incompressible fluid. To better understand the mechanisms of the instability induced by the viscous fluid, based on the linearized two-dimensional Navier–Stokes equations, this system's differential equations are transferred to solve the characteristic equations with boundary conditions. A Matlab boundary value problem (BVP) solver bvp5c proposed in 2008 is an efficient tool to solve this problem by uncoupling the boundary conditions with unknown initial guess. Applying this approach to a rotor system allows the instability regions to be obtained. In this study, first, the radial and tangential velocities and pressure fluctuations along the radial direction of a disk filled with fluid were examined. Then, parametric analysis of the effect of the Reynolds number [math], filling ratio H, damping ratio C, and mass ratio m on the system's stability was conducted. Using this calculation method allowed the first exploration of some new laws regarding the instabilities. These results will benefit the further understanding of the existence of the first unstable region of a rotor partially filled with a viscous incompressible fluid.
  Citation: Physics of Fluids
  PubDate: 2023-04-05T10:25:44Z
  DOI: 10.1063/5.0147073
Resolvent-based analysis of hypersonic turbulent boundary layers
with/without wall cooling
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  Authors: Richard D. Sandberg
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The ability of the low-rank approximation of hypersonic turbulent boundary layers with/without wall cooling is examined with the linear resolvent operator in a compressible form. The freestream Mach number of the base flow is 5.86, and the friction Reynolds number is 420. The wall-to-recovery temperature ratio is set as 1.0 and 0.25, respectively, corresponding to an adiabatic wall condition and a cold-wall condition. Different from the resolvent analysis of incompressible turbulent boundary layers, the optimal response mode in the wave-parameter space exhibits a relatively subsonic and a relatively supersonic region [Bae et al., “Resolvent-based study of compressibility effects on supersonic turbulent boundary layers,” J. Fluid Mech. 883, A29 (2020)], divided by the freestream relative Mach number of unity. The features of energy distribution of the optimal response mode in space and scales are examined, and the energy spectra of streamwise velocity and temperature fluctuations, carried by the optimal response mode, are discussed with typical subsets of streamwise and spanwise wavelengths. This reveals the dynamics of the near-wall small-scale and outer larger-scale motions and the distinction in the relatively subsonic/supersonic region. Moreover, the coherent structures, including the velocity and temperature streaks, quasi-streamwise vortices, and large-scale/very-large-scale motions, are identified in the optimal response mode. Special attention is paid to the effects of wall cooling.
  Citation: Physics of Fluids
  PubDate: 2023-04-05T10:25:42Z
  DOI: 10.1063/5.0142371
Modeling and validation of coarse-grained computational fluid
dynamics–discrete element method for dense gas–solid flow simulation
in a bubbling fluidized bed
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  Authors: Mahmoud A. El-Emam, Ramesh Agarwal
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Computational fluid dynamics (CFD) combined with the discrete element method (DEM) are powerful tools for analyzing dense gas–solid flows. However, the computational cost of CFD–DEM will be unfeasibly great when simulating large-scale engineering applications with billions of particles. Accordingly, the coarse-grained (CG) CFD–DEM method is applied to solve this problem. This investigated method replaces several smaller particles with larger ones called parcels, aiming to reduce the number of particles and fully consider the collision of particles between composition parcels and the collision of particles within composition parcels. First, high-speed photography verifies the numerical simulation's reliability. Then, the CG CFD–DEM was used to analyze the transient spatial distribution, transient average velocity, pressure drop, bed height, and the mixing state of particles in a dense gas–solid fluidized bed. The CG CFD–DEM was also compared with the CFD–DEM results, which showed a good agreement with the calculation results and proved the accuracy and applicability of the method. Finally, the computation time of the CG CFD–DEM was evaluated, showing a significant decrease in computation time with an increasing coarse ratio (k). This investigation can provide theoretical reference for the numerical simulation of the CG CFD–DEM method in dense gas–solid flow.
  Citation: Physics of Fluids
  PubDate: 2023-04-05T10:24:23Z
  DOI: 10.1063/5.0146264
Interactions between a heavy particle, air, and a layer of liquid
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  Authors: E. M. Jolley, F. T. Smith
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  As an aircraft flies through cloud at temperatures below freezing, it encounters ice particles and supercooled droplets, which results in the accretion of ice onto its surfaces and hence deformation of its aerodynamic shape. This can, in worst cases, cause series accidents. Here, we focus on tackling the common situation where there is a thin layer of water on the aircraft surface and the particles are similarly thin such as to be able to interact with the water layer. Three-way interaction occurs between air, water, and body motion: under suitable assumptions (including that the Reynolds and Froude numbers are large, and that the body is much denser than the air), the model allows the shape of the layer interface and pressure profile beneath the body to be calculated for a given body position. Simultaneously, this in turn allows the forces on the body to be calculated and hence the motion of the particle to be computed in full. The result is a wide range of possible motions of the particle, including both “sink” cases (the particle enters the water and becomes submerged) and “skim” cases (where the particle is launched back off the surface of the water following contact). The latter cases have analogy with traditional “stone skimming/skipping” games. Repeated skims and significant wakes are accommodated rationally.
  Citation: Physics of Fluids
  PubDate: 2023-04-05T10:24:22Z
  DOI: 10.1063/5.0145552
Effect of liquid elasticity on nonlinear pressure waves in a viscoelastic
bubbly liquid
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The importance of viscoelasticity of biological media that are used in medical ultrasounds has been discussed in the literature. Furthermore, the use of microbubbles in biological media drastically improves the efficiency of both diagnostic and therapeutic ultrasounds. Weakly nonlinear wave equations for ultrasound propagation in liquids containing microbubbles have long been studied, although the viscoelasticity of the liquid phase has been ignored for simplicity. In this study, we derived a nonlinear wave equation for ultrasound propagation in a viscoelastic liquid containing microbubbles by considering the effect of the elasticity of the liquid. Additionally, we evaluated how the elasticity of the liquid modifies the nonlinear, dissipation, and dispersion effects of the ultrasound in a few tissue models (i.e., liver, muscle, breast cancer, fat, and skin models and that without shear elasticity). The results revealed that liquid shear elasticity decreases the nonlinear and dissipation effects and increases the dispersion effect, and this tendency is more significantly observed in the breast cancer tissue compared with other tissues. Furthermore, we numerically solved the nonlinear wave equation and investigated the changes in ultrasonic wave evolution with and without shear elasticity.
  Citation: Physics of Fluids
  PubDate: 2023-04-05T10:23:13Z
  DOI: 10.1063/5.0131091
Suppression of cross-infection in actual dental office using viscoelastic
polyacrylate solution as a handpiece coolant
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  Authors: Alexander L. Yarin
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  In the present work, aqueous solutions of NaPAA [poly (sodium acrylate)] or PAA [polyacrylic acid] are used as the coolants for a dental handpiece to evaluate their suppressive effect on the aerosolization and bacteria (Staphylococcus epidermidis) transmission in an actual dental environment. Both polymer solutions significantly suppressed the formation of aerosols (
  Citation: Physics of Fluids
  PubDate: 2023-04-05T10:23:12Z
  DOI: 10.1063/5.0146829
The interplay of geometry and coarsening in multicomponent lipid vesicles
under the influence of hydrodynamics
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  Authors: Elena Bachini, Veit Krause, Axel Voigt
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  We consider the impact of surface hydrodynamics on the interplay between membrane curvature and lipid composition in coarsening processes on model systems for biomembranes. This includes the influence on scaling laws and equilibrium configurations, which are investigated by computational studies of a surface two-phase flow problem with additional phase-dependent bending terms. These additional terms geometrically favor specific configurations. We find that the effect of hydrodynamics strongly depends on the composition. In situations where the composition allows a realization of a geometrically favored configuration, hydrodynamics enhances the evolution toward this configuration. We restrict our model and numerics to stationary surfaces of varying curvature and validate the numerical approach with various benchmark problems and convergence studies.
  Citation: Physics of Fluids
  PubDate: 2023-04-05T10:20:23Z
  DOI: 10.1063/5.0145884
Transportation and deformation of high-speed aluminum nanoparticles in
inert gas with molecular dynamics study
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  In addition to complex deformation, high-speed nanoparticles in gas are also accompanied by significant size and interfacial effects. In this work, we simulate the transportation behavior of high-speed aluminum nanoparticles in helium gas with the classical molecular dynamics method. The evolution of aerothermodynamic quantities of solid particles and liquid particles is revealed, and different temperature rise effects are found. Furthermore, the melting of aluminum particles induced by high aerodynamic drag force is discovered, and the melting threshold conditions are proposed. In low-density (0.002 g/cm3) and high-density (0.02 g/cm3) gas, the initial velocity at which particles start to melt is 6 and 4 km/s, respectively. During the deformation of solid particles, the evolution of dislocation motion is discussed, and the evolution of the development characteristics of the molten layer is given. During the deformation of the liquid particles, vibration deformation and bag deformation modes are observed. The threshold conditions for deformation mode transitions are also given. Only in high-density gas, bag deformation occurs when the initial velocity of particles (D > 5 nm) exceeds 6 km/s. The local mechanical quantity of gas is used to explain the variation of the drag force of the particles. Moreover, the drag force model is corrected according to temperature and deformation effects. Within a certain period, the model results overestimate the drag force, and the error with the simulation results is about 25%. This provides a model reference for high-speed nanoparticle dynamics and two-phase flow problems.
