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THERMODYNAMICS (30 journals)

Showing 1 - 28 of 28 Journals sorted alphabetically
Advances in Heat Transfer     Full-text available via subscription   (Followers: 27)
Applied Thermal Engineering     Hybrid Journal   (Followers: 38)
Araucaria. Revista Iberoamericana de FilosofĂ­a, PolĂ­tica y Humanidades     Open Access  
Archives of Thermodynamics     Open Access   (Followers: 11)
Chemical Thermodynamics and Thermal Analysis     Open Access   (Followers: 3)
Condensed Matter Physics     Open Access   (Followers: 2)
Diffusion Foundations     Full-text available via subscription   (Followers: 4)
European Journal of Mechanics - B/Fluids     Hybrid Journal   (Followers: 5)
Experimental Heat Transfer     Hybrid Journal   (Followers: 18)
Experimental Thermal and Fluid Science     Hybrid Journal   (Followers: 35)
Fluids     Open Access  
Heat and Mass Transfer     Hybrid Journal   (Followers: 29)
Heat Transfer Engineering     Hybrid Journal   (Followers: 38)
High Temperature     Hybrid Journal   (Followers: 2)
HTM Journal of Heat Treatment and Materials     Full-text available via subscription   (Followers: 3)
International Journal of Thermodynamics     Open Access   (Followers: 14)
International Journal of Thermophysics     Hybrid Journal   (Followers: 7)
Journal of Chemical Thermodynamics     Hybrid Journal   (Followers: 8)
Journal of Low Temperature Physics     Hybrid Journal   (Followers: 9)
Journal of Non-Newtonian Fluid Mechanics     Hybrid Journal   (Followers: 14)
Journal of Thermal Science     Hybrid Journal   (Followers: 22)
Journal of Thermal Spray Technology     Hybrid Journal   (Followers: 5)
Journal of Thermodynamics     Open Access   (Followers: 9)
Journal of Thermophysics and Heat Transfer     Hybrid Journal   (Followers: 94)
Low Temperature Physics     Hybrid Journal   (Followers: 6)
Metal Science and Heat Treatment     Hybrid Journal   (Followers: 36)
Quantitative InfraRed Thermography Journal     Hybrid Journal  
Thermophysics and Aeromechanics     Hybrid Journal   (Followers: 6)
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Number of Followers: 0  

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ISSN (Online) 2311-5521
Published by MDPI Homepage  [258 journals]
  • Fluids, Vol. 8, Pages 78: Influence of Morphological Parameters on the
           Flow Development within Human Airways

    • Authors: Andres Santiago Espinosa-Moreno, Carlos Alberto Duque-Daza, Diego Alexander Garzón-Alvarado
      First page: 78
      Abstract: Anatomical airways parameters, such as length, diameter and angles, have a strong effect on the flow dynamics. Aiming to explore the effect of variations of the bifurcation angle (BA) and carina rounding radius (CRR) of lower human airways on respiratory processes, numerical simulations of airflow during inhalation and exhalation were performed using synthetic bifurcation models. Geometries for the airways models were parameterized based on a set of different BA’s and several CRR’s. A range of Reynolds numbers (Re), relevant to the human breathing process, were selected to analyze airflow behavior. The numerical results showed a significant influence of BA and the CRR on the development of the airflow within the airways, and, therefore, affecting the following relevant features of the flow: the deformation of velocity profiles, alterations of pressure drop, flow patterns, and, finally, enhancement or attenuation of wall shear stresses (WSS) appearing during the regular respiratory process. The numerical results showed that increases in the bifurcation angle value were accompanied by pressure increases of about 20%, especially in the regions close to the bifurcation. Similarly, increases in the BA value led to a reduction in peak shear stresses of up to 70%. For the ranges of angles and radii explored, an increase in pressure of about 20% and a reduction in wall shear stress of more than 400% were obtained by increasing the carina rounding radius. Analysis of the coherent structures and secondary flow patterns also revealed a direct relationship between the location of the vortical structures, the local maxima of the velocity profiles and the local vorticity minima. This relationship was observed for all branches analyzed, for both the inhalation and exhalation processes of the respiratory cycle.
      Citation: Fluids
      PubDate: 2023-02-21
      DOI: 10.3390/fluids8030078
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 79: Time-Dependent Numerical Modelling of Acoustic
           Cavitation in Liquid Metal Driven by Electromagnetic Induction

