Publisher: AIP (Total: 27 journals) [Sort by number of followers]

Physics of Fluids
Journal Prestige (SJR): 1.19 Citation Impact (citeScore): 3 Number of Followers: 37 Hybrid journal (It can contain Open Access articles) ISSN (Print) 10706631  ISSN (Online) 10897666 Published by AIP [27 journals] 
 Two touching/selfassembly droplets in uniform Stokes flow: Viscous energy
dissipation of the flow in droplets Authors: Kui Song, Zheng Zhou
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
Viscous energy dissipation of the flow in two touching or selfassembly droplets in uniform Stokes flow is investigated in this paper. Based on the Stokes solution, the energy dissipation per unit time of the two droplets is calculated and validated by comparing with the result of one droplet Stokes flow, and then a theoretical model to calculate the energy dissipation is established. The investigation reveals that the energy dissipation per unit time of either droplet increases with the increasing droplet viscosity at constant continuous fluid viscosity and reaches a peak value when the two viscosities are equal. Moreover, the energy dissipation per unit time of either droplet changes with the sizes of both droplets. The total energy dissipation per unit time of the two droplets is less than the sum of the energy dissipation per unit time of the two droplets before their contact or selfassembly, and in particular, it reaches the minimum value which is about 1/6 of the result of one droplet flow when the two droplets’ sizes are equal. Two droplets’ contact or selfassembly will minimize the energy dissipation of droplets, so it can save energy for the flow system. This study proposes a new perspective for droplet selfassembly study and can promote droplet collision and coalescence studies and then bring benefits to relevant applications.
Citation: Physics of Fluids
PubDate: 20190118T07:31:11Z
DOI: 10.1063/1.5063659
 Authors: Kui Song, Zheng Zhou
 Selective upstream influence on the unsteadiness of a separated turbulent
compression ramp flow Authors: Kevin M. Porter, Jonathan Poggie
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
A study of the shockwave/boundarylayer interaction induced by a compression ramp was carried out using highfidelity simulations. The objective was to investigate the influence of upstream disturbances on lowfrequency separation unsteadiness. Two computations were performed for a 24° compression ramp at Mach 2.25, one highly resolved case and one reducedresolution case. The reducedresolution case was run for an extended duration to capture many cycles of lowfrequency unsteadiness. Basic flow characteristics, including statistics on the boundary layer, wall pressure, and skin friction, were computed. Frequency spectra were calculated to confirm the presence of lowfrequency unsteadiness. The influence of upstream disturbances on largescale separation unsteadiness was investigated using correlations, filtering, and conditional averaging based on the position of the primary separation shock. Lowfrequency unsteadiness was found to be related to structures near the wall (y/δ < 0.5) with a time scale greater than 20δ/U∞, whereas higher frequency separation motion could be attributed to turbulent boundary layer structures with a time scale on the order of δ/U∞. The finding that the separation region responds selectively to certain largescale, nearwall perturbations in the incoming flow supports a model of separation unsteadiness in which external forcing by certain components of boundary layer turbulence drives a weakly damped global mode of the separation bubble. This contrasts with suggestions that have been made in the literature that the separation region oscillates on its own, as in an amplified global mode.
Citation: Physics of Fluids
PubDate: 20190117T05:45:49Z
DOI: 10.1063/1.5078938
 Authors: Kevin M. Porter, Jonathan Poggie
 Jet formation during the gas penetration through a thin liquid layer
 Authors: Mingbo Li, Liang Hu, Hanghang Xu, Wenyu Chen, Haibo Xie, Xin Fu
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
A free bubble reaching the liquid surface usually bursts and then forms a liquid jet with drops ejected. While bubblemediated jetting is a topic widely studied, few investigations deal with the jet produced by a growing bubble. Here, we report and characterize a novel phenomenon, named periodic bubblingbursting, that can develop when a continuous stream of gas penetrates through a thin liquid layer. This behavior is complex with a characteristic frequency and can be divided into three stages from bubbling to cavity collapse and jetting. We show that increasing the liquid layer thickness and gas velocity leads to a larger bubble. However, the effect is strongly coupled with the orifice diameter and a scaling law of the bubble rupture radius is derived. Subsequently, we demonstrate that the collapsing cavities exhibit shape similarity and deduce the dependence of pinchoff height and opening angle of the conical cavity on the bubble rupture radius and liquid layer thickness. This enables us to disentangle three different neckpinching mechanisms at play in pinchoff. Accordingly, gravity shapes the cavity and participates in the capillary wave selection that strongly modulates the jet formation. With increasing layer thickness, the jet first becomes fat and small and then ends up thinner and higher, detaching more and smaller droplets. We present a simple scaling law for the jet velocity which involves the liquid layer thickness to the power 1/2. Finally, a phase diagram for jet breakup and no breakup is built with respect to the initial Weber and Bond numbers.
Citation: Physics of Fluids
PubDate: 20190117T05:45:46Z
DOI: 10.1063/1.5066593
 Authors: Mingbo Li, Liang Hu, Hanghang Xu, Wenyu Chen, Haibo Xie, Xin Fu
 Aerodynamic forces on projectiles used in various sports
 Authors: Kunjal Shah, Ravi Shakya, Sanjay Mittal
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
The aerodynamics of projectiles used in various sports is investigated via experiments in a low speed wind tunnel. Force measurements are carried out on actual artifacts at speeds in the range of 1575 m/s. The sports considered include golf, field hockey, soccer, baseball, tennis, cricket, volleyball, and badminton. Both synthetic and duckfeather models of shuttlecocks used in badminton are considered. The variation of the coefficient of drag, CD, with Reynolds number, Re, is quite different for the two models. The deformation of the synthetic model increases significantly with an increase in speed, leading to a decrease in CD with an increase in Re. The duckfeather model, on the other hand, does not undergo such severe deformations. Force measurements for a baseball are carried out for three different orientations of its seam with the freestream flow. Variation of CD with Re for two internationally approved brands of golf balls is presented for the first time in the open literature. The data are compared with those for a ball used in field hockey, which also has dimples on its surface, albeit of different sizes and distributions. Force measurements are carried out on a new cricket ball as well as one whose surface is manually roughened to resemble a ball that has been in play for about 40 overs (=240 deliveries). The study brings out the regimes of conventional and reverseswing and their dependence on the surface roughness of the ball. Experiments on balls with differential roughness of the two hemispheres of the ball are utilized to study the “contrastswing.” Particle Image Velocimetry measurements are carried out for the 3Dprinted model of a new cricket ball to explore the phenomena of conventional and reverseswing. Experiments on a tennis ball bring out the role of the fuzz in the transition of the boundary layer on its surface; a nearconstant CD for the entire range of Re that is studied is observed. The brands of a soccer ball and volleyball that are tested exhibit very similar behaviour. In the supercritical regime, an increase in CD is followed by its decrease with an increase in Re.
Citation: Physics of Fluids
PubDate: 20190117T05:40:50Z
DOI: 10.1063/1.5064700
 Authors: Kunjal Shah, Ravi Shakya, Sanjay Mittal
 Adaptivepassive control of flow over a sphere for drag reduction
 Authors: Seokbong Chae, Seungcheol Lee, Jooha Kim, Jae Hwa Lee
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
A new adaptivepassive control device is introduced to optimally reduce the drag on a sphere over a wide range of Reynolds numbers, Re = 0.4 × 105–4.4 × 105. The device, called an adaptive moving ring (AMR), is designed to change its size (i.e., protrusion height) adaptively depending on the wind speed (i.e., the Reynolds number) without energy input. An empirical model is formulated to accurately predict the drag coefficient as a function of the size of AMR and the Reynolds number. Based on the model, we estimate how the optimal size of AMR should vary with the Reynolds number to maximize the drag reduction. Following the estimation of the optimal size, the optimally tuned AMR reduces its protrusion height with increasing Reynolds number, and the drag decreases monotonically by up to 74% compared to that of a smooth sphere. The drag reduction by AMR is attributed to different mechanisms depending on the Reynolds number. For low Reynolds numbers, the locally separated flow at large AMR is energized by the disturbance induced by AMR and reattaches to the sphere surface, forming a large recirculation region. Then, the main separation is delayed downstream due to the increased nearwall momentum. On the other hand, at high Reynolds numbers, no recirculation zone is formed at AMR due to its low protrusion height, but a secondary separation bubble is generated on the rear sphere surface. Therefore, the boundarylayer flow becomes turbulent, and the main separation is significantly delayed, resulting in more drag reduction than for low Reynolds numbers.