  Citation: Physics of Fluids
  PubDate: 2023-04-05T10:20:22Z
  DOI: 10.1063/5.0141084
Fast mathematical modeling of partial-breach dam-break flow using a
time-series field-reconstruction deep learning approach
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  Authors: Xiaohui Yan, Ruigui Ao, Abdolmajid Mohammadian, Jianwei Liu, Fu Du, Yan Wang
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Mathematical modeling of dam-breach flow can provide a better understanding of dam failure events, which in turn helps people to reduce potential losses. In the present study, the smooth particle hydrodynamics (SPH) modeling approach was employed to simulate the three-dimensional (3D) partial-breach dam-break flow using two different viscosity models: the artificial viscosity and sub-particle-scale models. The validated and best-performing SPH model was further employed to conduct numerical experiments for various scenarios, which generated a comprehensive dataset. The current work also presents a novel time-series field-reconstruction deep learning (DL) approach: Time Series Convolutional Neural Input Network (TSCNIN) for modeling the transient process of partial-breach dam-break flow and for providing the complete flow field. This approach was constructed based on the long short-term memory and convolutional neural network algorithms with additional input layers. A DL-based model was trained and validated using the numerical data, and tested using two additional unseen scenarios. The results demonstrated that the DL-based model can accurately and efficiently predict the transient water inundation process, and model the influence of dam-break gaps. This study provided a new avenue of simulating partial-breach dam-break flow using the time-series DL approaches and demonstrated the capability of the TSCNIN algorithm in reconstructing the complete fields of transient variables.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T12:24:47Z
  DOI: 10.1063/5.0142335
The vibration of a spring damped vibrator under a long cylindrical squeeze
film force
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Squeeze film effects between two concentric cylinders are common and important in many mechanical systems. The models for cylindrical squeeze film force and vibrating system with squeeze film effects have not been well understood. In this paper, we model a cylindrical squeeze film force starting from the simplification of the full Navier–Stokes equations and then get an expression for the squeeze film force. We study the influence of a harmonic exciting force on a spring damped vibrator with squeeze film effect by establishing a non-linear mathematical model, and we investigate the behavior of the system numerically. As a forced vibrating system, the fundamental frequency of the response is consistent with the exciting frequency; the response of the resultant non-linear system shows obvious superharmonic phenomenon; the number of the superharmonic peaks increase with Reynolds number, and the superharmonic components mainly come from the non-linearity of the flow convection.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T04:46:31Z
  DOI: 10.1063/5.0141629
Heat addition with variable area: Methodology for preliminary design of
the scramjet combustion chamber
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  Authors: R. Carneiro, A. Passaro, P. G. P. Toro
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Well-known analytical approaches are commonly adopted for the preliminary design of scramjet engines. In this context, the combustion process in the combustion chamber can be modeled by considering heat addition to the airflow at supersonic speed. The one-dimensional Rayleigh flow theory can be applied to estimate the behavior of thermodynamic properties and velocities when the combustion chamber has a constant cross-sectional area and no mass is added within the duct. However, the temperature and pressure predicted by using constant area combustion chambers are too high, implying the necessity of modifications in the cross-sectional area of the chamber to avoid thermal choking and excessive pressure gradients. In this case, the unidimensional Rayleigh theory does not fit anymore. This work proposes an analytical methodology to estimate the airflow thermodynamic properties and velocities for scramjet combustion chambers with cross sections of variable areas by using an iterative algorithm that employs the Rayleigh flow area ratio theory. The analytical results were compared with the two-dimensional computational fluid dynamics analysis using the Reynolds-averaged Navier–Stokes method for both inviscid and viscous flow and considering turbulence effects. The proposed analytical model to estimate the flow behavior in the scramjet combustion chamber predicted results in agreement with the physics of the problem and with the results obtained via numerical simulation. The analytical model cannot predict oscillations in the flow properties caused by the expansion waves and their reflections. Still, the behavior and intensity of the properties are well captured along the entire length of three combustion chambers with variable area. The proposed algorithm is also applied to determine the angle of the combustion chamber that allows guaranteeing a constant, or a quasi-constant, static pressure along the length of the combustion chamber, approaching better the ideal thermodynamic Brayton cycle. The proposed model is suitable for preliminary scramjet designs and can be used to solve other problems involving variable area ducts.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T04:46:29Z
  DOI: 10.1063/5.0138781
Appearance of the −5/3 scaling law in spatially intermittent flows
with strong vortex shedding
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Our previous investigations [“Extreme events and non-Kolmogorov -5/3 spectra in turbulent flows behind two side-by-side square cylinders,” J. Fluid Mech. 874, 677–698 (2019); “Energy transfer in turbulent flows behind two side-by-side square cylinders,” J. Fluid Mech. 903, A4 (2020)] revealed that an extended −5/3 energy spectrum can be found in the highly intermittent flow accompanied by strong vortex shedding (i.e., composed of both large-scale laminar and turbulent motions). To shed light on the emergence of the −5/3 energy spectra, we perform direct numerical simulations of a single-cylinder wake. In the highly intermittent upstream region, albeit a significant −5/3 scaling law can be found, the energy spectra and the corresponding Kolmogorov constant Ck are distinctly different from those in the downstream almost turbulent region and the case of grid turbulence with a similar local Reynolds number. However, the conditional Kolmogorov constant acquires the expected value and the conditional energy spectra are in good agreement with those in the far downstream region. Ck is found to have a power-law dependence on the intermittency factor γ, [math]. This study, the first of its kind, demonstrates that in the highly intermittent flow, the composed turbulent motions are responsible for the emergence of the −5/3 scaling law, which implies the characteristics of the composed turbulent motions resemble those in the fully turbulent flow.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T04:46:28Z
  DOI: 10.1063/5.0141076
Influence of abdominal aortic aneurysm shape on hemodynamics in human
aortofemoral arteries: A transient open-loop study
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  Authors: Sumit Kumar, B. V. Rathish Kumar, S. K. Rai
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  New imaging methods have enabled the detection of unruptured abdominal aortic aneurysms (AAA). It is necessary to develop appropriate mathematical models for rupture prediction to allow a proper patient treatment plan. To provide valid hemodynamic parameters, high-fidelity numerical models with patient-specific boundary conditions are needed. Researchers have pointed out in recent research articles and reviews that those morphological parameters, such as shape, dilation ratio, neck angle, common iliac bifurcation angle, and AAA type, consistently correlate with the rupture mechanism. However, it is unclear how morphological indicators affect hemodynamics-based computational fluid dynamics predictions. The present work investigates the influence of AAA shape on local and global hemodynamics parameters and rupture predictions. Five cases of magnetic resonance imaging scan-based data for patient-specific aortofemoral artery modeling are explored. The inflow conditions are patient-specific, and an open loop system has been considered to model all five cases. Hemodynamics parameters in pulsating conditions, such as wall shear stress (WSS), velocity contour, time average WSS (TAWSS), oscillatory shear index (OSI), vorticity, and streamlines, are computed and investigated. Both maximum dilation diameter and aneurysm neck angle are found to have substantial effects on local hemodynamics parameters. The magnitude of WSS, TAWSS, and OSI increases and decreases non-linearly with a change in maximum diameter during the cardiac process. Also, aneurysms with doubly titled and completely saccular shape show complex streamlines, low WSS, and high residence time in the sac area of the wall.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T02:55:27Z
  DOI: 10.1063/5.0139085
The vortex ring state of a rotor and its comparison with the collapse of
an annular jet in counterflow
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  Authors: D. J. Pickles, R. B. Green, A. Busse
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The vortex ring state (VRS) of a rotor is associated with the development of the trailed vortex system in powered descending flight, where the topology of the vortex wake changes from its usual helical form into a toroidal form. In the VRS, the toroidal vortex ring envelops the entire rotor, and it sheds and reforms in an unsteady manner. In previous attempts to understand the basic phenomenology of the VRS, the focus was on the role of the trailed vortices in the transition to the VRS: computational and experimental work utilized rotor models to generate the trailing vortex wake, and mechanisms for the emergence of the VRS were postulated based on the interaction of the trailed vortices. In this paper, a different approach is taken: a set of experiments on a core annular jet flow are described, where the jet flow in counterflow is used to simulate a rotor in powered descent. It is shown that this leads to the formation of a flow field that shares many of the features of the VRS of a rotor system. This brings into question the role of the rotor blade trailing vortices in the development of the rotor wake VRS, and it is proposed instead that the interaction between the mean flow and counterflow drives the VRS phenomenon.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T02:37:42Z
  DOI: 10.1063/5.0143406
A mixing enhancement mechanism for a hydrogen transverse jet coupled with
a shear layer for gas turbine combustion
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  Abstract: Physics of Fluids, Volume HFDP2022, Issue 1, April 2023.
  The mixing characteristics of hydrogen and air are vital to combustion performance. Excellent hydrogeni–air mixing is required to avoid hot spots in the reactivity of hydrogen in a combustion chamber. The present study aims to explore a mixing enhancement mechanism for a hydrogen transverse jet in which a rib is added in front of the jet. A schlieren technique is used to visualize the flow field of the improved hydrogen jet, and the combustion performance of the improved flame stabilizer is studied. The results show that the penetration depth and mixing performance of the hydrogen jet are improved. At its outset, the hydrogen jet flows like a free jet downstream of the rib. The flow pattern of the hydrogen jet is then changed by the shear layer between the low-velocity region and the mainstream. Ideal mixing performance is ultimately achieved under the strong effect of the mainstream. Combustion experiments show that the mixing and combustion performance are greatly improved by the rib in front of the jet. This study provides an important theoretical basis for the design of gaseous fuel combustors.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:53:29Z
  DOI: 10.1063/5.0142960@phf.2023.HFDP2022.issue-1
A mixing enhancement mechanism for a hydrogen transverse jet coupled with
a shear layer for gas turbine combustion
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The mixing characteristics of hydrogen and air are vital to combustion performance. Excellent hydrogeni–air mixing is required to avoid hot spots in the reactivity of hydrogen in a combustion chamber. The present study aims to explore a mixing enhancement mechanism for a hydrogen transverse jet in which a rib is added in front of the jet. A schlieren technique is used to visualize the flow field of the improved hydrogen jet, and the combustion performance of the improved flame stabilizer is studied. The results show that the penetration depth and mixing performance of the hydrogen jet are improved. At its outset, the hydrogen jet flows like a free jet downstream of the rib. The flow pattern of the hydrogen jet is then changed by the shear layer between the low-velocity region and the mainstream. Ideal mixing performance is ultimately achieved under the strong effect of the mainstream. Combustion experiments show that the mixing and combustion performance are greatly improved by the rib in front of the jet. This study provides an important theoretical basis for the design of gaseous fuel combustors.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:53:29Z
  DOI: 10.1063/5.0142960
Modeling of dispersion of aerosolized airborne pathogens exhaled in indoor
spaces
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  Authors: Praveen Sharma, Supreet Singh Bahga, Amit Gupta
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Since the beginning of the COVID19 pandemic, there has been a lack of data to quantify the role played by breathing-out of pathogens in the spread of SARS-Cov-2 despite sufficient indication of its culpability. This work aims to establish the role of aerosol dispersion of SARS-Cov-2 virus and similar airborne pathogens on the spread of the disease in enclosed spaces. A steady-state fluid solver is used to simulate the air flow field, which is then used to compute the dispersion of SARS-Cov-2 and spatial probability distribution of infection inside two representative classrooms. In particular, the dependence of the turbulent diffusivity of the passive scalar on the air changes per hour and the number of inlet ducts has been given due consideration. By mimicking the presence of several humans in an enclosed space with a time-periodic inhalation–exhalation cycle, this study firmly establishes breathing as a major contributor in the spread of the pathogen, especially by superspreaders. Second, a spatial gradient of pathogen concentration is established inside the domain, which strongly refutes the well-mixed theory. Furthermore, higher ventilation rates and proximity of the infected person to the inlet and exhaust vents play an important role in determining the spread of the pathogen. In the case of classrooms, a ventilation rate equivalent to 9 air changes or more is recommended. The simulations show that the “one-meter distance rule” between the occupants can significantly reduce the risk of spreading infection by a high-emitter.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:52:11Z
  DOI: 10.1063/5.0142869
A high-order generalized differential quadrature method with lattice
Boltzmann flux solver for simulating incompressible flows
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  This paper presents a high-order generalized differential quadrature method with lattice Boltzmann flux solver (LBFS-GDQ) for simulating incompressible isothermal flows. In this method, high-order polynomials are adopted to approximate both the solution and fluxes globally across the computational domain. Solution derivatives and flux divergence are conveniently computed by the GDQ method. At the interior solution points, the viscous and inviscid fluxes are evaluated simultaneously via LBFS. Treatments to prevent the global accuracy from being contaminated by the streaming error of LBFS are proposed and studied, including the choice for the local streaming spacing and interpolation methods for the local reconstruction. The present method inherits the advantages of both GDQ and LBFS, i.e., global spectral accuracy, direct evolution of macroscopic variables, and convenient implementation of boundary conditions. Numerical experiments with a wide selection of incompressible flow problems confirm the excellent accuracy, efficiency, and flexibility of the proposed method.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:52:10Z
  DOI: 10.1063/5.0146130
Experimental study on the impulsively started motion of a close-to-neutral
buoyancy freely decelerating sphere
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  Authors: Pablo Lopez-Gavilan, Antonio Barrero-Gil, Angel Velazquez