    • Authors: Georgi Djambazov
      First page: 79
      Abstract: The numerically simulated method of using electromagnetic field from an alternating current is a patented method to create in liquid metal, under the conditions of resonance, acoustic waves of sufficient strength to cause cavitation and implosion of gas bubbles, leading to beneficial degassing and grain refinement. The modelling stages of electromagnetics are described below along with acoustics in liquids, bubble dynamics, and their interactions. Sample results are presented for a cylindrical container with liquid aluminium surrounded by an induction coil. The possibility of establishing acoustic resonance and sustaining the bubble oscillation at a useful level is demonstrated. Limitations of the time-dependent approach to this multi-physics modelling problem are also discussed.
      Citation: Fluids
      PubDate: 2023-02-22
      DOI: 10.3390/fluids8030079
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 80: Surrogate Models for Heat Transfer in
           Oscillating Flow with a Local Heat Source

    • Authors: Simon Knecht, Denislav Zdravkov, Albert Albers
      First page: 80
      Abstract: Simulative optimization methods often build on an iterative scheme, where a simulation model is solved in each iteration. To reduce the time needed for an optimization, finding the right balance between simulation model quality, and simulation time is essential. This is especially true for transient problems, such as fluid flow within a hydromechanical system. Therefore, we present an approach to building steady-state surrogate models for oscillating flow in a pipe with a local heat source. The main aspect is to model the fluid as a solid with an orthotropic heat transfer coefficient. The values of this coefficient are fitted to reproduce the temperature distribution of the transient case by parametric optimization. It is shown that the presented approach is feasible for different sets of parameters and creates suitable surrogate models for oscillating flow within a pipe with a local heat source. In future works, the presented approach will be transferred from the simplified geometry under investigation to industrial problems.
      Citation: Fluids
      PubDate: 2023-02-22
      DOI: 10.3390/fluids8030080
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 81: Theoretical Estimates of the Critical Reynolds
           Number in the Flow around the Sphere on the Basis of Theory of Stochastic
           Equations and Equivalence of Measures

    • Authors: Artur V. Dmitrenko, Vladislav M. Ovsyannikov
      First page: 81
      Abstract: The aim of this investigation is to show the solution for the critical Reynolds number in the flow around the sphere on the basis of theory of stochastic equations and equivalence of measures between turbulent and laminar motions. Solutions obtained by numerical methods (DNS, LES, RANS) require verification and in this case the theoretical results have special value. For today in the scientific literature, there is J. Talor’s implicit formula connecting the critical Reynolds number with the parameters of the initial fluctuations in the flow around the sphere. Here the derivation of the explicit formula is presented. The results show a satisfactory correspondence of the obtained theoretical dependence for the critical Reynolds number to the experiments in the flow around the sphere.
      Citation: Fluids
      PubDate: 2023-02-23
      DOI: 10.3390/fluids8030081
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 82: Influence of Vertically Treaded Outsoles on
           Interfacial Fluid Pressure, Mass Flow Rate, and Shoe–Floor Traction
           during Slips

    • Authors: Shubham Gupta, Subhodip Chatterjee, Arnab Chanda
      First page: 82
      Abstract: Accidental injuries due to slips and falls are considered serious threats to public safety. Sufficient friction at the footwear and flooring interface is essential to reduce slip-related risks. The presence of slippery fluidic contaminants, such as water, further reduces friction and increases the risks of slip-related accidents drastically. While the effect of floorings and contaminants on footwear traction has been measured extensively across a variety of footwear designs, only a few studies have explored the science of the outsole design and its role in providing sufficient traction. In this work, the tread design of a commonly encountered outsole pattern, i.e., with vertically oriented tread channels, was parametrically altered across its width and gap. Based on the impressions of an original footwear design, nine outsoles were fabricated. The induced fluid pressures, mass flow rates, and traction were quantified by using a computational fluid dynamics (CFD) framework and through slip testing experiments. Outsoles that had wide treads with small gaps decreased the overall slipping risk on dry floorings. As compared to the tread area, tread gaps were found to be a dominating parameter in providing adequate shoe–floor traction in wet slipping conditions. The methods, including the outcomes presented in this work, are anticipated to advance the understanding of the science behind footwear friction and help footwear manufacturers optimize outsole designs to reduce slip and fall risks.
      Citation: Fluids
      PubDate: 2023-02-27
      DOI: 10.3390/fluids8030082
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 83: Experimental Comparison of Methods to Evaluate
           Heat Generated by Magnetic Nanofluids Exposed to Alternating Magnetic