Citation: Physics of Fluids
PubDate: 20190117T05:40:47Z
DOI: 10.1063/1.5063908
 Authors: Seokbong Chae, Seungcheol Lee, Jooha Kim, Jae Hwa Lee
 Effect of yaw angle on flow structure and crossflow force around a
circular cylinder Authors: Ran Wang, Shaohong Cheng, David SK. Ting
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
Flow around an inclined circular cylinder at yaw angles of α = 0°, 30°, 45°, and 60° has been numerically studied using the delayed detached eddy simulation at a Reynolds number of 1.4 × 104. Periodic boundary conditions are utilized to minimize the end effect. The focus is to explore the effect of yaw angle on the flow structure and the spatial distribution of the crossflow forces. For the normal flow case, the modulation of the spanwise averaged lift force coefficient is found to be related to the unstable shear layer. For the inclined cases, contours of the sectional lift force coefficient show that the local vortex shedding staggers in time along the axial span at the early stage of the simulation, when the flow approaches the cylinder. After the flow reaches the quasiperiodic state, the axial difference disappears for α> 45° but not for α = 30°. In particular, the axial difference of the sectional lift force coefficient results in a nearzero value of the spanwise averaged lift force coefficient. The transition from a twodimensional flow to a threedimensional one is not captured in the current simulation. However, wake visualization indicates a mitigation of von Kármán vortex shedding when the yaw angle is greater than 30°. Although the Strouhal number is well predicted by the Independence Principle (IP), other flow properties are less agreeable with the prediction by IP.
Citation: Physics of Fluids
PubDate: 20190117T05:40:45Z
DOI: 10.1063/1.5079750
 Authors: Ran Wang, Shaohong Cheng, David SK. Ting
 Nonlinear interaction of thermogravitational waves and thermomagnetic
 Authors: Pinkee Dey, Sergey A. Suslov
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
The interacting thermogravitational and thermomagnetic instabilities arising in a vertical layer of nonisothermal ferrofluid placed in a horizontal magnetic field are investigated by means of a weakly nonlinear analysis. An expansion in disturbance amplitude leads to the reduction of a full problem to a system of coupled cubic Landau amplitude equations. Their solutions are analyzed and interpreted from a physical point of view. The details of an intricate competition between gravitational and magnetic buoyancy mechanisms are highlighted. The spatial structure of the resulting flow patterns is discussed. It is shown that the parametric existence regions determined for finite amplitude disturbances differ drastically from those predicted based on the analysis of infinitesimal perturbations. Subsequently, the crosslayer heat flux characteristics are discussed. It is shown that the coexistence of two physical mechanisms of convection can lead to a suppression of heat transfer rather than to its enhancement.
Citation: Physics of Fluids
PubDate: 20190117T05:31:05Z
DOI: 10.1063/1.5070092
 Authors: Pinkee Dey, Sergey A. Suslov
 Instability onset for submerged cylinders
 Authors: Leo M. GonzálezGutierrez, Juan M. Gimenez, Esteban Ferrer
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
This paper describes how the global stability of a circular cylinder is affected when submerged in a twophase gravitational flow. The flow behavior is governed by both the Reynolds and the Froude number, while the depth of the cylinder has been varied to create different scenarios for the stability analysis. The baseflow obtained by the numerical solution of the 2D NavierStokes equations has been analyzed, and the first bifurcation (i.e., Hopf type) has been explored for different depths, Reynolds numbers, and Froude numbers. In addition to the typical vortex shedding instabilities associated with the isolated cylinders, the presence of an interface between fluids creates new instabilities associated with the free surface which present more complex and deformed structures. According to the region of the parameter space studied here, two main causes of instabilities have been found: the ones provoked by vortex shedding on the cylinder wake (wake instabilities) at low Froude numbers and the ones produced by the free surface deformation (free surface instabilities) at high Froude numbers. When instabilities are related to vortex shedding, the critical Reynolds number and the frequency of the most unstable mode are comparable to the classical solution without free surface and gravity effects. In all cases, the shape of the most unstable mode is deformed and distorted according to the free surface location, while the critical Reynolds numbers and the frequency associated with the perturbation are both affected by the gravity and the free surface presence.
Citation: Physics of Fluids
PubDate: 20190117T05:31:05Z
DOI: 10.1063/1.5063327
 Authors: Leo M. GonzálezGutierrez, Juan M. Gimenez, Esteban Ferrer
 Lattice Boltzmann simulation of doublediffusive natural convection of
viscoplastic fluids in a porous cavity Authors: Gholamreza Kefayati
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
In this paper, a twodimensional double diffusive natural convection in a porous cavity filled with viscoplastic fluids is simulated. The dimensional and nondimensional macroscopic equations are presented, employing the Papanastasiou model for viscoplastic fluids and the Darcy–Brinkman–Forchheimer model for porous media. An innovative approach based on a modification of the lattice Boltzmann method is explained and validated with previous studies. The effects of the pertinent dimensionless parameters are studied in different ranges. The extensive results of streamlines, isotherms, and isoconcentration contours, yielded/unyielded regions, and local and average Nusselt and Sherwood numbers are presented and discussed.
Citation: Physics of Fluids
PubDate: 20190117T05:25:24Z
DOI: 10.1063/1.5074089
 Authors: Gholamreza Kefayati
 Machine learning methods for turbulence modeling in subsonic flows around
airfoils Authors: Linyang Zhu, Weiwei Zhang, Jiaqing Kou, Yilang Liu
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
In recent years, the datadriven turbulence model has attracted widespread concern in fluid mechanics. The existing approaches modify or supplement the original turbulence model by machine learning based on the experimental/numerical data, in order to augment the capability of the present turbulence models. Different from the previous researches, this paper directly reconstructs a mapping function between the turbulent eddy viscosity and the mean flow variables by neural networks and completely replaces the original partial differential equation model. On the other hand, compared with the machine learning models for the low Reynolds (Re) number flows based on direct numerical simulation data, high Reynolds number flows around airfoils present the apparent scaling effects and strong anisotropy, which induce large challenges in accuracy and generalization capability for the machine learning algorithm. We mainly concentrate on the high Reynolds number turbulent flows around the airfoils and take the results calculated by the computational fluid dynamics solver with the SpallartAllmaras (SA) model as training data to construct a highdimensional datadriven network model based on machine learning. The radial basis function neural network and the auxiliary optimization methods are adopted, and the individual models are built separately for the flow fields of the nearwall region, wake region, and farfield region. The training data in this paper is extracted from only three subsonic flow fields of NACA0012 airfoil. The datadriven turbulence model can be applied to various airfoils and flow states, and the predicted eddy viscosity, lift/drag coefficients, and skin friction distributions are all in good agreement with the results of the original SA model. This research demonstrates the promising prospect of machine learning methods in future studies about turbulence modeling.
Citation: Physics of Fluids
PubDate: 20190116T06:34:13Z
DOI: 10.1063/1.5061693
 Authors: Linyang Zhu, Weiwei Zhang, Jiaqing Kou, Yilang Liu
 Drag coefficient and formation length at the onset of vortex shedding
 Authors: Gaurav Chopra, Sanjay Mittal
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
The flow past a circular cylinder at a low Reynolds number (40 ≤ Re ≤ 180) is investigated. A stabilized finite element method is utilized to solve the incompressible flow equations in twodimensions. The critical Re for the onset of vortex shedding (Rec) is estimated to be 46.985. The variation of timeaveraged coefficient of drag ([math]) with Re is found to be nonmonotonic for Re> Rec. Unlike for the steady flow, the pressure component of [math] increases with an increase in Re in a short range of Re for Re> Rec. This increase is due to a significant rise in the peak suction, near the shoulder of the cylinder, of the timeaveraged flow, with Re. Several definitions of vortex formation length (Lf), proposed in the past, are reviewed and compared. A new definition, based on the fluctuation in the local kinetic energy of the flow, is proposed. The variation of Lf with Re is compared with Lw, the separation bubble length. Lf is found to be significantly larger than Lw for Re close to Rec. The difference between the two lengths decreases with an increase in Re. The meaning of Lf, in terms of flow physics, is explored. It is found that the vortices form in the near wake, even for Re close to Rec. They become stronger as they convect downstream and gain full strength at a location Lf downstream of the cylinder, beyond which they begin to decay.