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  An experimental study is presented that addresses the problem of a freely decelerating sphere in a still water tank. The diameter of the sphere was 0.04 m. Three different solid-to-fluid density ratios were considered: 0.78, 0.88, and 0.94. The submerged sphere was impulsively started upon being rammed by an actuator-mass system. Six initial velocities were considered: 0.91, 2.03, 2.54, 2.94, 3.29, and 3.78 m/s. The Reynolds number of the initial velocities based on the sphere diameter was 3.6 × 104, 8.1 × 104, 1.01 × 105, 1.17 × 105, 1.31 × 105, and 1.51 × 105 (subcritical). It was observed that both sphere dynamics and associated flow topology (identified via an optical system and a particle image velocimetry system, respectively) differed significantly from the case of an accelerating sphere. In the present case, a large vortex ring structure (both torus diameters of the order of the sphere's diameter) formed and attached to the sphere surface. This vortex ring followed the sphere motion all the way down the falling trajectory. From the data reduction standpoint, it was found that a suitably defined dimensionless acceleration parameter allowed for collapsing the kinematics variables of the sphere trajectory, namely, position, velocity, and acceleration, into a single ordinary differential equation.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:52:07Z
  DOI: 10.1063/5.0141322
Turbulent/turbulent interfacial layers of a shearless turbulence mixing
layer in temporally evolving grid turbulence
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Turbulent/turbulent interfacial (TTI) layers are investigated with direct numerical simulation of temporally evolving grid turbulence. The present study considers a temporally evolving wake of two parallel-bar grids with different mesh sizes, which generate homogeneous isotropic turbulent regions with large and small turbulent kinetic energies (TKE). A shearless mixing layer of turbulence forms between the large- and small-TKE regions. The TTI layer bounded by the large- or small-TKE region is identified with a passive scalar field, and the flow statistics are evaluated as functions of a position with respect to the TTI layer. Statistics of a velocity gradient tensor suggest that the center and edges of the TTI layer are dominated by vortex sheets and vortex tubes, respectively. Because of the configuration of these vortical structures, the flow toward the TTI layer in the layer-normal direction generates a compressive strain, which is important to sustain the thin layer structure. The mean velocity jump due to the compressive strain is about [math] and is observed over a length of about [math], where [math] and η are the Kolmogorov velocity and length scales, respectively. The thickness of the TTI layer is about [math], which hardly depends on time. The TTI layer has a large surface area when it is bounded by the large-TKE region. Consequently, the shearless mixing layer tends to entrain more amount of fluid from the large-TKE region than from the small-TKE region although the entrainment rate per unit surface area normalized by the Kolmogorov velocity is similar for both regions.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:50:00Z
  DOI: 10.1063/5.0141253
Aeroacoustic analysis of the tip-leakage flow of an ultrahigh bypass ratio
fan stage
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  Authors: Jean Al-Am, Vincent Clair, Alexis Giauque, Jérôme Boudet, Fernando Gea-Aguilera
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  A detailed aeroacoustic analysis of the flow induced by the clearance between the fan tip and the shroud is performed in a scale-model fan stage of an ultrahigh bypass ratio turbofan engine, which was designed to operate at transonic regimes. A wall-modeled large eddy simulation is performed at approach condition, which corresponds to a fully subsonic operating point. The contributions of the tip-gap noise to the total fan noise are investigated using the Ffowcs Williams and Hawkings analogy. The surface is split into two parts: the tip region and the rest of the blade in order to analyze the acoustic contributions of these two regions separately. It is shown that the tip-gap region generates a significant noise component above 2 kHz, which corresponds to approximately 1.2 times the blade passing frequency. Two separate tip-leakage vortices are identified in the vicinity of the fan tip. The dominant noise sources in the tip-gap region are observed at the trailing edge of the fan blade. The wall pressure spectra in the tip-gap region and the coherence of pressure fluctuations between monitor points at different positions show an acoustic contribution of the tip-leakage flow at two different frequency ranges. The first range corresponds to medium frequencies between 2 and 9 kHz, and the second range corresponds to high frequencies between 10 and 25 kHz. The analysis of dynamic mode tracking, fluctuating pressure and velocity spectra, and instantaneous flow fields relates specific vortices in the tip-gap flow to their spectral signature and paves the way for further analytical modeling of tip-gap noise sources.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:49:59Z
  DOI: 10.1063/5.0146143
Experimental and numerical study of flow field structure in U-shaped
channels with different bend sections
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Time-resolved particle image velocimetry is used to study the flow characteristics of rotating U-shaped channels with different types of bend sections: one with both inner and outer walls square, one with an inner circular wall and an outer square wall, and one with both inner and outer walls circular. The rotation number varies from 0 to 0.25, at a Reynolds number of 11 500. The present work aims at providing a detailed insight of the flow field occurring within a rotating U-shaped channel, typically resembling internal cooling channel embedded into first stages of turbine blades in aeroengines. A validated numerical simulation is carried out to determine the flow mechanism. A proper orthogonal decomposition and the Ω-criterion vortex identification method are used to study the vortical distribution and flow characteristics. The results show that a bend with both inner and outer square walls produces corner vortices on the outside of the bend section, and both the separation vortex and reattachment vortex are larger than those of the other two geometrical configurations. In the channels with a circular inner wall of the bend, separation is delayed, and both the separation vortex and reattachment vortex are smaller. When both walls of the bend are square, the peak Reynolds shear stress is twice than when they are both circular. With the increase in the rotation number, the size of vortical structures changes. The Coriolis force also changes the relative size of the secondary flow in the bend section, and the vortex near the leading surface becomes larger.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:48:29Z
  DOI: 10.1063/5.0142486
Optimization study on adaptive control performance of shock wave/boundary
layer interactions with different secondary recirculation configurations
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Shock wave/turbulent boundary layer interactions are widely observed in supersonic flows with many adverse effects on the flow field, resulting in increasing investigation on their control. This paper optimizes the secondary recirculation configuration based on our previous investigations. Six secondary recirculation configurations are designed, and the adaptive control schemes for these configurations are developed for incoming Mach numbers equaling 2.5, 3.0, and 3.5. The three-dimensional implicit Reynolds-Averaged Navier–Stokes equations employing the two-equation shear stress transport k–ω turbulence model are used to perform simulation calculations for each case. An evaluation approach is developed for the control performance and utilized to perform quantitative calculations. The calculation results are used to analyze the control effects of the separation zone volume, total pressure recovery coefficient, and peak wall heat flux for different configurations to find the best control configuration with the widest operating Mach number range. Finally, a configuration with a grid pattern distribution of suction holes, each with a length and width of 2.828 mm uniformly distributed over 52
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:48:27Z
  DOI: 10.1063/5.0142076
Numerical study of microjet and heat flux effects on flow separation and
heat transfer over a ramp
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  Authors: Mohammad Javad Pour Razzaghi, Yasin Masoumi, Seyed Mojtaba Rezaei Sani
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The control of flow and heat transfer has recently been of great interest to engineering researchers in light of computational technology advances. Microjets are used as control solutions to avoid flow separation and increase heat transfer. The present study evaluates a microjet over a ramp at microjet velocity ratios (jet to inflow velocity) of [math] = 1, 2, and 4 and heat flux ratios (heat flux to based heat flux) of [math] = 1, 2, and 3 to examine the flow separation area and heat transfer improvement numerically. The numerical velocity and temperature gradients were compared to earlier numerical and experimental works. Then, the flow over the ramp was analyzed at the aforementioned microjet velocity and heat flux ratios. Moreover, streamlines, bed pressure, fluid temperature, and bed Nusselt number were evaluated. It was found that a microjet with the optimal velocity could not only diminish the separation bubble but also improve heat transfer. A rise in the velocity ratio from 2 to 4 led to a nearly 33% decrease in the separation bubble and an approximately 20% rise in the Nusselt number. In addition, the microjets enhanced heat transfer by up to 50%.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:47:11Z
  DOI: 10.1063/5.0142658
Computational and experimental studies of wave–structure interaction:
Wave attenuation by a floating breakwater
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  Authors: N. N. Peng, W. K. Lau, O. W. H. Wai, K. W. Chow
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Interactions between surface gravity waves and a floating rigid body are complex, as waves may reflect from, break on, and be transmitted behind the body. Studies of these phenomena are critically important in improving the safety and functional efficiency of offshore structures. Here, the wave attenuation performance and motions of a type of floating breakwater (FB) are studied through numerical and experimental approaches. A numerical wave tank (NWT) is developed based on the software OpenFOAM and properties of wave channel from a laboratory. In the NWT, the air–water interface is captured by the volume of fluid method. The motions of FB are tracked by the six degrees of freedom model. A mooring system model is developed to simulate the constraints of the FB. Large eddy simulation turbulence modeling is implemented for the wave breaking processes. A model FB with a scale of 1:20 is tested in both the experimental and numerical wave channel. Wave heights at the back/front of the FB and the constraint forces of the mooring wires are measured. The numerical models are validated by comparing the results with experimental measurements. The variations of transmission/reflection coefficients, energy dissipation rate, and maximum mooring force are calculated. Changes of the response amplitude operators with the ratio of FB width to wavelength ([math]) and wave steepness are analyzed. The wave transmission coefficient will drop below 0.8 if the value of [math] is larger than 0.3, but will go over 0.95 if [math] is less than 0.1. Wave steepness has a large influence on FB motions and the mooring system. The effect of Stokes drift is observed by the shift of position of the FB.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:47:08Z
  DOI: 10.1063/5.0142991
Effects of polymer additives on the entrainment of turbulent water jet
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  We present an experimental study on the effects of polymer additives on the entrainment of a circular water jet and their dependence on the polymer concentration [math] (in the range of 0–40 ppm) and Weissenberg number Wi (in the range of 2.0–85.6), at the Reynolds number Re = 7075. Extensive particle image velocimetry measurements were performed between 0 and 74D (D is the inner diameter of the pipe) downstream of the nozzle. Our results clearly show that the polymer-laden jet exhibits two regimes along the flow direction compared to the pure water case. In the first regime, close to the jet exit, the jet spreading rate is smaller (entrainment is suppressed) and the centerline mean velocity decays more slowly. However, as the polymer-laden jet evolves further downstream, the entrainment rate is enhanced by up to 33% compared to that of the water jet. In this entrainment enhancement regime, the polymer-laden jet evolves into a new self-similar state. The turbulent intensities and Reynolds shear stress of different [math] and Wi collapse onto each other, and they are also much stronger compared to that of the water jet. We have also extended the integral entrainment analysis to the polymer-laden jet by adding a polymer stress term to the momentum equation. Our results show that the enhancement of the entrainment originates from the stronger production of the Reynolds shear stress in the polymer-laden jets, implying that the entrainment rate is intimately related to the energy-containing vortices in the polymer-laden jets.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:43:54Z
  DOI: 10.1063/5.0146313
Phase proper orthogonal decomposition of non-stationary turbulent flow
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  Authors: Azur Hodžić, Fabien Evrard, Berend van Wachem, Clara M. Velte
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  A phase proper orthogonal decomposition (phase POD) method is demonstrated utilizing phase averaging for the decomposition of spatiotemporal behavior of statistically non-stationary turbulent flows in an optimized manner. The proposed phase POD method is herein applied to a periodically forced statistically non-stationary lid-driven cavity flow, implemented using the snapshot proper orthogonal decomposition algorithm. Space-phase modes are extracted to describe the dynamics of the chaotic flow, in which four central flow patterns are identified for describing the evolution of the energetic structures as a function of phase. The modal building blocks of the energy transport equation are demonstrated as a function of the phase. The triadic interaction term can here be interpreted as the convective transport of bi-modal interactions. Non-local energy transfer is observed as a result of the non-stationarity of the dynamical processes inducing triadic interactions spanning across a wide range of mode numbers.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:43:47Z
  DOI: 10.1063/5.0143780
Laminar separation bubble on a rotating cylinder in uniform flow
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  Authors: Gaurav Chopra, Sanjay Mittal
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  We study the effect of spin ([math]) of a cylinder, placed in uniform flow, on the transition of the boundary layer. Large Eddy Simulation, with the Sigma turbulence model to account for the sub-grid scales, is carried out using a stabilized finite element formulation. The Reynolds numbers ([math] and [math]) lie in the high-subcritical regime for a non-rotating cylinder where the boundary layer separates in a laminar state and does not reattach. Magnus effect is observed at low α wherein separation is delayed on the retreating side and preponed on the advancing side, resulting in a lift force that increases with increase in α. At a certain critical α, the boundary layer on the advancing side transitions to a turbulent state, causing it to reattach. A laminar separation bubble (LSB) forms, significantly delaying the final separation and increasing suction. At [math], this suction overcomes that on the retreating side, leading to a reversal in the direction of lift force, referred to as the inverse Magnus effect. The LSB is accompanied by weakened vortex shedding at increased frequency. The spatial extent of the LSB and the magnitude of reverse lift, at a given Re, decreases with increase in α. The lift force changes direction yet again at a certain α marking the end of the inverse Magnus effect regime and beginning of the second Magnus effect regime. The LSB vanishes beyond a certain spin rate, and the boundary layer directly transitions to a turbulent state.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:40:39Z
  DOI: 10.1063/5.0141336
Numerical study on flow stall and kinetic energy conversion of
low-specific-speed centrifugal pump
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Stall is a common phenomenon in centrifugal pumps under low-flow conditions; it has a significant impact on fatigue and can even damage mechanical structural components. Computational fluid dynamics was used to perform high-precision numerical calculations to describe multiple operating conditions in the computational domain. The accuracy of these numerical simulations was verified by comparing the results with the single-flow channel flow patterns captured by time-resolved particle image velocimetry and the external characteristics of the centrifugal pump. On this basis, the unsteady spatiotemporal evolution of the vortex structure under stall conditions and the kinetic energy conversion relationship were determined. The stall vortex under the rotating stall condition has a relative motion with the impeller in the circumferential direction between channels, with the characteristic propagation frequency fcs = 0.71 Hz. For stationary stall conditions, the critical stall condition has a greater kinetic energy dissipation compared with the deep stall condition, with energy differences being more than three times larger at the blade leading edge, where the stall vortex is formed.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:40:38Z