    • Authors: Elisabetta Sieni, Simonetta Geninatti Crich, Maria Rosaria Ruggiero, Lucia Del Bianco, Federico Spizzo, Roberta Bertani, Mirto Mozzon, Marco Barozzi, Michele Forzan, Paolo Sgarbossa
      First page: 83
      Abstract: The paper aims to compare different methods able to estimate the specific loss power (SLP) generated by three different types of magnetic nanoparticles, MNPs, dispersed in a suspension fluid, e.g., octane or water. The nanoparticles were characterized morphologically in terms of shape and size, chemically for composition and their physical properties like magnetization and SLP were studied. We evidenced the differences in SLP evaluation due to the applied method, particularly in the presence of thermally induced phenomena such as aggregation or precipitation of MNPs that can affect the heating curve of the samples. Then, the SLP determination methods less sensible to this phenomenon appear to be the ones that use the initial slope when the sample is in quasi-adiabatic condition. Finally, we propose a comparison of those methods based on the pros and cons of their use for the SLP determination of magnetic nanofluids. In particular, the analysis of the behavior of the heating curve is useful to evaluate the useful amplitude of the interval analysis for the initial slope methods.
      Citation: Fluids
      PubDate: 2023-02-27
      DOI: 10.3390/fluids8030083
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 84: Modal Representation of Inertial Effects in
           Fluid–Particle Interactions and the Regularity of the Memory Kernels

    • Authors: Giuseppe Procopio, Massimiliano Giona
      First page: 84
      Abstract: This article develops a modal expansion (in terms of functions exponentially decaying with time) of the force acting on a micrometric particle and stemming from fluid inertial effects (usually referred to as the Basset force) deriving from the application of the time-dependent Stokes equation to model fluid–particle interactions. One of the main results is that viscoelastic effects induce the regularization of the inertial memory kernels at t=0, eliminating the 1/t-singularity characterizing Newtonian fluids. The physical origin of this regularization stems from the finite propagation velocity of the internal shear stresses characterizing viscoelastic constitutive equations. The analytical expression for the fluid inertial kernel is derived for a Maxwell fluid, and a general method is proposed to obtain accurate approximations of it for generic complex viscoelastic fluids, characterized by a spectrum of relaxation times.
      Citation: Fluids
      PubDate: 2023-02-28
      DOI: 10.3390/fluids8030084
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 85: Adjoint-Based High-Fidelity Concurrent
           Aerodynamic Design Optimization of Wind Turbine

    • Authors: Sagidolla Batay, Bagdaulet Kamalov, Dinmukhamed Zhangaskanov, Yong Zhao, Dongming Wei, Tongming Zhou, Xiaohui Su
      First page: 85
      Abstract: To evaluate novel turbine designs, the wind energy sector extensively depends on computational fluid dynamics (CFD). To use CFD in the design optimization process, where lower-fidelity approaches such as blade element momentum (BEM) are more popular, new tools to increase the accuracy must be developed as the latest wind turbines are larger and the aerodynamics and structural dynamics become more complex. In the present study, a new concurrent aerodynamic shape optimization approach towards multidisciplinary design optimization (MDO) that uses a Reynolds-averaged Navier–Stokes solver in conjunction with a numerical optimization methodology is introduced. A multidisciplinary design optimization tool called DAFoam is used for the NREL phase VI turbine as a baseline geometry. Aerodynamic design optimizations in terms of five different schemes, namely, cross-sectional shape, pitch angle, twist, chord length, and dihedral optimization are conducted. Pointwise, a commercial mesh generator is used to create the numerical meshes. As the adjoint approach is strongly reliant on the mesh quality, up to 17.8 million mesh cells were employed during the mesh convergence and result validation processes, whereas 2.65 million mesh cells were used throughout the design optimization due to the computational cost. The Sparse Nonlinear OPTimizer (SNOPT) is used for the optimization process in the adjoint solver. The torque in the tangential direction is the optimization’s merit function and excellent results are achieved, which shows the promising prospect of applying this approach for transient MDO. This work represents the first attempt to implement DAFoam for wind turbine aerodynamic design optimization.
      Citation: Fluids
      PubDate: 2023-02-28
      DOI: 10.3390/fluids8030085
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 86: A Highly Scalable Direction-Splitting Solver on
           Regular Cartesian Grid to Compute Flows in Complex Geometries Described by
           STL Files