Citation: Physics of Fluids
PubDate: 20190116T06:31:37Z
DOI: 10.1063/1.5075610
 Authors: Gaurav Chopra, Sanjay Mittal
 Evaluating interfacial shear and strain stress during droplet deformation
in micropores Authors: Tobias Wollborn, Laura Luhede, Udo Fritsching
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
The formulation of high quality emulsions is a key challenge in many industrial applications. The premix emulsification process in porous membranes enables the generation of tailored emulsions with fine and narrow droplet size distributions under low shear and energy input. However, the droplet deformation and breakup process within porous structures is a complex mechanism and single breakup events are hard to relate to the local stress conditions and the pore geometry. This relation however is required for the proper design of membrane structures with specific emulsification behavior (i.e., avoidance of stress peaks). Thus, in this contribution, the stress residence time behavior of single droplets during deformation and breakup in idealized micropores is investigated for different Capillary numbers and droplet sizes. The interface stress induced droplet deformation and breakup process is to be analyzed in a generic flow configuration. The results show that interface stresses are applied by the wall interface (walldroplet interface) and by the liquidliquid (continuousdroplet interface) interface and that both stress contributions have to be considered separately in order to understand the droplet deformation and breakup process. Only at the liquidliquid interface, stress induced deformation is possible. The analysis of the stress conditions delivers a correlation between the stress residence time behavior and the interface deformation, which can be directly related to the pore geometry. As a result, main deformation and breakup trends are derived. This enables better opportunities for proper membrane design and handling of shear sensitive media in the premix emulsification process.
Citation: Physics of Fluids
PubDate: 20190116T06:31:35Z
DOI: 10.1063/1.5064858
 Authors: Tobias Wollborn, Laura Luhede, Udo Fritsching
 On the closure problem of the effective stress in the EulerianEulerian
and mixture modeling approaches for the simulation of liquidparticle
suspensions Authors: Rashid Jamshidi, Panagiota Angeli, Luca Mazzei
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
We address the closure problem of the phasic effective stress tensors in the EulerianEulerian and mixture models, considering suspensions of identical particles dispersed in Newtonian liquids. First, after briefly describing the modeling approaches, we review the key mechanisms generating phasic stress and discuss the shortcomings of some constitutive expressions in reproducing important experimental observations. For dilute suspensions, these include the mixture viscosity rise with solid concentration whilst for dense suspensions, the occurrence of particle migration and the change of mixture rheology from Newtonian to nonNewtonian. We then use computational fluid dynamics simulations to compare results based on various stress tensor closures. In a first case study, the simulation results of a laminar flow in a horizontal pipe of a dilute suspension of particles dispersed in a Newtonian liquid are compared to experimental data obtained from the literature. We show that both the EulerianEulerian and mixture models can predict pressure drops accurately but only if they are coupled with suitable experimental closures for the mixture rheology. In a second case study, we simulate the laminar flow of a dense suspension of identical particles dispersed in a Newtonian liquid through an abrupt expansion. We show that the particle concentration profile in the upstream tube, which develops owing to shearinduced particle migration, strongly affects the flow patterns downstream of the expansion. This migration must be modeled via an appropriate closure for the solid effective stress tensor; this allows capturing the sophisticated flow patterns in the expansion section.
Citation: Physics of Fluids
PubDate: 20190116T06:31:33Z
DOI: 10.1063/1.5081677
 Authors: Rashid Jamshidi, Panagiota Angeli, Luca Mazzei
 Prevention of network destruction of partially hydrolyzed polyacrylamide
(HPAM): Effects of salt, temperature, and fumed silica nanoparticles Authors: Ehsan Aliabadian, Milad Kamkar, Zhangxin Chen, Uttandaraman Sundararaj
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
Polymer flooding is one of the most effective enhanced oil recovery (EOR) methods. High temperature and a high salt content in oil reservoirs significantly decrease the performance of polymer flooding. In this work, the viscoelastic properties of a partially hydrolyzed polyacrylamide (HPAM) solution with and without salt (NaCl) and at two different temperatures (35 °C and 70 °C) were evaluated using rheological approaches. Two fumed silica nanoparticles (NPs) featuring different surface chemistries were used, and their ability to prevent destruction of the polymer network structure against salt addition and temperature increase was investigated. Linear rheological tests (frequency sweep, creep, and creep recovery) and nonlinear rheological tests (large amplitude oscillatory shear) were employed to evaluate the network structure of these systems. The results showed that either adding salt or increasing the temperature destroyed the mechanical integrity of the HPAM 3dimensional elastic network. However, the introduction of both types of NPs at a sufficient concentration maintained the network structure of HPAM solutions in the small deformation region. In the large deformation region, it was shown that the extent of intracycle shearthickening behavior in the HPAM solution (T = 35 °C and without any salt) decreased by incorporating salt or by increasing the temperature. Moreover, upon incorporating either of the NPs to the HPAM solution, the nonlinear viscoelastic behavior dramatically changed, and the critical strain (linear to nonlinear transition) decreased to a much lower strain amplitude. The outcomes of this study will help petroleum scientists to design more efficient EOR methods.
Citation: Physics of Fluids
PubDate: 20190116T06:31:29Z
DOI: 10.1063/1.5080100
 Authors: Ehsan Aliabadian, Milad Kamkar, Zhangxin Chen, Uttandaraman Sundararaj
 Vortex shedding from a circular cylinder in shearthinning Carreau fluids
 Authors: Shantanu Bailoor, JungHee Seo, Rajat Mittal
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
Results from numerical simulations of twodimensional, shearthinning Carreau fluid flow over an unconfined circular cylinder are presented in this paper. Parametric sweeps are performed over the various Carreau model parameters, and trends of the timeaveraged force coefficients and vortex characteristics are reported. In general, increased shearthinning results in lower viscous forces on the body but greater pressure forces, resulting in a complex nonmonotonic drag response. Lift forces generally increased with shearthinning due to the dominant pressure contribution. The decrease in fluid viscosity also led to shorter vortex formation lengths and the consequent rise in the Strouhal frequency of vortex shedding. It is expected that these results will be useful for verification of computational models of unsteady nonNewtonian flows.
Citation: Physics of Fluids
PubDate: 20190116T06:17:49Z
DOI: 10.1063/1.5086032
 Authors: Shantanu Bailoor, JungHee Seo, Rajat Mittal
 Twodimensionalization of a threedimensional bluff body wake
 Authors: LiHao Feng, GuoPeng Cui, LiYang Liu
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
The threedimensional flow characteristics of a circular cylinder with synthetic jet control are numerically studied using large eddy simulation. The Reynolds number based on the diameter of the cylinder is Re = 500. The control effects and underlying mechanism are revealed to show how the synthetic jet changes the threedimensional wake pattern. Analysis of the dynamic control process indicates that the blowing stroke helps the shear layer to assemble vorticity, and then, the suction stroke accelerates the detachment of the concentrated vorticity. The vortex shedding process will be gradually dominated by symmetric actuation of the synthetic jets. Thus, the asymmetric vortex shedding mode could be changed into a symmetric mode several periods after actuation at certain excitation frequencies, leading to significant suppression of lift fluctuations. A periodic pressure variation at the leeward surface of the circular cylinder caused by the changes of the separation point for the flow over a circular cylinder and recirculation region results in a large drag fluctuation. The excitation phase influences only the control process, but not the final state, while the excitation frequency plays an important role in the formation of different wake patterns. It is also found that the synthetic jet can completely suppress the formation of streamwise vortices due to the threedimensional instability suppression and reduce the deformation of spanwise vortices, resulting in a conversion of the original threedimensional flow into a twodimensional one. Such twodimensionalization can be achieved for both asymmetric and symmetric wake patterns, indicating that it is not influenced by the excitation phase and frequency as long as the actuation is twodimensional.
Citation: Physics of Fluids
PubDate: 20190115T04:11:43Z
DOI: 10.1063/1.5066422
 Authors: LiHao Feng, GuoPeng Cui, LiYang Liu
 RayleighTaylor instability of a miscible interface in a confined domain
 Authors: T. Lyubimova, A. Vorobev, S. Prokopev
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
On the basis of the phasefield approach, we investigate the simultaneous diffusive and convective evolution of an isothermal binary mixture of two slowly miscible liquids that are confined in a horizontal plane layer. We assume that two miscible liquids are brought into contact, so the binary system is thermodynamically unstable and the heavier liquid is placed on top of the lighter liquid, so the system is gravitationally unstable. Our model takes into account the nonFickian nature of the interfacial diffusion and the dynamic interfacial stresses at a boundary separating two miscible liquids. The numerical results demonstrate that the classical growth rates that characterise the initial development of the RayleighTaylor instability can be retrieved in the limit of the higher Peclet numbers (weaker diffusion) and thinner interfaces. The further nonlinear development of the RayleighTaylor instability, characterised, e.g., by the size of the mixing zone, is however limited by the height of the plane layer. On a longer time scale, the binary system approaches the state of thermodynamic and hydrodynamic equilibrium. In addition, a novel effect is found. It is commonly accepted that the interface between the miscible liquids slowly smears in time due to diffusion. We however found that when the binary system is subject to hydrodynamic transformations the interface boundary stretches, so its thickness changes (the interface becomes thinner) on a much faster convective time scale. The thickness of the interface is inversely proportional to the surface tension, and the stronger surface tension limits the development of the RayleighTaylor instability.