  DOI: 10.1063/5.0143316
Effects of spacing ratio on vortex-induced vibration of twin tandem
diamond cylinders in a steady flow
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Vortex-induced vibration of twin tandem square cylinders at an inclined angle of 45° to the fluid, i.e., twin diamond cylinders of mass ratio m* = 3, is numerically investigated at Reynolds number Re = 100 and reduced velocity Ur = 3–18. This paper focuses on the effects of cylinders' spacing ratio L* (=L/B, where L is cylinders' center-to-center spacing and B is the characteristic length) ranging from 2 to 6 on the oscillation responses of two-degree-of-freedom cylinders. The results indicate that the wake structure experiences two gap flow patterns, the reattachment and co-shedding regimes, and eight different wake modes. At a small spacing (L* = 2–3), the reattachment regime occurs for the lower or higher Ur with the approximate range of 3 and 16–18. Meanwhile, the reattachment regime mainly occurs for other ranges of Ur at L* = 2–6. The more significant oscillation of each spacing appears in the cross-flow direction, and the maximum cross-flow amplitude of the upstream cylinder is smaller than that of the downstream cylinder. Additionally, although significant cross-flow oscillations occur at small spacings (L* = 2–3) with the Ur ≈ 5–9 and 12–14, the intrinsic mechanisms are entirely different. For the cross-flow oscillation characteristics of larger spacings (L* = 4–6), they are virtually similar.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:38:05Z
  DOI: 10.1063/5.0146395
Dynamics of flexible fibers in confined shear flows at finite Reynolds
numbers
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  We carry out a numerical study on the dynamics of a single non-Brownian flexible fiber in two-dimensional confined simple shear (Couette) flows at finite Reynolds numbers. We employ the bead-spring model of flexible fibers to extend the fluid particle dynamics (FPD) method that was originally developed for rigid particles in viscous fluids. We implement the extended FPD method using a multiple-relaxation-time scheme of the lattice Boltzmann method. The numerical scheme is validated first by a series of benchmark simulations that involve fluid–solid coupling. The method is then used to study the dynamics of flexible fibers in Couette flows. We only consider the highly symmetric cases where the fibers are placed on the symmetry center of Couette flows, and we focus on the effects of the fiber stiffness, the confinement strength, and the finite Reynolds number (from 1 to 10). A diagram of the fiber shape is obtained. For fibers under weak confinement and a small Reynolds number, three distinct tumbling orbits have been identified: (1) Jeffery orbits of rigid fibers—the fibers behave like rigid rods and tumble periodically without any visible deformation; (2) S-turn orbits of slightly flexible fibers—the fiber is bent to an S-shape and is straightened again when it orients to an angle of around 45° relative to the positive x-direction; and (3) S-coiled orbits of fairly flexible fibers—the fiber is folded to an S-shape and tumbles periodically and steadily without being straightened anymore during its rotation. Moreover, the fiber tumbling is found to be hindered by increasing either the Reynolds number or the confinement strength, or both.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:38:03Z
  DOI: 10.1063/5.0141027
Deformations of an active liquid droplet
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  Authors: R. Kree, A. Zippelius
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  A fluid droplet, in general, deforms if subject to active driving, such as a finite slip velocity or active tractions on its interface. Starting from Stokes equations, we show that these deformations and their dynamics can be computed analytically in a perturbation theory in the inverse of the surface tension γ, by using an approach based on vector spherical harmonics. We consider squirmer models and general active tractions, such as inhomogeneous surface tensions, which may result from the Marangoni effects. In the lowest order, the deformation is of order [math], yet it affects the flow fields inside and outside of the droplet in order to [math]. Hence, a correct description of the flow has to allow for shape fluctuations, —even in the limit of large surface tension. We compute stationary shapes and relaxation times and compare our results to an approach, which discards all effects of deformations on surface tensions. This approach leads to the same propulsion velocity but to significantly different flow fields.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:35:27Z
  DOI: 10.1063/5.0143700
Dynamics of electrified liquid metal surface using shallow water model
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  Authors: Kentaro Hara, Mikhail N. Shneider
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  A shallow water model that incorporates surface tension and electric field effects is developed to investigate the dynamics of an electrified liquid surface. The computational model is verified against the Zakharov–Kuznetsov equation and is applied to study the growth and damping of the electrified liquid surface. A linear wave analysis is performed under a shallow water theory assuming an analytic solution of the electric field, similar to the Tonks–Frenkel instability. The electrified liquid surface grows or dampens based on the balance of the electric field, surface tension, and gravitational forces. The numerical results obtained from the electrified shallow water solver are in good agreement with the theoretical analysis.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:34:19Z
  DOI: 10.1063/5.0145930
Effect of fuel temperature on mixing characteristics of a kerosene jet
injected into a cavity-based supersonic combustor
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  To explain the phenomenon observed in previous experiments of kerosene-ignition failure in scramjet combustors as the kerosene temperature increases, we numerically investigate the mixing characteristics of a kerosene jet injected into a cavity-based supersonic combustor at different injection temperatures by using a compressible two-phase flow large-eddy simulation based on the Eulerian–Lagrangian approach. The results indicate that, upon injecting kerosene at high temperatures, the flow field preceding the leading edge of the cavity is similar to a typical gas jet in supersonic crossflow. The wall counter-rotating vortex pair (CVP) develops more fully and eventually becomes the main vortex pair. This evolution of the wall CVP modifies the cavity shear layer and alters the local flow-field characteristics near the cavity. Upon injecting kerosene at high temperatures, its evaporation rate increases sharply and the cavity recirculation zone enlarges, which causes more kerosene vapor to be entrained into the cavity. Because the kerosene-vapor temperature is lower than that of the low-speed fluid in the cavity, a significant amount of kerosene vapor entering the cavity not only makes the mass fraction of kerosene in the cavity exceed the fuel stoichiometric mass fraction but also reduces the temperature in the cavity, which negatively impacts the ignition process. The ignition delay time is much longer when the injection temperature is high, which is consistent with the inability of the initial flame kernel to form in the experiment.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:34:17Z
  DOI: 10.1063/5.0145494
Understanding stable/unstable miscible [math] reaction front and mixing in
porous medium
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  Authors: Priya Verma, Vandita Sharma, Manoranjan Mishra
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The transport phenomena of [math] type reactive miscible front undergoing radial displacement in a porous medium are numerically investigated. For a stable displacement when the viscosity of fluids A, B, and C is same, the dependence of various reaction characteristics on the Damköhler number (Da) is analyzed. The total reaction rate is found to be a non-monotonic function of time depending upon Da, while the total amount of product follows the temporal scaling [math]. The viscosity contrast in the system renders unstable flow and results in a hydrodynamic instability called viscous fingering. The effect of hydrodynamics on the reaction product formation is discussed. An insight into the reaction characteristics due to interaction of chemical reaction and instability is obtained for various log-mobility ratios [math]. It is observed that the onset of instability, as well as the mixing of the fluids, depends on whether the reaction generates a high or less viscous product or equivalently, the sign of [math], keeping Rb fixed. Furthermore, the relation between the first moment of averaged reaction rate for stable and unstable displacement is influenced by the sign of [math] and Da. The coupling of convection and diffusion on the chemo-hydrodynamic instability is presented, and the existence of the frozen fingers in this reactive fluid system is reported. Our numerical results allow us to understand how instability and chemical reaction interplay to affect the reaction characteristics and the mixing of fluids.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:28:20Z
  DOI: 10.1063/5.0143853
Numerical study of periodic flame flashback in a cavity-based scramjet
combustor
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  Authors: Shengzu Guo, Xu Zhang, Qili Liu, Lianjie Yue
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The periodic flame flashback phenomenon in an ethylene-fueled cavity-based scramjet combustor was numerically investigated by a three-dimensional unsteady Reynolds-averaged Navier–Stokes solver with two-step kinetics. The air inflow stagnation temperature is 1225 K, and its Mach number is 2.6. Spectral analyses revealed the combustion oscillations with flame flashbacks maintained in the separated scramjet mode with the establishment/vanishment of flow separation near the fuel injector, differing from previous studies of flame flashbacks connected to the ramjet/scramjet mode transitions. A mechanism with four evolution stages was proposed to elucidate the flow-flame interaction. In stage I, a rapid flame flashback upstream and shock-train extension were caused by the high-temperature induced auto-ignition tendency of well-mixed unburned gas in the near-sidewall low-speed region. In stage II, the combustion-induced back pressure and shock train gradually achieved an aerodynamic balance. The combustion flow barely changed in stage III. Meanwhile, a simplified model suggested that the gradual temperature rises occurring upstream of the cavity and away from the sidewall were caused by spanwise heat conduction. The higher temperatures would cause upstream flame propagation with enhanced heat release due to auto-ignition. However, the enhanced heat release occurred mostly in the subsonic flow, resulting in pressure decreases according to one-dimensional flow equations. A smaller near-sidewall separation was produced by the lower back-pressures, which prompted the rapid flame recession downstream in stage IV. Moreover, a simplified flame-spreading model was proposed to illuminate the flame propagation nature. The comparison of flame speeds with theoretical estimations indicated that the current flame was in the regime of turbulent flame propagation, rather than the C–J detonation or deflagration speculated in previous studies.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:28:17Z
  DOI: 10.1063/5.0142210
Creeping flow of non-Newtonian fluid through membrane of porous
cylindrical particles: A particle-in-cell approach
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  Authors: Amit Kumar Saini, Satyendra Singh Chauhan, Ashish Tiwari
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The present study is an attempt to deal with hydrodynamic and thermal aspects of the incompressible Carreau fluid flow past a membrane consisting of uniformly distributed aggregates of porous cylindrical particles enclosing a solid core which aims to provide a comprehensive study of the impact of non-Newtonian nature of Carreau fluid in the filtration process through membranes. The non-Newtonian characteristic of Carreau fluid is adopted to describe the mechanism of the pseudoplastic flow through membranes. The layout of the fluid flow pattern is separated into two distinct areas in which the area adjacent to the solid core of the cylindrical particle is considered as porous. However, the region surrounding the porous cylindrical particle is taken as non-porous (clear fluid region). The Brinkman equation governs the porous region, whereas the non-porous region is regulated by the Stokes equation. The nonlinear governing equations of the Carreau fluid flow in the different regions are solved using an asymptotic series expansion in terms of the small parameters, such as Weissenberg number [math] and a non-dimensional parameter [math], for the higher permeability of the porous material. For large permeability, the expression of velocity is derived, and the same has been used to compute the hydrodynamic permeability, Kozeny constant, and temperature profile. The numerical scheme (NDSolve in Mathematica) is used to solve the singularly perturbed boundary value problems in the case of small permeability of the porous medium [i.e., [math]]. The graphical analysis illustrating the outcomes of the effects of varying control parameters such as the power-law index, viscosity ratio parameter, permeability of the porous medium, Weissenberg number, and Nusselt number on the membrane permeability, Kozeny constant and temperature profile are discussed comprehensively and validated with previously published works on the Newtonian fluid in the limiting cases. The notable determination of the present study is that the Carreau fluid parameters, such as the Weissenberg number, power-law index, and viscosity ratio parameter, have a significant impact on the velocity, and hence, the membrane permeability, Kozeny constant, and temperature profile. The results showed a significant increase in the flow velocity and hydrodynamic permeability as the dominance of elastic forces over viscous forces increased in the case of high permeability [math]. The velocity gets a slight reduction for lower permeability of the porous material [math]; however, the hydrodynamic permeability behaves similar to the higher permeability of the porous material. The findings of the proposed work may be instrumented in analyzing various processes, including wastewater treatment filtration processes, and blood flow through smooth muscle cells. The proposed work, however, requires experimental verification.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:26:49Z
  DOI: 10.1063/5.0143317
An improved asymptotic expansion method for fluid flow and convective heat
transfer in cone-and-disk geometries with rotating cone
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  Authors: Igor V. Shevchuk
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  In this paper, an improved asymptotic expansion method has been developed to simulate fluid flow and convective heat transfer in a conical gap at small conicity angles up to 4°. Unlike previous works, the improved asymptotic expansion method was applied to the self-similar system of Navier–Stokes equations for small conicity angles. The characteristic Reynolds number varied in the range from 0.001 to 2.0. A detailed validation of the improved asymptotic expansion method compared to the self-similar solution performed for the case of cone rotation with a fixed disk demonstrated its significant advantages compared to previously known asymptotic expansion methods. For the first time, novel approximate analytical solutions were obtained for the tangential and axial velocity components, the swirling angle of the flow, tangential shear stresses on the surface of a fixed disk, as well as static pressure distribution varying in the gap height, which perfectly coincide with the self-similar solution. The accuracy of the improved asymptotic expansion method in the numerical calculation of the Nusselt number in the range of Prandtl numbers from Pr = 0.71 to Pr = 10 significantly exceeds the accuracy of the previously known asymptotic expansion methods. This enables expanding the range of Reynolds and Prandtl numbers, for which the improved asymptotic expansion method has approximately the same accuracy as the self-similar solution. The fact is confirmed that the account for the radial thermal conductivity in the energy equation in the case of small conicity angles up to 4° leads to insignificant deviations of the Nusselt number (maximum 1.5%).