    • Authors: Antoine Morente, Aashish Goyal, Anthony Wachs
      First page: 86
      Abstract: We implement the Direction-Splitting solver originally proposed by Keating and Minev in 2013 and allow complex geometries to be described by a triangulation defined in STL files. We develop an algorithm that computes intersections and distances between the regular Cartesian grid and the surface triangulation using a ray-tracing method. We thoroughly validate the implementation on assorted flow configurations. Finally, we illustrate the scalability of our implementation on a test case of a steady flow through 144,327 spherical obstacles randomly distributed in a tri-periodic box at Re=19.2. The grid comprises 6.8 billion cells and the computation runs on 6800 cores of a supercomputer in less than 48 h.
      Citation: Fluids
      PubDate: 2023-02-28
      DOI: 10.3390/fluids8030086
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 87: Improving Homogeneity of 3D-Printed Cementitious
           Material Distribution for Radial Toolpath

    • Authors: Mingyang Li, Zhixin Liu, Jin Yao Ho, Teck Neng Wong
      First page: 87
      Abstract: The 3D cementitious material printing method is an extrusion-based additive manufacturing strategy in which cementitious materials are extruded through a dynamic nozzle system to form filaments. Despite its ability to fabricate structures with high complexity and efficiency, the uneven material distribution during the extrusion and deposition process is often encountered when a radial toolpath is introduced. This limits the design freedom and printing parameters that can be utilized during radial toolpath printing. Here, we report a facile strategy to overcome the existing challenges of cementitious material non-homogeneity by rationally developing new nozzle geometries that passively compensate the differential deposition rate encountered in conventional rectangular nozzles. Using two-phase numerical study, we showed that our strategy has the potential of achieving a homogeneous mass distribution even when the nozzle travel speed is unfavorably high, while filament from a rectangular nozzle remains highly non-homogenous. The material distribution unevenness can be reduced from 1.35 to 1.23 and to 0.98 after adopting trapezoid and gaussian nozzles, indicating improvements of 34.3% and 94.2%, respectively. This work not only outlines the methodology for improving the quality of corner/curved features in 3DCMP, but also introduces a new strategy which can be adopted for other extrusion-based fabrication techniques with high material inertia.
      Citation: Fluids
      PubDate: 2023-03-01
      DOI: 10.3390/fluids8030087
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 88: Conjugate Heat Transfer in Thermal Inkjet

    • Authors: S. G. Mallinson, G. D. McBain, B. R. Brown
      First page: 88
      Abstract: The mass of individual droplets ejected from a thermal inkjet printhead increases with increasing local temperature near the ejector nozzles. The amount of ink deposited on the page and so the printed image density depends on the droplet mass. Thus, printhead temperature nonuniformity results in printed image density variations that can be unacceptable to the end users of the printed output. Such temperature variations arise from a combination of the ink fluid flow and the heat transfer in both the ink and the solid components in the printhead. Conjugate heat transfer (CHT) in thermal inkjet printheads is investigated here using validated numerical simulations. A typical thermal inkjet printhead is considered here for the first time, with cold ink drawn through the solid structural components by the ejector nozzle refill. The effect of the width of the feedhole above the printhead chip on the temperature field within the chip is analyzed. Validation of the simulation model required the derivation of novel analytical solutions for the relatively simple problems of fully developed forced convection in a differentially heated planar channel and conduction against convection in plug flow. The results from numerical simulations of these two problems are found to compare well with the newly derived analytical solutions. CHT in flow over a backward-facing step with a heated downstream wall was also simulated as part of the validation process, and good agreement was observed with earlier numerical studies. For the main part of the study, it was found that increasing the width of the feedhole reduces the gradients in temperature on the surface of the printhead chip, thus reducing temperature-related printing defects.
      Citation: Fluids
      PubDate: 2023-03-01
      DOI: 10.3390/fluids8030088
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 89: Horizontal Stratified Air–Foam–Water
           Flows: Preliminary Modelling Attempts with OLGA