Citation: Physics of Fluids
PubDate: 20190115T04:11:40Z
DOI: 10.1063/1.5064547
 Authors: T. Lyubimova, A. Vorobev, S. Prokopev
 A versatile lattice Boltzmann model for immiscible ternary fluid flows
 Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
We propose a lattice Boltzmann colorgradient model for immiscible ternary fluid flows, which is applicable to the fluids with a full range of interfacial tensions, especially in nearcritical and critical states. An interfacial force for Nphase systems is derived and then introduced into the model using a body force scheme, which helps reduce spurious velocities. A generalized recoloring algorithm is applied to produce phase segregation and ensure immiscibility of three different fluids, where an enhanced form of segregation parameters is derived by considering the existence of Neumann’s triangle and the effect of the equilibrium contact angle in a threephase junction. The proposed model is first validated by two typical examples, namely, the YoungLaplace test for a compound droplet and the spreading of a droplet between two stratified fluids. It is then used to study the structure and stability of double droplets in a static matrix. Consistent with the theoretical stability diagram, seven possible equilibrium morphologies are successfully reproduced by adjusting the interfacial tension ratio. By simulating nearcritical and critical states of double droplets where the outcomes are very sensitive to the model accuracy, we show that the present model is advantageous to threephase flow simulations and allows for accurate simulation of nearcritical and critical states. Finally, we investigate the influence of interfacial tension ratio on the behavior of a compound droplet in a threedimensional shear flow, and four different deformation and breakup modes are observed.
Citation: Physics of Fluids
PubDate: 20190115T04:11:37Z
DOI: 10.1063/1.5056765
 Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
 Effect of the Reynolds number on turbulence kinetic energy exchanges in
flows with highly variable fluid properties Authors: D. Dupuy, A. Toutant, F. Bataille
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
Spatial and spectral energy exchanges associated with the turbulence kinetic energy per unit mass, or the halftrace of the velocity covariance tensor, are studied in an anisothermal low Mach number turbulent channel flow. The temperatures of the two channel walls are 293 K and 586 K. This generates a strong temperature gradient in the wallnormal direction. The effect of the temperature gradient on the energy exchanges is investigated using two direct numerical simulations of the channel, at the mean friction Reynolds numbers 180 and 395. The temperature gradient creates an asymmetry between the energy exchanges at the hot and cold sides due to the variations of the local fluid properties and low Reynolds number effects. The low Reynolds number effects are smaller at higher Reynolds numbers, reducing the asymmetry between the hot and cold sides. We also decomposed the energy exchanges in order to study separately the meanproperty terms, as found in the constantproperty isothermal case, and the thermal terms, specific to flows with variable fluid properties. The significant thermal terms have a similar effect on the flow. Besides, low Reynolds number effects have a negligible impact on thermal terms and only affect meanproperty terms.
Citation: Physics of Fluids
PubDate: 20190114T06:08:29Z
DOI: 10.1063/1.5080769
 Authors: D. Dupuy, A. Toutant, F. Bataille
 A simple technique to achieve meniscusfree interfaces
 Authors: PeiHsun Tsai, Tetuko Kurniawan, AnBang Wang
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
Liquid wetting on a container wall forms a meniscus that causes the reading uncertainty of interface measurement, which was considered as an “inevitable” interference in experiments. For minimizing the meniscus interference, the dynamic instead of the static contact angle should be focused on and θr ≤ 90° ± ε ≤ θa is the guideline to achieve a meniscusfree interface for a practical experiment, where θr, θa, and ε are the receding and advancing contact angles, and image measuring uncertainty, respectively. A simple and systematic technique to achieve the meniscusfree interface has been proposed and visualized.
Citation: Physics of Fluids
PubDate: 20190114T06:08:27Z
DOI: 10.1063/1.5080659
 Authors: PeiHsun Tsai, Tetuko Kurniawan, AnBang Wang
 Smoothed particle hydrodynamics (SPH) for complex fluid flows: Recent
developments in methodology and applications Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
Computer modeling of complex fluid flows usually presents great challenges for conventional gridbased numerical methods. Smoothed particle hydrodynamics (SPH) is a meshfree Lagrangian particle method and has special advantages in modeling complex fluid flows, especially those with large fluid deformations, fluidstructure interactions, and multiscale physics. In this paper, we review the recent developments of SPH in methodology and applications for modeling complex fluid flows. Specifically, in methodology, some important issues including modified SPH particle approximation schemes for improving discretization accuracy, different particle regularization techniques, and various boundary treatment algorithms for solid boundary, free surface, or multiphase interface are described. More importantly, the SPH method with ideas from the dissipative particle dynamics for complex fluids in macro or mesoscales is discussed. In applications, different complex fluid flows, including biological flows, microfluidics and droplet dynamics, nonNewtonian fluid flows, free surface flows, multiphase flows, and flows with fluidstructure interaction, are reviewed. Some concluding remarks in SPH modeling of complex fluid flows are provided.
Citation: Physics of Fluids
PubDate: 20190110T09:28:36Z
DOI: 10.1063/1.5068697
 Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
 Void collapse generated mesoscale energy localization in shocked
energetic materials: Nondimensional parameters, regimes, and criticality
of hotspots Authors: N. K. Rai, H. S. Udaykumar
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
The formation of hotspots due to collapse of voids leads to enhanced sensitivity of heterogeneous energetic (HE) materials. Several mechanisms of void collapse have been identified, but the regimes in which these mechanisms dominate have not been clearly delineated using scaling arguments and dimensionless parameters. This paper examines void collapse in cyclotetramethylenetetranitramine (HMX) to demarcate regimes where plastic collapse and hydrodynamic jetting play dominant roles in influencing hotspot related sensitivity. Using scaling arguments, a criticality envelope for HMX is derived in the form [math], i.e., as a function of shock pressure Ps and void size Dvoid, which are controllable design parameters. Once a critical hotspot forms, its subsequent growth displays a complex relationship to Ps and Dvoid. These complexities are explained with scaling arguments that clarify the physical mechanisms that predominate in various regimes of hotspot formation. The insights and scaling laws obtained can be useful in the design of HE materials.
Citation: Physics of Fluids
PubDate: 20190110T05:41:46Z
DOI: 10.1063/1.5067270
 Authors: N. K. Rai, H. S. Udaykumar
 Decaying compressible turbulence with thermal nonequilibrium
 Authors: Sualeh Khurshid, Diego A. Donzis
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
The interaction of decaying turbulence with thermal nonequilibrium (TNE) is studied using direct numerical simulations. The focus is on energy exchanges and decay rates in decaying flows with initial vibrational excitation. A key finding is the identification of different regimes in the interaction and the nondimensional parameter (β) that controls it. The latter accounts for the degree of initial TNE as well as the ratio of timescales of turbulence and vibrational relaxation. For β < 1, TNE is essentially frozen and turbulence is largely unaffected by the decay of vibrational energy. For β> 1, TNE relaxation is relatively fast and produces an increase in translational–rotational energy, which, through changes in transport coefficients, leads to a temporary increase in dissipation leading to faster turbulence decay rates. Theoretical arguments are put forth to determine the asymptotic limits of this effect. TNE relaxation is also affected by turbulent fluctuations in unexpected ways. For example, although initial conditions are always vibrationally hot, the flow may undergo vibrationally cold transients, which are explained through simple models. The results presented here help explain disagreement between previous experimental and numerical data.
Citation: Physics of Fluids
PubDate: 20190110T05:41:40Z
DOI: 10.1063/1.5080369
 Authors: Sualeh Khurshid, Diego A. Donzis
 Mixing processes in the transonic, accelerated wake of a central injector
 Authors: J. Richter, M. Beuting, C. Schulz, B. Weigand
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
The compressible accelerated mixing layer of a central injector was thoroughly investigated experimentally to provide a data set that can be used for validating numerical simulations. A dropshaped central injector was mounted upstream of a rectangular convergentdivergent nozzle, through which air was accelerated to a Mach number of 1.7. The freestream Reynolds number at the point of injection was 6.245 × 104. Four different measurement techniques—shorttime illuminated schlieren imaging, laser schlieren, laserinduced thermal acoustics, and laserinduced fluorescence (LIF)—were applied to visualize the flow structures and to measure the predominant frequency of periodic flow features, the Mach number and temperature, and the injectant distribution. Instantaneous images show that the mixing layer was dominated by a series of alternating vortices. The mixing layer’s selfsimilarity could be proven by means of injectant mass fraction profiles, which were derived from LIF measurements. The growth rate of the mixing layer was shown to approximately follow the 1 2power law. It was concluded from comparison to literature data that the growth rate is primarily determined by the freestream Reynolds number, whereas the freestream Mach number (compressibility effects) and the injectant amount play a minor role. These experimental data were used to validate threedimensional (3D) unsteady Reynoldsaveraged NavierStokes simulations using the shearstress transport turbulence model. It was shown that the vortex shedding frequency and the mixing layer growth rate as well as the wake velocity deficit were underestimated by the simulations. This indicates that the flow physics of vortex formation were not entirely reproduced.