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:26:46Z
  DOI: 10.1063/5.0146556
Lateral instability in fruit flies is determined by wing–wing
interaction and wing elevation kinematics
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  Authors: Illy Perl, Roni Maya, Oron Sabag, Tsevi Beatus
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Understanding the uncontrolled passive dynamics of flying insects is important for evaluating the constraints under which the insect flight control system operates and for developing biomimetic robots. Passive dynamics is typically analyzed using computational fluid dynamics (CFD) methods, relying on the separation of the linearized hovering dynamics into longitudinal and lateral parts. While the longitudinal dynamics are relatively understood across several insect models, our current understanding of the lateral dynamics is lacking, with a nontrivial dependence on wing–wing interaction and on the details of wing kinematics. Particularly, the passive stability of the fruit fly, D. melanogaster, which is a central model in insect flight research, has so far been analyzed using simplified quasi-steady aerodynamics and synthetic wing kinematics. Here, we perform a CFD-based lateral stability analysis of a hovering fruit fly, using accurately measured wing kinematics, and considering wing–wing interaction. Lateral dynamics are unstable due to an oscillating–diverging mode with a doubling time of 17 wingbeats. These dynamics are determined by wing–wing interaction and the wing elevation kinematics. Finally, we show that the fly's roll controller, with its one wingbeat latency, is consistent with the lateral instability. This work highlights the importance of accurate wing kinematics and wing–wing interactions in stability analyses and forms a link between such passive instability and the insects' controller.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:25:27Z
  DOI: 10.1063/5.0138255
Deformation characteristics of compound droplets with different
morphologies during transport in a microchannel
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  A numerical investigation of the deformation of compound microdroplets transported inside a circular microchannel is described in this article. Two droplet morphologies are considered (shell-core and Janus), which correspond to nonequilibrium and equilibrium states, respectively, based on the balancing of the three interfacial tensions at the triple line. Numerical simulations coupled with a three-phase volume-of-fluid method are performed on axisymmetric models to consider both the absence and presence of a triple line. In addition to adaptive mesh refinement on the interfaces, topology-oriented refinement is used to resolve thin films between the shell and core droplets. After experimental validation, the effects of flow rates, physical properties, and confinement conditions are considered. In the reference frame of the droplets, there are five inner vortexes inside the shell-core droplet, while only three are present inside the Janus droplet, the same as single-phase droplets. For shell-core droplets, the aspect ratio of the shell droplet decreases with the capillary number of the continuous phase and droplet sizes, while sudden jumps are identified when the thin film forms between the shell and core interfaces. Conversely, the aspect ratio of the core droplet increases and then decreases when the shape of the core droplets is influenced by the flow and space confinements. With Janus droplets, the aspect ratio decreases with the capillary number. The axial length of the front portion decreases with the capillary number and then reaches a plateau with small variations, while that of the rear portion increases nearly linearly.
  Citation: Physics of Fluids
  PubDate: 2023-04-04T01:25:27Z
  DOI: 10.1063/5.0146560
Erratum: “Diffusion of gravity waves by random space inhomogeneities in
pancake-ice fields. Theory and validation with wave buoys and synthetic
aperture radar” [Phys. Fluids 33, 096601 (2021)]
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  Authors: Piero Olla, Giacomo De Carolis, Francesca De Santi
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:24:30Z
  DOI: 10.1063/5.0146637
Publisher’s Note: “Effect of polymer viscosity and viscoelasticity on
tooth cooling and aerosolization during dental procedures” [Phys. Fluids
35, 023112 (2023)]
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  Authors: Alexander L. Yarin
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:24:29Z
  DOI: 10.1063/5.0151268
On the incorporation of conservation laws in machine learning tabulation
of kinetics for reacting flow simulation
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  Authors: Thomas Readshaw, W. P. Jones, Stelios Rigopoulos
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Tabulation of chemical mechanisms with artificial neural networks (ANNs) offers significant speed benefits when computing the real-time integration of reaction source terms in turbulent reacting flow simulations. In such approaches, the ANNs should be physically consistent with the reaction mechanism by conserving mass and chemical elements, as well as obey the bounds of species mass fractions. In the present paper, a method is developed for satisfying these constraints to machine precision. The method can be readily applied to any reacting system and appended to the existing ANN architectures. To satisfy the conservation laws, certain species in a reaction mechanism are selected as residual species and recalculated after ANN predictions of all of the species have been made. Predicted species mass fractions are set to be bounded. While the residual species mass fractions are not guaranteed to be non-negative, it is shown that negative predictions can be avoided in almost all cases and easily rectified if necessary. The ANN method with conservation is applied to one-dimensional laminar premixed flame simulations, and comparisons are made with simulations performed with direct integration (DI) of chemical kinetics. The ANNs with conservation are shown to satisfy the conservation laws for every reacting point to machine precision and, furthermore, to provide results in better agreement with DI than ANNs without conservation. It is, thus, shown that the proposed method reduces accumulation of errors and positively impacts the overall accuracy of the ANN prediction at negligible additional computational cost.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:23:30Z
  DOI: 10.1063/5.0143894
Publisher's Note: “Effect of radial velocity profiles on axial
dispersion in packed beds: Transient formulation” [Phys. Fluids 35,
033604 (2023)]
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  Authors: Carlos D. Luzi, Osvaldo M. Martinez, Guillermo F. Barreto
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:23:29Z
  DOI: 10.1063/5.0150423
The impact of secondary flow intensity on heat transfer efficiency of the
wire-to-plate electrohydrodynamics devices
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The performance optimization of electrohydrodynamics (EHD) induced heat transfer enhancement has attracted much interest in recent decades. Although various EHD device designs have been proposed, coupling optimization based on comprehensive parameters, including Reynolds number, voltage, and electrode spacing, is still absent, and the overall heat transfer efficiency is rarely considered. In this study, the heat transfer efficiency of a wire-to-plate EHD device in a wide range of secondary flow intensity NEHD = 0.4–5 is investigated. Here, NEHD is a dimensionless parameter that integrates Reynolds number, voltage, electrode radius, etc. The average Nusselt number Nu rather than the enhancement rate ER is selected for optimization. It is demonstrated that NEHD = 2 is the optimal secondary flow intensity in both single-electrode and multiple-electrode configurations. The too-weak or too-strong secondary flow will lead to a decrease in the heat transfer efficiency. The underlying physics is revealed by the barrier effect and oversize vortex. An optimal electrode spacing of l > 0.014 m is proposed in the multiple-electrode configuration. A strong interaction between adjacent vortices will significantly decrease the heat transfer efficiency when l 0.014 m, then arrange as many electrodes as possible in the channel.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:22:31Z
  DOI: 10.1063/5.0143629
Numerical model of the Gross–Pitaevskii equation for rotating
Bose–Einstein condensates using smoothed-particle hydrodynamics
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  This study proposed a new numerical scheme for vortex lattice formation in a rotating Bose–Einstein condensate (BEC) using smoothed particle hydrodynamics (SPH) with an explicit real-time integration scheme. Specifically, the Gross–Pitaevskii equation was described as a complex representation to obtain a pair of time-dependent equations, which were then solved simultaneously following discretization based on SPH particle approximation. We adopt the fourth-order Runge–Kutta method for time evolution. We performed simulations of a rotating Bose gas trapped in a harmonic potential, showing results that qualitatively agreed with previously reported experiments and simulations. The geometric patterns of formed lattices were successfully reproduced for several cases, for example, the hexagonal lattice observed in the experiments of rotating BECs. Consequently, it was confirmed that the simulation began with the periodic oscillation of the condensate, which attenuated and maintained a stable rotation with slanted elliptical shapes; however, the surface was excited to be unstable and generated ripples, which grew into vortices and then penetrated inside the condensate, forming a lattice. We confirmed that each branch point of the phase of wavefunctions corresponds to each vortex. These results demonstrate our approach at a certain degree of accuracy. In conclusion, we successfully developed a new SPH scheme for the simulations of vortex lattice formation in rotating BECs.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:22:29Z
  DOI: 10.1063/5.0143556
Piston driven shock waves in non-homogeneous planar media
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  Authors: Menahem Krief
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  In this work, we analyze in detail the problem of piston driven shock waves in planar media. Similarity solutions to the compressible hydrodynamics equations are developed, for a strong shock wave, generated by a time dependent pressure piston, propagating in a non-homogeneous planar medium consisting of an ideal gas. Power law temporal and spatial dependency is assumed for the piston pressure and initial medium density, respectively. The similarity solutions are written in both Lagrangian and Eulerian coordinates. It is shown that the solutions take various qualitatively different forms according to the value of the pressure and density exponents. We show that there exist different families of solutions, for which the shock propagates at a constant speed, accelerates, or slows down. Similarly, we show that there exist different types of solutions, for which the density near the piston is either finite, vanishes, or diverges. Finally, we perform a comprehensive comparison between the planar shock solutions and Lagrangian hydrodynamic simulations, by setting proper initial and boundary conditions. A very good agreement is reached, which demonstrates the usefulness of the analytic solutions as a code verification test problem.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:19:10Z
  DOI: 10.1063/5.0145896
Internal explosions and their effects on humans
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  Authors: Ioannis W. Kokkinakis, Dimitris Drikakis
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  This paper concerns indoor explosions and the impact of blasts on humans. The standard approach from the engineering and medical communities is that blast overpressure is the criterion that determines trauma and injury. However, this study shows that the wind force generated behind the blast can affect humans more significantly, even for relatively low blast overpressures. Although the present findings also apply to external explosions, we chose the indoor case as this is a more complex problem. We present high-order simulations for an explosion equivalent to 2.5 lbs of trinitrotoluene in a simplified indoor environment comprising three rooms and a corridor. The explosion magnitude could correspond to a malicious act, such as someone carrying a rucksack with the above explosive. The study reveals that the force generated can be up to 60 times the human's weight, even in the spaces adjacent to the room where the explosion occurred. The blast effects will be fatal for humans in the room where the explosion occurs. The impact on human organs, such as the lungs, brain, and gastrointestinal system, will vary in the adjacent spaces. The likelihood of primary injury increases from the repeated shockwaves due to their continuous reflections of the walls, impacting the lungs and gastrointestinal tract significantly and causing eardrums to burst and brain hemorrhage. Secondary blast injuries will occur due to the debris and high airspeeds behind the blast. Corridors and locations facing the doors are particularly dangerous. The simulations show a common asymptotic decay behavior of the wind force and blast overpressure across rooms at later times. The study concludes that forces resulting from the high airspeeds that develop are likely to cause greater injury than the blast overpressure itself.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:19:09Z
  DOI: 10.1063/5.0146165
Numerical simulations of the flow and aerosol dispersion in a violent
expiratory event: Outcomes of the “2022 International Computational
Fluid Dynamics Challenge on violent expiratory events”
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  Authors: Jordi Pallares, Alexandre Fabregat, Akim Lavrinenko, Hadifathul Akmal bin Norshamsudin, Gabor Janiga, David F. Fletcher, Kiao Inthavong, Marina Zasimova, Vladimir Ris, Nikolay Ivanov, Robert Castilla, Pedro Javier Gamez-Montero, Gustavo Raush, Hadrien Calmet, Daniel Mira, Jana Wedel, Mitja Štrakl, Jure Ravnik, Douglas Fontes, Francisco José de Souza, Cristian Marchioli, Salvatore Cito
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  This paper presents and discusses the results of the “2022 International Computational Fluid Dynamics Challenge on violent expiratory events” aimed at assessing the ability of different computational codes and turbulence models to reproduce the flow generated by a rapid prototypical exhalation and the dispersion of the aerosol cloud it produces. Given a common flow configuration, a total of 7 research teams from different countries have performed a total of 11 numerical simulations of the flow dispersion by solving the Unsteady Reynolds Averaged Navier–Stokes (URANS) or using the Large-Eddy Simulations (LES) or hybrid (URANS-LES) techniques. The results of each team have been compared with each other and assessed against a Direct Numerical Simulation (DNS) of the exact same flow. The DNS results are used as reference solution to determine the deviation of each modeling approach. The dispersion of both evaporative and non-evaporative particle clouds has been considered in 12 simulations using URANS and LES. Most of the models predict reasonably well the shape and the horizontal and vertical ranges of the buoyant thermal cloud generated by the warm exhalation into an initially quiescent colder ambient. However, the vertical turbulent mixing is generally underpredicted, especially by the URANS-based simulations, independently of the specific turbulence model used (and only to a lesser extent by LES). In comparison to DNS, both approaches are found to overpredict the horizontal range covered by the small particle cloud that tends to remain afloat within the thermal cloud well after the flow injection has ceased.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:15:51Z
  DOI: 10.1063/5.0143795
Evolution of unsteady vortex structures in the tip region of an axial
compressor rotor
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The evolution of unsteady vortex structures in the tip region of an axial compressor rotor is investigated based on delayed detached eddy simulation. The vortex structures are identified by the [math] method, and the velocity fields are visualized by the particle tracing method. The results show that the evolution of the tip leakage vortex (TLV) can be divided into three phases: the generation phase, the development phase, and the dissipation phase. The unsteadiness of the flow field mainly appears in the dissipation phase as a consequence of the unsteady secondary tip leakage. There are three primary unsteady vortex structures caused by the tip leakage flow (TLF), and these vortex structures are related to each other as a feedback loop. The intermittent formation of the vortex ropes leads to the breakdown of the TLV and thus results in the roll-up of the backflow vortex (BFV) due to the radial velocity gradient. The secondary leakage of the BFV locally enhances the TLF jet and affects the formation of the vortex ropes in turn. This feedback loop causes the unsteady behavior of the TLF and has great impacts on the performance and stability of the compressors.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:15:49Z
  DOI: 10.1063/5.0141818
Deep dual recurrence optical flow learning for time-resolved particle
image velocimetry
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Motion fields estimated from image data have been widely used in physics and engineering. Time-resolved particle image velocimetry (TR-PIV) is considered as an advanced flow visualization technique that measures multi-frame velocity fields from successive images. Contrary to conventional PIV, TR-PIV essentially estimates a velocity field video that provides both temporal and spatial information. However, performing TR-PIV with high computational efficiency and high computational accuracy is still a challenge for current algorithms. To solve these problems, we put forward a novel deep learning network named Deep-TRPIV in this study, to effectively estimate fluid motions from multi-frame particle images in an end-to-end manner. First, based on particle image data, we modify the optical flow model known as recurrent all-pairs field transforms that iteratively updates flow fields through a convolutional gated recurrent unit. Second, we specifically design a temporal recurrent network architecture based on this optical flow model by conveying features and flow information from previous frame. When N successive images are fed, the network can efficiently estimate N – 1 motion fields. Moreover, we generate a dataset containing multi-frame particle images and true fluid motions to train the network supervised. Eventually, we conduct extensive experiments on synthetic and experimental data to evaluate the performance of the proposed model. Experimental evaluation results demonstrate that our proposed approach achieves high accuracy and computational efficiency, compared with classical approaches and related deep learning models.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:09:52Z
  DOI: 10.1063/5.0142604
Pressure gradient effect on flame–vortex interaction in lean premixed
bluff body stabilized flames
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  Authors: Y. Yalcinkaya, A. G. Gungor
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  This investigation considers the effect of axial pressure gradient on the dynamics of flame–vortex interaction for a lean premixed bluff body stabilized flame. Large eddy simulations (LESs) of four different combustor geometries generated through combustor wall adjustments that resulted in mild to strong pressure gradients are studied. A bluff body stabilized combustor for a propane/air flame is analyzed first. The results are compared with all available experimental data with the purpose of validating the LES methodology used in OpenFOAM and obtaining a base solution for the study of the pressure gradient effect on flame–vortex interaction. The role of the pressure gradient on flame structure, emission characteristics, vortex dynamics, and flame stability is presented. The mild favorable pressure gradient due to the decelerated flow in diffuser configurations influences flame–vortex dynamics by suppressing flame-induced vorticity sources, baroclinic torque and dilatation, and hence resulting in augmented hydrodynamic instabilities. The sustained hydrodynamic instabilities maintain the large flame wrinkles and sinusoidal flame mode in the wake region. The nourished near-lean blowoff dynamics also affect the emission characteristics, and the emission of species increases. However, the accelerated flow in the nozzle configuration amplifies the flame-induced vorticity sources that preserve the flame core, resulting in a more organized, symmetric, and stable flame. Ultimately, the combustion performance and operation envelope in the lean premixed flames can be increased by maintaining the flame stability and suppressing the limiting lean blowoff dynamics and emissions with the help of a strong favorable pressure gradient generated through adjusting the combustor geometry.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:09:49Z
  DOI: 10.1063/5.0140026
Study of interscale interactions for turbulence over the obstacle arrays
from a machine learning perspective
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Investigating interactions between large- and small-scale motions is essential for understanding turbulence over rough boundaries. The present work applies XGBoost models to predict the spatial distribution of ejections and sweeps and quantify their statistical dependence on scale-decomposed velocity fields. Based on large eddy simulation, the models are trained and validated at 20 horizontal planes in turbulence over two types of obstacle arrays. At each height, a default XGBoost model X0 and four comparison models ([math], and [math]) are trained. The model X0 is trained by the set with four scale-decomposed velocity fields [math], where u and w are the streamwise and vertical velocity fluctuations and subscripts L and S refer to above-canyon and sub-canyon scales, while the comparison models are trained by subsets of the scale-decomposed velocity fields. The results indicate that the model X0 predicts the spatial distributions of both ejection and sweep events well, with the structure underestimation being less than 8% within the canopy layer and 3% above it. Along the vertical direction, the relative importance of scale-decomposed velocity fields on the prediction of ejections and sweeps is quantified by the feature importance and prediction errors. The feature importance profiles reveal that both sweeps and ejections are most strongly related to [math] within the canopy, but ejections have a stronger dependence on [math] well above the canopy. For the comparison models, those trained with [math] (namely, [math] and [math]) give better predictions within the canopy layer, whereas those trained with [math] (namely, [math] and [math]) perform better above the canopy. This study shows that a machine-learning-based approach can be designed to quantify the relative importance of different scale-decomposed velocity fields on predicting ejections and sweeps and to detect vertical changes of such relative importance.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:06:51Z
  DOI: 10.1063/5.0138440
Detecting ultrafast turbulent oscillations in near-nozzle discharged
liquid jet using x-ray phase-contrast imaging with MHz frequency
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  Authors: Omer Faruk Atac
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Characteristics of a discharged liquid jet in near-nozzle are determined by the in-flow turbulences generated by the evolution of inflow vortices and cavitation. High-fidelity simulations have indicated that such physical processes can generate ultrafast turbulent fluctuations (in the range of MHz) originating from the nature of turbulence by the interaction between the large and small-scale turbulence in the flow. Detecting ultrafast turbulent oscillations while resolving small-scale turbulences in the optically dense near-nozzle liquid jet has not been observed through experimental methods so far. In this study, therefore, ultrafast x-ray phase-contrast imaging, which can provide a clear image in the near-field using a high-energy x-ray source, was applied to observe the fluctuation of flow velocity in the near-field to obtain the ultrafast turbulent oscillations at the discharged jet. To capture the ultrafast variance of flow velocity originating from the nature of turbulence, the high imaging frequency was applied up to 1.2 MHz. With the implemented methodology, turbulence intensity distributions of discharged liquid jets were measured for various injection pressures and nozzle geometries. Such turbulence intensity results were also correlated with the initial dispersion angle of the spray. In addition, the turbulence length scales, which can be detected through the current methodology, were estimated and discussed considering standard-length scales. The results showed that the current experimental method introduced in this study can provide important insights into the turbulence characteristics of spray by resolving Taylor scale turbulences and can provide valuable validation data and boundary conditions for reliable spray simulations.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:06:49Z
  DOI: 10.1063/5.0143351
Influences of the cavity leakage flow on shrouded stator performance at
different inlet boundary layer shapes
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  Authors: Xiaozhi Kong, Tianshuo Huang, Yuxin Liu, Yuze Sun, Huawei Lu
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  By using the validated numerical method to simulate the annular shrouded stator with the inter-stage seal cavity, the stator aerodynamic performance related to the cavity leakage flow was examined. The current study also indicated the interactions between the leakage flow and the mainstream. Discussions on the developments of secondary flow movement and hub corner separation were conducted, and the performances at different incoming boundary layer shapes were assessed using both the total pressure loss coefficient and the entropy-based loss coefficient. The results indicate that the cavity leakage flow creates a new vortex close to the leading edge of blade and is crucial to the passage vortex development and the concentrated shedding vortex size. At the same time, the cavity leakage flow weakens the transverse deflection of flow near the end wall and strengthens the three-dimensional flow effect. The two loss coefficients for the shrouded stator with seal cavity change little by thickening the boundary layer. The proper boundary layer skew as well as the interacted cavity leakage flow have stronger resistance to the deflection of the passage vortex and the transverse pressure gradient.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:04:51Z
  DOI: 10.1063/5.0146925
Attenuation of Tollmien–Schlichting waves using resonating
surface-embedded phononic crystals
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  Authors: T. Michelis, A. B. Putranto, M. Kotsonis
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  A novel method for control of convective boundary layer instabilities using metamaterial concepts is investigated. Attenuation of Tollmien–Schlichting (TS) waves with surface-embedded one-dimensional phononic crystals (PCs) is theoretically and numerically modeled, capitalizing on the inherent frequency band stop of PCs. The PC is tuned to the targeted TS wave characteristics through the use of analytical models derived from transfer matrix and interface response theories, verified using a finite elements analysis. The interaction between TS waves and a single PC is investigated using coupled two-dimensional fluid structure interaction simulations in the frequency domain. It is shown that TS waves are either amplified or attenuated depending on whether the PC free-face surface displacement and unsteady perturbation pressure at the wall are in-phase or out-of-phase, respectively. The perturbation pressure acts solely as the driver for the mechanical oscillation of the PC. The emerging hydrodynamic coupling between TS waves and the PC is found to be governed by a combination of the Orr mechanism and wall-normal velocity linear superposition near the wall. Finally, a metasurface comprised of an array of streamwise-distributed PCs is evaluated, resulting in an amplitude growth delay of 11.3% of the TS wavelength along the metasurface extent.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T02:04:49Z
  DOI: 10.1063/5.0146795
Research on the seepage properties of coal with different particle size