Citation: Physics of Fluids
PubDate: 20190110T05:41:38Z
DOI: 10.1063/1.5055749
 Authors: J. Richter, M. Beuting, C. Schulz, B. Weigand
 A comparison of frequency downshift models of wave trains on deep water
 Authors: John D. Carter, Diane Henderson, Isabelle Butterfield
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
Frequency downshift (FD) in wave trains on deep water occurs when a measure of the frequency, typically the spectral peak or the spectral mean, decreases as the waves evolve. Many FD models rely on wind or wave breaking. We consider seven models that do not include these effects and compare their predictions with four sets of experiments that also do not include these effects. The models are the (i) nonlinear Schrödinger equation (NLS), (ii) dissipative NLS equation (dNLS), (iii) Dysthe equation, (iv) viscous Dysthe equation (vDysthe), (v) Gordon equation (Gordon), which has a free parameter, (vi) IslasSchober equation (IS), which has a free parameter, and (vii) a new model, the dissipative GramstadTrulsen (dGT) equation. The dGT equation has no free parameters and addresses some of the difficulties associated with the vDysthe equation. We compare a measure of overall error and the evolution of the spectral amplitudes, means, and peaks. We find the following: (i) The NLS and Dysthe equations do not accurately predict the spectral amplitudes. (ii) The Gordon equation, which is a successful model of FD in optics, does not accurately model FD in water waves, regardless of the choice of free parameter. (iii) The dNLS, vDysthe, dGT, and IS (with optimized free parameter) models do a reasonable job predicting the measured spectral amplitudes, but none captures all spectral evolutions. (iv) The vDysthe, dGT, and IS models most accurately predict the observed evolution of the spectral peak and the spectral mean. (v) The IS and vDysthe models have the smallest overall errors.
Citation: Physics of Fluids
PubDate: 20190110T05:32:36Z
DOI: 10.1063/1.5063016
 Authors: John D. Carter, Diane Henderson, Isabelle Butterfield
 Capillary surface wave formation and mixing of miscible liquids during
droplet impact onto a liquid film Authors: Nuri Erdem Ersoy, Morteza Eslamian
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
In order to advance the understanding of the process of droplet impact on wet surfaces, realized in various applications such as dropletbased coating methods (inkjet printing, aerosoljet, and spray coating), we studied the impact of a dyed water droplet onto a clear water film. The color contrast in images allowed investigation of mixing process of the like liquids during the rapid dynamic stage and beyond. Four Weber numbers (We), in the range of 121–304, and four dimensionless film thickness to droplet diameter ratios (h*), in the range of 0.092–0.367, were considered. The aforementioned numbers correspond to the film thickness of 0.4–1.6 mm, droplet size of 4.36 mm, and impact velocity of 1.4–2.2 m/s. While the experimental database is rather comprehensive and can be used for further detailed analysis, here we focused on lessexplored topics of capillary surface waves formed outside the crater and found the wave characteristics and their role in mixing. Within the range of parameters studied here, we found that the outer capillary surface waves were created as a result of perturbing the liquid film by droplet impact, but the wave characteristics such as frequency (400500 Hz) were not a strong function of the impact We number. We also observed six mixing mechanisms of miscible liquids, including the expansion/compression waves and turbulence created upon impact, stable crown wall formation with an acute wall angle, which causes a tsunamitype of flow, unstable crown leading to fingering and splashing, capillary waves, and molecular diffusion.
Citation: Physics of Fluids
PubDate: 20190110T05:28:56Z
DOI: 10.1063/1.5064640
 Authors: Nuri Erdem Ersoy, Morteza Eslamian
 A geometrical criterion for absolute instability in separated boundary
layers Authors: Mateus P. Avanci, Daniel Rodríguez, Leonardo S. de B. Alves
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
Laminar separation bubbles on airfoils and lowpressure turbines are generally expected to be dominated by convective inflectional instability. However, absolute instability is possible under certain circumstances, which may lead to important changes in the laminarturbulent transition, reattachment processes, and their impact on the aerodynamics. This paper revisits the absolute/convective instability properties of different families of boundarylayer velocity profiles with a reversed flow region. A new methodology is employed in the analysis, which incorporates an additional equation to the classic Rayleigh’s equation governing inviscid instability. This allows for the direct recovery of the zerogroupvelocity disturbance waves that govern the absolute/convective behavior, at an unprecedented low computational cost that enabled the large parametric study performed here. Present results show that while the peak reversed flow or wallnormal extent of the reversed flow impact the instability character, criteria based on any of them are generally not valid. A new criterion is proposed, based on the relative position of the inflection point: inviscid inflectional instability becomes of an absolute kind when the inflection point is located inside of the recirculation region. Absolutely unstable velocity profiles are identified with peak reversed flow as low as 10% of the freestream velocity, a value substantially smaller than thresholds previously proposed in the literature.
Citation: Physics of Fluids
PubDate: 20190109T06:53:42Z
DOI: 10.1063/1.5079536
 Authors: Mateus P. Avanci, Daniel Rodríguez, Leonardo S. de B. Alves
 The analytic description of hydraulic jump in the linear theory of the
shear shallowwater flows Authors: Evgeny V. Semenko
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
This article is devoted to the analytic description of the hydraulic jump in frames of the linear theory of the shear shallowwater flows. Linearization on the piecewiseconstant solution is used, and the solution of linear problem is constructed using the Fourier transform, such as in the classical shock wave theory. It allows the solution to be analyzed analytically, in particular, to predict the absence or presence of the hydraulic jump toe oscillations (in terms of shock wave theory—to separate the cases of stability and neutral stability). The simplified example of flows is analyzed, and the formulas for the mentioned separation are obtained.
Citation: Physics of Fluids
PubDate: 20190109T06:53:41Z
DOI: 10.1063/1.5072772
 Authors: Evgeny V. Semenko
 Twodimensional modal and nonmodal instabilities in
straightdivergingstraight channel flow Authors: Mamta Jotkar, Rama Govindarajan
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
A systematic study of a twodimensional viscous flow through the straightdivergingstraight (SDS) channel defined by two straightwalled sections of different widths and a divergent section inbetween is presented here. It has the plane Poiseuille flow (PPF) and the symmetric sudden expansion flow as the limiting cases. The topology of steady laminar flows and its bifurcations are characterized in the multiparametric space formed by the divergence angle, the expansion ratio, and the Reynolds number. Three different steady flow regimes with two symmetric zones of recirculation, two asymmetric zones of recirculation, and the one with an additional third recirculation zone are observed with increasing Reynolds number. Modal stability analysis shows that the asymmetric flows remain stable at least up to Re = 300, regardless of the divergence angle and expansion ratio. Nonmodal stability analyses are applied to SDS flows in the three topology regimes. A remarkable potential for transient amplification due to the Orr mechanism is found even for relatively low Reynolds numbers, which is related to the flow topology. The optimal energy amplification grows exponentially with the Reynolds number, as opposed to the substantially weaker Re2 scaling known for the liftup mechanism dominant for PPF. This scaling holds for all divergence angles and is further increased by the expansion ratio, resulting in energy amplifications Gmax ∼ 104 for Reynolds numbers as low as Re ∼ 300. Present results suggest that the subcritical transition due to transient growth is the most likely scenario for SDS flows at low Reynolds numbers.
Citation: Physics of Fluids
PubDate: 20190109T06:53:37Z
DOI: 10.1063/1.5055053
 Authors: Mamta Jotkar, Rama Govindarajan
 Coalescence dynamics of unequal sized drops
 Authors: Hiranya Deka, Gautam Biswas, Suman Chakraborty, Amaresh Dalal
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
In this study, coalescence dynamics of two unequal sized drops of the same liquid have been investigated using the coupled level set and volume of fluid method. A broad range of fluid properties is considered with two orders of magnitude variation of Ohnesorge numbers and Atwood number ranging between 0.01 and 0.9976. The pinchoff process and controlling parameters that lead to satellite generation have been investigated. The capillary waves are generated as a result of the sharp curvature produced near the contact region. Here we demonstrate that the capillary waves propagating along the interface of the lower drop can affect the eventual pinchoff of the satellite. The local curvature of the neck plays a crucial role in the pinchoff process. A sharper axial curvature of the neck increases the local capillary pressure which restricts the pinchoff. The critical diameter ratio above which a satellite pinches off during the coalescence of two freefalling drops increases with increasing relative strength of the viscous force and the gravity force. The critical ratio can be as low as 1.2 at a lower relative strength of the viscous and the gravity forces. The coalescence of two unequal sized drops may produce much smaller satellite drops, on account of coalescence in successive steps, with or without intermediate detachment.
Citation: Physics of Fluids
PubDate: 20190108T07:04:45Z
DOI: 10.1063/1.5064516
 Authors: Hiranya Deka, Gautam Biswas, Suman Chakraborty, Amaresh Dalal
 Wake structures behind a rotor with superhydrophobiccoated blades at low
Reynolds number Authors: Hongseok Choi, Jungjin Lee, Hyungmin Park
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
We experimentally investigate the flow structures generated by a rotor with the superhydrophobic coating applied on the blade surface in static water. Considered Reynolds number based on the rotating velocity and blade tip to tip distance is 96 000, and spray coating of hydrophobic nanoparticles is used to produce a superhydrophobic surface. We focus on the changes in both instantaneous and time/ensembleaveraged flows measured with a stereoscopic particle image velocimetry. The vortical structures behind a rotor is characterized by the periodic shedding of hub and tip vortices, whose interactions induce a coneshaped lowspeed region where higher velocities are induced over it. These are closely connected to the spatial distribution of velocity fluctuation. With superhydrophobic surface, the organized formation of vortical structures is disturbed due to the slip on the blade surface, that is, the accumulation of vorticity on the surface is delayed or not strong. Thus the conical region shrinks toward the rotation axis, and the vortex strength is reduced. As a result, about 20% reduction in the turbulent kinetic energy is achieved in the wake, followed by smaller decrease (∼6%) in the streamwise momentum flux. Also, it is found that superhydrophobic surface on the pressure side is more effective, in terms of turbulence reduction. This is the first study to investigate the effect of superhydrophobic surface on the flow around a rotating body, and we think the results will be useful to extend the application of superhydrophobic surface.
Citation: Physics of Fluids
PubDate: 20190108T07:04:42Z
DOI: 10.1063/1.5054039
 Authors: Hongseok Choi, Jungjin Lee, Hyungmin Park
 Laser induced fluorescence studies on the distribution of surfactants
during drop/interface coalescence Authors: Teng Dong, Weheliye Hashi Weheliye, Panagiota Angeli
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
The spatiotemporal distribution of fluorescent surfactants on the merging interfaces during the coalescence of an aqueous drop with an organic/aqueous flat interface was studied experimentally with highspeed laser induced fluorescence. The aqueous phase was a 46% glycerol solution, while the organic phase was a 5 cSt silicone oil. A fluorescently tagged surfactant was used at a concentration of 0.001 mol/m3 in the aqueous phase. To vary the concentration of surfactants on the interfaces, the drop and the flat interface were left to stand for different times before the coalescence experiments (different interface ages). It was found that when a drop rested on the interface, the surfactants adsorbed on the interfaces were swept outwards by the draining liquid film between the drop and the flat interface and reached a peak value at 0.75Rh away from the centre of the film, where Rh is the horizontal drop radius. After the film rupture, the concentration of the surfactants at the tip of the meniscus increased. Once the film had retracted, the concentration of the surfactants peaked at the meniscus at the bottom of the drop. As the liquid in the drop started to merge with its homophase, the drop formed a cylinder from the upward capillary waves on the drop surface. The surfactant concentration was found to be low at the top of the liquid cylinder as the interface was stretched by the convergence of the capillary waves. Subsequently, the cylinder began to shrink and the top part of the drop acquired a high surfactant concentration.
Citation: Physics of Fluids
PubDate: 20190108T07:04:39Z
DOI: 10.1063/1.5059554
 Authors: Teng Dong, Weheliye Hashi Weheliye, Panagiota Angeli
 Dynamics and stability of a powerlaw film flowing down a slippery slope
 Authors: Symphony Chakraborty, Tony WenHann Sheu, Sukhendu Ghosh
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
A powerlaw fluid flowing down a slippery inclined plane under the action of gravity is deliberated in this research work. A Newtonian layer at a small strain rate is introduced to take care of the divergence of the viscosity at a zero strain rate. A lowdimensional twoequation model is formulated using a weightedresidual approach in terms of two coupled evolution equations for the film thickness h and a local velocity amplitude or the flow rate q within the framework of lubrication theory. Moreover, a longwave instability is shown in detail. Linear stability analysis of the proposed twoequation model reveals good agreement with the spatial OrrSommerfeld analysis. The influence of a wallslip on the primary instability has been found to be nontrivial. It has the stabilizing effect at larger values of the Reynolds number, whereas at the onset of the instability, the role is destabilizing which may be because of the increase in dynamic wave speed by the wall slip. Competing impressions of shearthinning/shearthickening and wall slip velocity on the primary instability are captured. The impact of slip velocity on the travelingwave solutions is discussed using the bifurcation diagram. An increasing value of the slip shows a significant effect on the traveling wave and free surface amplitude. Slip velocity controls both the kinematic and dynamic waves of the system, and thus, it has the profound passive impact on the instability.
Citation: Physics of Fluids
PubDate: 20190108T07:04:36Z
DOI: 10.1063/1.5078450
 Authors: Symphony Chakraborty, Tony WenHann Sheu, Sukhendu Ghosh
 Electroosmotic pumping through a bumpy microtube: Boundary perturbation
and detection of roughness Authors: JieChao Lei, Chien C. Chang, ChangYi Wang
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
To machine precision, a microduct cannot be fabricated without producing surface roughness. It is of essential importance to examine the effects and predict the level of roughness on electroosmotic (EO) pumping for ducts of fundamental shapes. In this study, we consider a bumpy microtube with its wall shape modeled by the product of two sinusoidal functions. Boundary perturbation is carried out with respect to the amplitude roughness ε (relative to the Debye length) up to the secondorder by considering the DebyeHückel approximation and viscous Stokes equation for the electrolyte transport. Besides the amplitude roughness ε, the key parameters include the azimuthal wave number n and the axial wave number α of the bumpiness, as well as the nondimensional electrokinetic width K. It is shown that the EO pumping rate Q is modified by a secondorder term −ε2πχ, namely, Q = Q0 − ε2πχ, where Q0 denotes the pumping rate through the smooth tube. The net effect χ = χ1 + χ2 comprises two components: χ1 = χ1(K) < 0 increases with increasing K, representing a pure gain, while χ2 has no definite sign and is a complex function of K, n, and α. In particular, χ is negative at small α whilst being positive at large α, and the dividing line of signs also depends on K. For small α (1), χ decreases with increasing n at large K (>20). For a given number of oscillations Ac = nα (>1), there exists an intermediate n at which the EO pumping rate is maximized at small K (
Citation: Physics of Fluids
PubDate: 20190107T06:22:43Z
DOI: 10.1063/1.5063869
 Authors: JieChao Lei, Chien C. Chang, ChangYi Wang
 Families of reversing and nonreversing Taylor vortex flows between two
cooscillating cylinders with different amplitudes Authors: Mehdi Riahi, Saïd Aniss, Mohamed Ouazzani Touhami
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
This paper deals with the centrifugal instability of timemodulated TaylorCouette flow for the case in which the inner and outer cylinders are cooscillating around zero mean with the angular velocities Ωin = Ω0 cos(ωt) and Ωout = [math]Ω0 cos(ωt), respectively (Ω0, ω, and [math] denote, respectively, the amplitude, the frequency of the modulated rotation, and the amplitudes ratio). The smallgap equations for the stability of this flow with respect to axisymmetric disturbances are derived and solved on the basis of Floquet theory. We recover in the case [math] = 0 where the outer cylinder is stationary while the inner is modulated the two wellknown reversing and nonreversing Taylor vortex flows. Attention is focused on the evolution of these timedependent flows when one allows the oscillation of the outer cylinder. It turns out that an increase in the parameter [math] leads to the discovery of families of reversing and nonreversing flows and other interesting bifurcation phenomena including codimensiontwo bifurcation points. In addition, a proper tuning of this parameter [math] provides a control of the onset of instability as well as the nature of the primary bifurcation. Moreover, it is shown that when [math]> 1, the instability is suppressed in low frequencies and the flow is always stable in good agreement to what is obtained by a quasisteady approach where transient instability is detected. This latter is attributed to the fluid inertia taking place when the cylinders are reversing their rotation’s direction. However, no effect of the parameter [math] is observed in high frequencies where the instability develops in thin boundary Stokes layers near the oscillating cylinders.