proppant under cyclic loading
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The selection of proppant particle size significantly impacts the gas output and gas production period of the extracting coalbed methane (CBM). This study combines theoretical analysis and permeability testing, based on the in situ stress distribution characteristics of the coal seam in Wangjiazhai Coal Mine, Guizhou Province, conducted on artificial fractures with different particle size proppant combinations during the cyclic loading and unloading. The findings indicate that the coal sample with two particle sizes of proppant has more permeability and smaller stress sensitivity coefficient than the coal sample with a single particle size proppant; as effective stress increases, the coal sample with the maximum permeability and the smallest stress sensitivity coefficient is placed with a proppant ratio of 20/40 mesh to 40/70 mesh of 1–3. The stress sensitivity coefficient and the permeability decrease with an increase in the number of confining pressure cycles. The increase in the proppant embedding depth has a hysteresis phenomenon with the increase in the effective stress, and the coal sample with a proppant ratio of 20/40 mesh to 40/70 mesh of 1–3 has the smallest embedded depth. The proppant will cause damage to the fracture surface of the coal seam. This study provides technical support for efficiently extracting the CBM resources that are difficult to exploit in Guizhou Province.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T01:59:58Z
  DOI: 10.1063/5.0143895
Hydrodynamics of the projectile entering the water under the ice hole
constraint environment
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The study of the water entry of the projectile passing through the ice hole can solve the special issue of water entry under marine environmental constraints. We conducted experiments to validate the effect of the ice hole constraint on the dynamics of the water entry cavity and then used the numerical simulations to investigate the cavity dynamics of the projectile passing through ice holes with different sizes and rotation degrees. The results show that the ice hole affects the evolution of the water entry cavity and the motion state of the projectile. The splash crown flows back and then contacts the projectile surface when passing through the small-sized ice hole. Cavity collapses before the pinch-off. The splash crown flows back at the hole as the hole size increases, the cavity morphology is complete, and the projectile's movement is more stable at the initial stage of water entry and after deep cavity pinch-off. Special oblique jets form when passing through irregular holes. The impact of the oblique jet on the cavity increases as the rotation degree increases. The type of hole has little effect on the water entry dynamics of the projectile, but has a significant effect on the cavity morphology and the jet motion near the hole. The size of the hole has a great effect on the motion stability of the projectile.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T01:59:53Z
  DOI: 10.1063/5.0146980
Numerical investigation of compressible cryogenic cavitating flows by a
modified mass transport model
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The objectives of this study are to propose exact numerical methods for the compressible cryogenic cavitating flows and investigate the cavitation behaviors and vortex structures. A numerical modeling framework including large eddy simulations, vapor–liquid equations of state, and a modified mass transport model is presented in this paper. The modified transport model is proposed based on the convective heat transfer in which the convective heat transfer coefficient is associated with the material properties and local temperature. To validate the applicability of the modified model, the liquid nitrogen cavitating flows in the inertial and thermal modes (σ ≈ 0.50, Tthroat = 77.24 K and Tthroat = 85.23 K) are simulated, respectively. Meanwhile, the influence of thermodynamic effects on compressibility is investigated. The numerical method is further utilized to visualize the detailed cavity and vortex structures in different cavitating flow patterns (Tthroat ≈ 77 K, σ = 0.58, 0.39, 0.18). The results show that the predicted cavity structures with the modified mass transport model agree better with the corresponding experimental data. For the thermal mode, since the significant thermal effects restrain the development of cavity, the area of the low sound speed region is smaller than that of the inertial model. The value of the minimum sound speed is larger, so that the Mach number in the cavitation region is reduced. Therefore, the compressibility of the liquid nitrogen cavitation in the thermal mode is weaker. For different cavitating flow patterns, the core region of attached cavities near the throat remains stable during an evolutionary cycle. Compared to the attached cavity region, since some hairpin vortices break into many small-scale discrete vortices, the multi-scale effect of vortex distribution is more remarkable in the shedding cavity region.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T01:48:30Z
  DOI: 10.1063/5.0142186
Numerical and experimental study of the effects of tangential to axial
velocity ratio and structural parameters inside the nozzle on spray
characteristics
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Pressure swirl nozzles are widely applied in spray cooling, dust removal, and fuel injection. To better connect the nozzle structure with the internal flow to analyze their influence on spray parameters, this paper designs a nozzle structure and uses experimental measurement and computational fluid dynamics simulation methods to investigate the influence of the nozzle's tangential velocity to axial velocity ratio (vτin/vzin) and the swirl diversion channel eccentric distance (dl) on the spray parameters. A phase Doppler particle analyzer was used in the experiment study to determine the spray axial velocity (vz) and sault mean diameter (D32). In the simulation investigation, the Eulerian multiphase flow model was used to calculate the multiphase flow field of the spray. The results showed that dl and vτin/vzin both have an obviously linear relationship to the peak location (rpeak) of each spray parameter. It means that dl plays similar roles as the vτin/vzin, which can enhance the swirl strength inside the nozzle and increase the spray cone angle. The rpeak of liquid phase volume fraction (αw) and D32 of the droplet particle are always greater than the rpeak of vz. The analysis of the flow field inside the spray orifice indicates that as the vτin/vzin rises, the liquid in the nozzle orifice tends to move farther from the central axis, causing atomization to occur more upstream. This study serves as a reference for the flow analysis and structure design of the pressure swirl nozzle.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T01:46:17Z
  DOI: 10.1063/5.0140753
Effect of nozzle upscaling on coaxial, gas-assisted atomization
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  Authors: Simon Wachter, Thorsten Zirwes, Tobias Jakobs, Nikolaos Zarzalis, Dimosthenis Trimis, Thomas Kolb, Dieter Stapf
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Mass flow scaling of gas-assisted coaxial atomizers from laboratory to industrial scale is of major interest for a wide field of applications. However, there is only scarce knowledge and research concerning the effect of atomizer scale-up on liquid breakup and spray characteristics. The main objective of this study is therefore to derive basic principles for liquid jet breakup using upscaled nozzles to increase the liquid mass flow rate [math]. For that purpose, atomizers with the same geometrical setup but increased sizes have been designed and experimentally investigated for [math], 50, 100, and 500 kg/h, while the aerodynamic Weber number Weaero and gas-to-liquid ratio GLR have been kept constant. The primary jet breakup was recorded via high-speed imaging, and the liquid core length LC and the frequency of the Kelvin–Helmholtz instability fK were extracted. Applying these results as reference data, highly resolved numerical simulations have been performed to gain a deeper understanding of the effect of mass flow scaling. In the case of keeping Weaero and GLR constant, it has been shown by both experiments and simulations that the breakup morphology, given by a pulsating liquid jet with the disintegration of fiber-type liquid fragments, remains almost unchanged with the degree of upscaling n. However, the normalized breakup length [math] has been found to be considerably increased with increasing n. The reason has been shown to be the decreased gas flow velocity vgas at the nozzle exit with n, which leads to a decreased gas-to-liquid momentum flux ratio j and an attenuated momentum exchange between the phases. Accordingly, the calculated turbulence kinetic energy of the gas flow and the specific kinetic energy in the liquid phase decrease with n. This corresponds to a decreased fKHI with n or [math], respectively, which has been confirmed by both experiments and simulations. The same behavior has been shown for two liquids with different viscosities and at different Weaero. The obtained results allow a first-order estimate of the liquid breakup characteristics, where the influence of nozzle upscaling can be incorporated into j and Reliq in terms of n.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T01:43:34Z
  DOI: 10.1063/5.0141156
Experimental study on the lateral migration of a bubble contaminated by
surfactant in a linear shear flow
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Adding a small amount of surfactant to a gas–liquid two-phase flow can markedly change the dynamic behavior of its bubbles. In this study, the lateral motion of a single bubble (deq = 1.99–3.33 mm, Reb = 200–420) contaminated by surfactant and rising in a linear shear flow is experimentally studied. Sodium dodecyl sulfate (SDS) is chosen as the surfactant with concentrations ranging from 10 to 50 ppm. A curved screen is used to generate a stable linear shear flow, and particle image velocimetry is used to measure the quality of the flow field. Bubble motion parameters, including trajectory, aspect ratio, instantaneous velocity, and terminal velocity, are captured using the shadow method with charge-coupled device cameras. The lift coefficient [math] is obtained by a quasi-steady-state analysis. The results show that the presence of surfactant inhibits the lateral migration of bubbles rising in a shear flow and that increasing the SDS concentration and bubble equivalent diameter strengthens this inhibition effect. That is, the [math] and the net lateral migration distance decreased with SDS concentration and bubble equivalent diameter. In addition, the variation trends of the quasi-steady drag coefficient, bubble terminal velocity, and bubble oscillation frequency with bubble equivalent diameter and SDS concentration also were analyzed.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T01:43:32Z
  DOI: 10.1063/5.0140708
A second-order slip/jump boundary condition modified by nonlinear
Rayleigh–Onsager dissipation factor
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  A newly heuristic form of second-order slip/jump boundary conditions (BCs) for the Navier–Stokes–Fourier (NSF) equations is proposed from the viewpoint of generalized hydrodynamic equations (GHE) to extend the capability of the NSF equations for moderately rarefied gas flows. The nonlinear Rayleigh–Onsager dissipation function appearing in the GHE, which contains useful information about the nonequilibrium flow fields of interest, is introduced into the proposed BCs named the simplified generalized hydrodynamic (SGH) BCs as a correction parameter. Compared with the classical Maxwell/Smoluchowski (MS) BCs, the SGH BCs may be more sensitive to capture the nonequilibrium information of flows adaptively and produce physically consistent solutions near the wall. Subsequently, the SGH BCs are implemented in the NSF equations for planar micro-Couette gas flows over a wide range of Knudsen numbers. The results indicate that the SGH BCs make impressive improvements against the MS BCs for diatomic and monatomic gases at the slip region and early transition regime, particularly in terms of capturing precisely the temperature and normal heat flux profiles in the flow and the temperature jump on the wall. More importantly, the SGH BCs conducted in NSF equations with less computational cost still can obtain well-pleased results comparable to the non-Newton–Fourier equations, such as several Burnett-type equations and regularized 13-moment equations, and even perform better than these models near the wall compared with direct simulation Monte Carlo data for the Couette flows to some extent.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T01:43:29Z
  DOI: 10.1063/5.0138433
An exact solution for directional cell movement over Jeffrey slime layer
with surface roughness effects
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  Authors: Zeeshan Asghar, Ahmed Elmoasry, Wasfi Shatanawi, Muhammad Asif Gondal
  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  The role of marine microbes in the aquatic ecosystem is dynamic. The current work explores the fluid mechanics of gliding organisms near a porous boundary. Surface roughness effects are utilized on the lower substrate. The ooze layer between the two-dimensional sheet (micro-swimmers) and the rough substrate is considered a non-Newtonian Jeffrey fluid. The laminar flow of incompressible slime is generated by organism movement. Darcy's law is applied to capture the porous effects. This law is compatible with our study since the laminar flow of slime is driven via bacterial movement. The lubrication assumption is utilized on Navier–Stokes equations. The closed-form solution of a reduced differential equation is calculated. The unknowns present in the boundary conditions are refined by the root-finding algorithm. Finally, the organism speed, flow rate, energy losses, and streamlines are visually represented. These obtained results are elaborated, and key points are mentioned at the end.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T01:40:12Z
  DOI: 10.1063/5.0143053
Electrothermally excited plasma droplet evolution on the laser-patterned
surface
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  Abstract: Physics of Fluids, Volume 35, Issue 4, April 2023.