Citation: Physics of Fluids
PubDate: 20190104T07:50:46Z
DOI: 10.1063/1.5064656
 Authors: Mehdi Riahi, Saïd Aniss, Mohamed Ouazzani Touhami
 Stability analysis of a flexible rotor partially filled with two liquid
phases Authors: Guangding Wang, Huiqun Yuan
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
This paper deals with the dynamic stability of a flexible rotor partially filled with two liquid phases. On the basis of the NavierStokes equations for the incompressible flow, a twodimensional analytical model is developed for fluid motion. The perturbation method is employed to obtain the linearized NavierStokes and continuity equations. According to the boundary conditions of fluid motion, the fluid force exerted on the rotor is calculated. Then, combining the structural static equilibrium equation with the equations describing the fluid forces, the whirling frequency equation of the system, which is used to predict the system stability, is obtained. The stability and critical spinning speed of the coupled fluidstructure system are analyzed. To demonstrate the validity of the developed model, the analysis results are compared with the results reported in the previous study. The two analysis results are in good agreement. Finally, the effects of some main parameters on system stability are discussed.
Citation: Physics of Fluids
PubDate: 20190104T07:11:04Z
DOI: 10.1063/1.5054683
 Authors: Guangding Wang, Huiqun Yuan
 Flow separation control over a rounded ramp with spanwise alternating wall
actuation Authors: Charles Moulinec, David R. Emerson
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
An implicit largeeddy simulation is carried out to study turbulent boundarylayer separation from a backwardfacing rounded ramp with active wall actuation control. This method, called spanwise alternating distributed strips control, is imposed onto the flat plate surface upstream of a rounded ramp by alternatively applying outofphase control and inphase control to the wallnormal velocity component in the spanwise direction. As a result, the local turbulence intensity is alternatively suppressed and enhanced, leading to the creation of vertical shearlayers, which is responsible for the presence of largescale streamwise vortices. These vortices exert a predominant influence on the suppression of the flow separation. The interaction between the largescale vortices and the downstream recirculation zone and free shearlayer is studied by examining flow statistics. It is found that in comparison with the noncontrolled case, the flow separation is delayed, the reattachment point is shifted upstream, and the length of the mean recirculation zone is reduced up to 8.49%. The optimal control case is achieved with narrow inphase control strips. An indepth analysis shows that the delay of the flow separation is attributed to the activation of the nearwall turbulence by the inphase control strips and the improvement of the reattachment location is mainly due to the largescale streamwise vortices, which enhance the momentum transport between the main flow and separated region.
Citation: Physics of Fluids
PubDate: 20190104T07:08:24Z
DOI: 10.1063/1.5055948
 Authors: Charles Moulinec, David R. Emerson
 New normalized Rortex/vortex identification method
 Authors: Xiangrui Dong, Yisheng Gao, Chaoqun Liu
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
A new vortex identification criterion, named [math], is proposed for the normalization of Rortex, using the idea of the Omega method ([math]). [math] is a normalized function from 0 to 1, which measures the relative rotation strength on the plane perpendicular to the local rotation axis. The advantages of the proposed [math] method can be summarized as follows: (1) [math] is from 0 to 1 and can be further used in statistics and correlation analysis as a physical quantity. (2) [math] can distinguish the rotational vortices from the shear layers, discontinuity structures, and other nonphysical structures. (3) [math] is quite robust and can be always set as [math] to capture vortex structures in different cases and at different time steps.
Citation: Physics of Fluids
PubDate: 20190104T07:04:44Z
DOI: 10.1063/1.5066016
 Authors: Xiangrui Dong, Yisheng Gao, Chaoqun Liu
 Numerical simulations of the forced oscillation of a wire in Newtonian and
shearthinning fluids Authors: Cameron C. Hopkins, John R. de Bruyn
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
Forced oscillations of a wire vibrating in Newtonian and shearthinning fluids described by the Carreau model are studied numerically. Twodimensional simulations were performed using a commercial finite element modeling software package. When subjected to a sinusoidal driving force, the wire exhibits resonant behavior that depends on the viscosity of the surrounding fluid. The simulations of the wire vibrating in a Newtonian fluid were extremely well described by the theory developed by Retsina et al. [“The theory of a vibratingrod densimeter,” Appl. Sci. Res. 43, 127–158 (1986); “The theory of a vibratingrod viscometer,” Appl. Sci. Res. 43, 325–346 (1987)]. Our simulations of a wire vibrating in a Carreau fluid also revealed resonant behavior, but the shear rate and viscosity in the fluid varied significantly in both space and time. The behavior of a wire vibrating in a Carreau fluid can be described by the Newtonian theory if the constant viscosity in that theory is set equal to the nonNewtonian fluid viscosity averaged spatially around the circumference of the wire and temporally over one period of oscillation.
Citation: Physics of Fluids
PubDate: 20190103T09:24:12Z
DOI: 10.1063/1.5063591
 Authors: Cameron C. Hopkins, John R. de Bruyn
 Simultaneous mapping of single bubble dynamics and heat transfer rates for
SiO2/water nanofluids under nucleate pool boiling regime Authors: Dhairya Bhatt, Prasad Kangude, Atul Srivastava
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
Dependence of single vapor bubble dynamics and heat transfer rates on varying concentration of SiO2 nanoparticles for a range of subcooled conditions (0–9 °C) has been experimentally studied under nucleate pool boiling configuration. Noninvasive measurements have been carried out using rainbow schlieren deflectometry. Results on bubble dynamics showed that the bubble diameter and aspect ratio decrease with increasing subcooling levels as well as concentration of nanofluids. The frequency of bubble oscillations was found to increase first and then decrease with increasing subcooling levels while it decreases monotonically with increasing nanofluid concentration. Bubble departure frequency increased significantly for nanofluids, while it decreased with increasing subcooling levels. Condensation effects at the bubble interface were reflected in the form of redistribution of colors around it. Schlieren images clearly revealed a spread in the spatial extent of the thermal boundary layer region caused by the suspended nanoparticles around the vapor bubble as well as near the heated substrate. This phenomenon has been considered as one of the factors that tends to alter the condensation effects and, in turn, affects the bubble dynamics. Quantitative analysis of schlieren images revealed that the natural convective heat flux increases with increasing subcooling levels, while it decreases with increasing nanoparticle concentration. Deterioration in the natural convection phenomenon in the presence of suspended nanoparticles has been attributed to the reduced strength of thermal gradients adjacent to the heater substrate. On the other hand, evaporative heat flux was observed to decrease with increasing subcooling levels and increase with increasing concentration of nanofluids.
Citation: Physics of Fluids
PubDate: 20190103T06:46:43Z
DOI: 10.1063/1.5050980
 Authors: Dhairya Bhatt, Prasad Kangude, Atul Srivastava
 Discrete unified gas kinetic scheme for flows of binary gas mixture based
on the McCormack model Authors: Yue Zhang, Lianhua Zhu, Peng Wang, Zhaoli Guo
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
The discrete unified gas kinetic scheme (DUGKS) was originally developed for singlespecies flows covering all the regimes, whereas the gas mixtures are more frequently encountered in engineering applications. Recently, the DUGKS has been extended to binary gas mixtures of Maxwell molecules on the basis of the Andries–Aoki–Perthame kinetic (AAP) model [P. Andries et al., “A consistent BGKtype model for gas mixtures,” J. Stat. Phys. 106, 993–1018 (2002)]. However, the AAP model cannot recover a correct Prandtl number. In this work, we extend the DUGKS to gas mixture flows based on the McCormack model [F. J. McCormack, “Construction of linearized kinetic models for gaseous mixtures and molecular gases,” Phys. Fluids 16, 2095–2105 (1973)], which can give all the transport coefficients correctly. The proposed method is validated by several standard tests, including the plane Couette flow, the Fourier flow, and the liddriven cavity flow under different mass ratios and molar concentrations. Good agreement between results of the DUGKS and the other wellestablished numerical methods shows that the proposed DUGKS is effective and reliable for binary gas mixtures in all flow regimes. In addition, the DUGKS is about two orders of magnitude faster than the direct simulation Monte Carlo for lowspeed flows in terms of the wall time and convergent iteration steps.
Citation: Physics of Fluids
PubDate: 20190103T06:43:04Z
DOI: 10.1063/1.5063846
 Authors: Yue Zhang, Lianhua Zhu, Peng Wang, Zhaoli Guo
 Effect of repeated immersions and contamination on plastron stability in
superhydrophobic surfaces Authors: Felix Vüllers, Sam PeppouChapman, Maryna N. Kavalenka, Hendrik Hölscher, Chiara Neto
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
Development of superhydrophobic surfaces is of great interest for dragreducing applications as air layers retained underwater greatly reduce fluidic drag. However, liquid flow over these surfaces can result in the collapse of the lubricating air layer. Here, we investigate the dynamic stability of retained air layers on three different superhydrophobic surfaces against repeated immersion and motion through various viscous liquids. The three surfaces investigated are a highly ordered polytetrafluoroethylene micropillar array, a twolevel hierarchical random polycarbonate nanofur, and a doublescale hierarchical Teflon AF wrinkled surface. Both repeated immersions and contamination by viscous liquids accelerated the rate of plastron decay on the pillar array and the nanofur, while the Teflon wrinkles remained dry. Five topographical features were identified as correlated to a dynamically stable retained air layer, and a relation between these stabilityenhancing parameters and the dragreducing capabilities is found. Furthermore, resistance of superhydrophobic surfaces against contamination is studied and the directionality of the CassietoWenzel wetting transition on airretaining surfaces is demonstrated. Together, an understanding of these properties allows for the rational design of new superhydrophobic surfaces fit for application.
Citation: Physics of Fluids
PubDate: 20190103T06:25:31Z
DOI: 10.1063/1.5064817
 Authors: Felix Vüllers, Sam PeppouChapman, Maryna N. Kavalenka, Hendrik Hölscher, Chiara Neto
 Instabilities in viscositystratified twofluid channel flow over an
anisotropicinhomogeneous porous bottom Authors: Geetanjali Chattopadhyay, Usha Ranganathan, Severine Millet
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
A linear stability analysis of a pressure driven, incompressible, fully developed laminar Poiseuille flow of immiscible twofluids of stratified viscosity and density in a horizontal channel bounded by a porous bottom supported by a rigid wall, with anisotropic and inhomogeneous permeability, and a rigid top is examined. The generalized Darcy model is used to describe the flow in the porous medium with the BeaversJoseph condition at the liquidporous interface. The formulation is within the framework of modified OrrSommerfeld analysis, and the resulting coupled eigenvalue problem is numerically solved using a spectral collocation method. A detailed parametric study has revealed the different active and coexisting unstable modes: porous mode (manifests as a minimum in the neutral boundary in the long wave regime), interface mode (triggered by viscositystratification across the liquidliquid interface), fluid layer mode [existing in moderate or O(1) wave numbers], and shear mode at high Reynolds numbers. As a result, there is not only competition for dominance among the modes but also coalescence of the modes in some parameter regimes. In this study, the features of instability due to twodimensional disturbances of porous and interface modes in isodense fluids are explored. The stability features are highly influenced by the directional and spatial variations in permeability for different depth ratios of the porous medium, permeability and ratio of thickness of the fluid layers, and viscositystratification. The two layer flow in a rigid channel which is stable to long waves when a highly viscous fluid occupies a thicker lower layer can become unstable at higher permeability (porous mode) to long waves in a channel with a homogeneous and isotropic/anisotropic porous bottom and a rigid top. The critical Reynolds number for the dominant unstable mode exhibits a nonmonotonic behaviour with respect to depth ratio. However, it increases with an increase in anisotropy parameter ξ indicating its stabilizing role. Switching of dominance of modes which arises due to variations in inhomogeneity of the porous medium is dependent on the permeability and the depth ratio. Inhomogeneity arising due to an increase in vertical variations in permeability renders short wave modes to become more unstable by enlarging the unstable region. This is in contrast to the anisotropic modulations causing stabilization by both increasing the critical Reynolds number and shrinking the unstable region. A decrease in viscositystratification of isodense fluids makes the configuration hosting a less viscous fluid in a thinner lower layer adjacent to a homogeneous, isotropic porous bottom to be more unstable than the one hosting a highly viscous fluid in a thicker lower layer. An increase in relative volumetric flow rate results in switching the dominant mode from the interface to fluid layer mode. It is evident from the results that it is possible to exercise more control on the stability characteristics of a twofluid system overlying a porous medium in a confined channel by manipulating the various parameters governing the flow configurations. This feature can be effectively exploited in relevant applications by enhancing/suppressing instability where it is desirable/undesirable.
Citation: Physics of Fluids
PubDate: 20190103T06:25:28Z
DOI: 10.1063/1.5065780
 Authors: Geetanjali Chattopadhyay, Usha Ranganathan, Severine Millet
 Viscous resistance in drop coalescence
 Authors: Md Mahmudur Rahman, Willis Lee, Arvind Iyer, Stuart J. Williams
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
Hydrodynamics of drop coalescence has been studied theoretically and numerically by solving the Navier Stokes equation considering a single fluid after the minimum bridge formation. Many experiments have been performed to document bridge growth over time with the use of high speed videography and electrical methods. However, internal fluid motion during coalescence has not been extensively studied, in part due to the spherical shape of the drops. This work observed overall fluid motion (except at the site of early coalescence) using particle image velocimetry for twodimensional (sandwiched drop) coalescence. Fluid motion inside the bulk drops is inertial, and governing fluid flow in the bridge region is one dimensional. At the merging interface, incoming liquids join and coflow in the perpendicular direction. These observations were extended to a threedimensional counterpart, and a scaling law was developed that was validated through experimentation. While flow in the bulk drops is inertial, the dominant resistance comes through a viscous effect in the merging interface region and at the lesser extent in the bridge region. Early dynamics of drop coalescence is dominated by the Ohnesorge number (Oh), and later dynamics are dependent on how drops are bounded.
Citation: Physics of Fluids
PubDate: 20190103T06:25:26Z
DOI: 10.1063/1.5064706
 Authors: Md Mahmudur Rahman, Willis Lee, Arvind Iyer, Stuart J. Williams
 Capillary wave method: An alternative approach to wave excitation and to
wave profile reconstruction Authors: Andrey Shmyrov, Aleksey Mizev, Anastasia Shmyrova, Irina Mizeva
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
The capillary wave method is a wellknown classical technique to measure surface tension and surface rheological properties. Despite the large number of theoretical works devoted to capillary waves, this technique has serious difficulties associated with its implementation, and therefore, it is not widely used by researchers. In this paper, we introduce our modifications of the existing method to overcome its drawbacks. First, a capillary wave is excited by pressure fluctuations generated locally at the interface. Being contactless, the proposed method is suitable for any liquid irrespective of its electrical properties. Second, the application of optical interferometry together with the spatial phase shifting method allows to quantify the surface profile with high accuracy. A new data processing algorithm makes it possible to subtract the parasitic deformation of the surface caused by external perturbations avoiding, thereby the thorough vibroisolation procedure. The relative error for surface measurements and surface tension calculations is 0.3%. The results of surface tension measurements of several liquids obtained by the modified method are in good agreement with the data determined by the Wilhelmy plate technique. The main advantage of our method is that is well suited for measurements of low liquid volumes, which makes it of particular interest in biological and chemistry applications. Additionally, our version of the examined method allows one to extend the frequency range to 103–104 Hz, where only the quasielastic light scattering technique is currently applicable.
Citation: Physics of Fluids
PubDate: 20190103T06:25:23Z
DOI: 10.1063/1.5060666
 Authors: Andrey Shmyrov, Aleksey Mizev, Anastasia Shmyrova, Irina Mizeva
 A new kinetic theory model of granular flows that incorporates particle
stiffness Authors: Yifei Duan, ZhiGang Feng
Abstract: Physics of Fluids, Volume 31, Issue 1, January 2019.
Granular materials of practical interest in general have finite stiffness; therefore, the particle collision is a process that takes finite time to complete. Softsphere Discrete Element Method (DEM) simulations suggest that there are three regimes for granular shear flows: inertial regime (or rapid flow regime), elastic regime (or quasistatic regime), and the transition regime (or elasticinertial regime). If we use tf to represent the mean free flight time for a particle between two consecutive collisions and tc to represent the binary collision duration, these regimes are implicitly related to the ratio tc/tf. Granular flows can be successfully predicted by the classical Kinetic Theory (KT) when they are in the inertial regime of low particleparticle collision frequencies and short time contacts (tc/tf ≈ 0). However, we find that KT becomes less accurate in the transition regime where the collision duration tc is no longer small compared with the collision interval tf (tc/tf> 0.05). To address this issue, we develop a softsphere KT (SSKT) model that takes particle stiffness k as an input parameter since tc/tf is mainly determined by k. This is achieved by proposing a modified expression for the collision frequency and introducing an elastic granular temperature Te. Compared with the classical KT that only considers the kinetic granular temperature Tk, a redefined total granular temperature (Tg = Tk + Te/3) that takes both kinetic fluctuation energy and elastic potential energy into consideration is used in the SSKT model. The model is developed for identical frictionless particles with the linearspringdashpot collision scheme; however, it can be extended to frictional systems as well after the modification of constitutive equations. We show that the proposed SSKT extends the applicability of the KT framework to the transition regime without losing significant accuracy. The rheological crossover has been explained physically, and the regime boundaries that separate the inertial regime and the elastic regime are quantitatively determined, showing good agreement with the previous regime map that was based on the DEM simulations. Our SSKT predictions also show that for unsteady flows such as homogeneous cooling, the particle stiffness could have a large impact on the granular flow behavior due to the energy transfer between Te and Tk.
Citation: Physics of Fluids
PubDate: 20190102T05:20:49Z
DOI: 10.1063/1.5051034
 Authors: Yifei Duan, ZhiGang Feng