  Droplet behavior involving electrothermal coupling fields has gradually attracted the attention of researchers, one of which includes electrosurgical scalpels that often contact biofluids. However, the evolution of bio-droplets exposed to the surface of electrosurgical scalpels is not yet well understood. Here, we experimentally studied the effect of different heating temperatures on plasma droplets on the laser-patterned surface (LPS) and the original surface (OS) under defined direct-current (DC) or alternating-current (AC) electric fields. The results show that at a lower heating temperature, the evolution of plasma droplets was dominated by electrolysis. Oxygen bubbles generated on the papillae on the LPS in the DC field inhibited the targeted adsorption of plasma proteins on this surface. In contrast, in the AC field, only a small number of bubbles was generated, which is not sufficient to inhibit protein adsorption, leading to the formation of coagulation on the papillae after heating. At higher heating temperatures, the rapid formation of coagulation resulted in the suppression of electrolysis. The plasma proteins were then transported by the Marangoni flow causing coagulation to reach a thickness of stress mutation. Stress release over the entire coagulation caused its edges to bend and then detach from the papillae. Thus, the LPS exhibited excellent anti-adhesive properties to plasma droplets under electrothermal excitations compared to the OS. This study provides valuable information for understanding the mechanisms of contact behavior between biofluids and electrosurgical scalpels and demonstrates great promise for their anti-adhesive performance.
  Citation: Physics of Fluids
  PubDate: 2023-04-03T01:40:10Z
  DOI: 10.1063/5.0147088
Numerical study of the mechanisms of the longitudinal pulsed detonation in
two-dimensional rotating detonation combustors
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  Abstract: Physics of Fluids, Volume HFDP2022, Issue 1, April 2023.
  A numerical study of the longitudinal pulsed detonation (LPD) is conducted in the present paper. The occurrence mechanism of the LPD, called shock wave amplification by coherent energy release, is verified preliminarily in this study. To be specific, upstream propagating shock waves, which originate from the outlet, induce a specific gradient of reactant distribution, and then detonation waves are ignited and evolve along the gradient in close succession. It is worth noting that the occurrence of LPD does not mean that the LPD will necessarily be sustained. The low injection pressure ratio PR (i.e., the ratio of inlet pressure to outlet pressure) = 1.3 is found to be conducive to the sustenance of the LPD instability in the baseline model. A lower PR (PR ≤ 1.2) or a slightly higher PR (1.4 ≤ PR ≤ 1.8) shall lead to an unstable detonation or quenching of detonations, while a much higher PR (PR > 1.8) contributes to the formation of stable canonical rotating detonation waves. In addition, the combustion regimes of five combustors of different heights at different PR are explored. As the combustion chamber height increases, the PR of the sustainable LPD is nearly linearly increasing, and its operating frequency decreases gradually. The calculation formula between the sustainable LPD propagating frequency and the natural acoustic resonance frequency of the combustor is employed and discussed, but in consideration of its imperfection, further investigation is required.
  Citation: Physics of Fluids
  PubDate: 2023-03-21T11:38:55Z
  DOI: 10.1063/5.0136290@phf.2023.HFDP2022.issue-1
Constitutive modeling of human cornea through fractional calculus approach
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  Authors: Dibyendu Mandal, Himadri Chattopadhyay, Kumaresh Halder
  Abstract: Physics of Fluids, Volume EYES2022, Issue 1, April 2023.
  In this work, the fractional calculus approach is considered for modeling the viscoelastic behavior of human cornea. It is observed that the degree of both elasticity and viscosity is easy to describe in terms of the fractional order parameters in such an approach. Modeling of the human cornea when subjected to simple stress up to the level of 250 MPa by fractional order Maxwell model along with the Fractional Kelvin Voigt Viscoelastic Model is reported. For the Maxwell governing fractional equation, two fractional parameters α and β have been considered to model the stress–strain relationship of the human cornea. The analytical solution of the fractional equation has been obtained for different values of α and β using Laplace transform methods. The effect of the fractional parameter values on the stress-deformation nature has been studied. A comparison between experimental values and calculated values for different fractional order of the Maxwell model equation defines the parameters which depict the real-time stress–strain relationship of the human cornea. It has been observed that the fractional model converges to the classical Maxwell model as a special case for α = β = 1.
  Citation: Physics of Fluids
  PubDate: 2023-03-17T03:04:43Z
  DOI: 10.1063/5.0138730@phf.2023.EYES2022.issue-1
Understanding multi-regime detonation development for hydrogen and syngas
fuels
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  Abstract: Physics of Fluids, Volume HFDP2022, Issue 1, April 2023.
  Autoignition and detonation development are foundational events in the combustion community and are fundamentally relevant to engine knocking and detonation propulsion. Autoignition-induced reaction front propagation modes have been extensively investigated, addressing the role of thermal and concentration inhomogeneities. In this work, we have further investigated the nonmonotonic response of detonation development to temperature gradients for low-carbon fuels (hydrogen and syngas) and have found additional detonation regimes, which can depict the panorama of reaction front propagation modes. Results show that separate detonation regimes can be observed when hotspot sizes are below some critical thresholds, with the first corresponding to the known “Bradley detonation peninsula” and the second newly identified featuring broader detonation regions. Despite this, distinct combustion characteristics are observed in the demarcation of detonation regimes between hydrogen and syngas fuels. Specifically, the upper branch of the first detonation regimes for hydrogen is sensitive to temperature gradients at various hotspot sizes, while it exhibits similar behaviors in the lower branch of the second one for syngas, which results in narrower detonation regions. Meanwhile, hydrogen possesses a larger critical hotspot size compared to syngas, and the underlying mechanism is ascribed to the chemical reactivity when hotspot autoignition and the difference of energy density between hotspot interior and exterior. Finally, various detonation regimes are summarized in dimensionless detonation diagrams, in which hydrogen and syngas show similar distributions of detonation peninsula. Despite this, those distinctions in the detonation characteristics between hydrogen and syngas can still be manifested quantitatively. The current work can provide useful insights into knocking inhabitation and detonation promotion.
  Citation: Physics of Fluids
  PubDate: 2023-03-09T12:28:16Z
  DOI: 10.1063/5.0139872@phf.2023.HFDP2022.issue-1
Analysis of pressure oscillations and wall heat flux due to hydrogen
auto-ignition in a confined domain
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  Authors: Xinbei Dou, Mohsen Talei, Yi Yang
  Abstract: Physics of Fluids, Volume HFDP2022, Issue 1, April 2023.
  This study investigates the impact of the near-wall temperature gradient on hydrogen auto-ignition characteristics using one-dimensional (1D) fully resolved simulations. Ten cases are simulated, one featuring normal combustion and the other nine simulating auto-ignitive combustion with different initial pressures, equivalence ratios, and near-wall temperature gradients. The simulations show that the near-wall temperature gradient greatly affects the onset and intensity of the auto-ignition event. For cases with the initial conditions of 833.3 K and 15 bar, a small near-wall temperature gradient delays the timing of auto-ignition and places the auto-ignition kernel further away from the wall, facilitating deflagration-to-detonation transition of the auto-ignitive flame. This leads to a large increase in pressure oscillations within the domain and heat flux to the wall. When the initial conditions are changed to 900 K and 20 bar, the magnitude of the near-wall temperature gradient also affects the number of auto-ignition events, leading to a significant impact on the wall heat flux. The results suggest that an accurate modeling of the near-wall temperature gradient is necessary for the simulations of hydrogen end-gas auto-ignition. This requires special considerations in the near-wall region and a careful selection of the wall heat transfer model in Computational Fluid Dynamics (CFD) tools, such as Reynolds-Averaged Navier–Stokes (RANS) and Large-Eddy Simulation (LES).
  Citation: Physics of Fluids
  PubDate: 2023-01-13T12:06:51Z
  DOI: 10.1063/5.0133045@phf.2023.HFDP2022.issue-1
Numerical investigation of wall effects on combustion noise from a
lean-premixed hydrogen/air low-swirl flame
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  Authors: Abhishek Lakshman Pillai
  Abstract: Physics of Fluids, Volume HFDP2022, Issue 1, April 2023.
  A hybrid computational fluid dynamics (CFD)/computational aero-acoustics (CAA) approach, in which large-eddy simulation (LES) and APE-RF (solution of the acoustic perturbation equations for reacting flows) are employed for the CFD and CAA, respectively, calling it the hybrid LES/APE-RF approach, is used to analyze the influence of a wall on the combustion noise from a lean-premixed gaseous hydrogen/air low-swirl turbulent jet flame. The wall boundary conditions pertaining to the APE-RF system are formulated to account for acoustic reflection from the wall. The results show that the sound pressure level (SPL) spectrum obtained from the LES/APE-RF is in good agreement with that measured in the experiment. In the LES/APE-RF, the SPL spectrum of combustion noise with the wall plate explicitly changes compared to that without the wall plate. Specifically, the presence of the wall plate tends to ease the peaks that appeared in the case without the wall plate and create a nearly constant SPL within a specific frequency band. The analysis of the heat release rate fluctuation reveals that these phenomena are caused by the absence of a single periodic oscillation of heat release rate. The presence of the wall plate creates an asymmetric flow around the flame and distorts the flame structure, thereby altering the flame fluctuation phenomena.
  Citation: Physics of Fluids
  PubDate: 2023-01-12T12:43:47Z
  DOI: 10.1063/5.0131974@phf.2023.HFDP2022.issue-1