    • Authors: William Ferretto, Igor Matteo Carraretto, Andrea Tiozzo, Marco Montini, Luigi Pietro Maria Colombo
      First page: 89
      Abstract: Water accumulation is a major problem in the flow assurance of gas pipelines. To limit liquid loading issues, deliquification by means of surfactant injection is a promising alternative to the consolidated mechanical methods. However, the macroscopic behavior of foam pipe flow in the presence of other phases has barely been explored. The goal of this work was to propose an approach to simulate air–water–foam flows in horizontal pipes using OLGA by Schlumberger, an industry standard tool for the transient simulation of multiphase flow. The simulation results were compared with experimental data for 60 mm and 30 mm ID (Inner Diameter) horizontal pipelines. Preliminary validation for two-phase air–water flow was carried out, which showed that correct flow pattern recognition is essential to accurately reproduce the experimental data. Then, stratified air–foam–water flows were investigated, assuming different models for the foam local velocity distribution. Foam rheology was considered through the Herschel–Bulkley model with the yield stress varying in time due to foam decay. The results showed good agreement for a uniform velocity profile and fresh foam properties in the case of the 60 mm ID pipeline, whereas for the 30 mm ID, which was characterized by significantly higher velocities, a linear velocity profile and 2000 s foam aging provided the best agreement. In both cases, the pressure gradient was overestimated, and the mean absolute prediction error ranged from about 5% to 30%.
      Citation: Fluids
      PubDate: 2023-03-01
      DOI: 10.3390/fluids8030089
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 90: Prediction of Critical Heat Flux of Mixtures
           Flowing in Channels

    • Authors: Mirza M. Shah
      First page: 90
      Abstract: Boiling of mixtures in channels is involved in many chemical processes, and refrigerant mixtures are finding a rapidly increasing use in the refrigeration industries. Hence, reliable prediction of CHF (critical heat flux) is needed. There is no published method that has been verified over a wide range of parameters. In this study, available data for CHF in tubes and annuli were compared to general correlations for tubes and annuli, which are well-verified for single-component fluids. This paper presents the results of this comparison. It was found that general correlations for single-component fluids were in satisfactory agreement with data for mixtures. The data included mixtures of refrigerants and chemicals, reduced pressure 0.02 to 0.8597, mass flux 200 to 3798 kgm−2s−1, temperature glide 2.1 to 21.9 K, and critical quality −0.46 to 0.99. The data for annuli were predicted with a MAD (mean absolute deviation) of 13.6%, and the data for tubes had a MAD of 13.9% when compared to the Shah correlations for single-component fluids. Based on the results of the present data analysis and results of research on pool boiling, a range is identified in which pure fluid correlations can be used for mixtures. This will be useful in the design of heat exchangers involving the boiling of mixtures in channels.
      Citation: Fluids
      PubDate: 2023-03-02
      DOI: 10.3390/fluids8030090
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 91: Computational Fluid Dynamics Modelling of
           Two-Phase Bubble Columns: A Comprehensive Review

    • Authors: Giorgio Besagni, Nicolò Varallo, Riccardo Mereu
      First page: 91
      Abstract: Bubble columns are used in many different industrial applications, and their design and characterisation have always been very complex. In recent years, the use of Computational Fluid Dynamics (CFD) has become very popular in the field of multiphase flows, with the final goal of developing a predictive tool that can track the complex dynamic phenomena occurring in these types of reactors. For this reason, we present a detailed literature review on the numerical simulation of two-phase bubble columns. First, after a brief introduction to bubble column technology and flow regimes, we discuss the state-of-the-art modelling approaches, presenting the models describing the momentum exchange between the phases (i.e., drag, lift, turbulent dispersion, wall lubrication, and virtual mass forces), Bubble-Induced Turbulence (BIT), and bubble coalescence and breakup, along with an overview of the Population Balance Model (PBM). Second, we present different numerical studies from the literature highlighting different model settings, performance levels, and limitations. In addition, we provide the errors between numerical predictions and experimental results concerning global (gas holdup) and local (void fraction and liquid velocity) flow properties. Finally, we outline the major issues to be solved in future studies.
      Citation: Fluids
      PubDate: 2023-03-03
      DOI: 10.3390/fluids8030091
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 92: Detection and Recognition of the Underwater
           Object with Designated Features Using the Technical Stereo Vision System

    • Authors: Vadim Kramar, Aleksey Kabanov, Oleg Kramar, Sergey Fateev, Valerii Karapetian
      First page: 92
      Abstract: The article discusses approaches to solving the problems of detecting, recognizing, and localizing an object with given distinctive features in an aquatic environment using a technical stereo vision system, taking into account restrictions. The stereo vision system is being developed as part of the task in which the AUV, for the purpose of conducting a monitoring mission, follows from the starting point of its route along a given trajectory in order to detect and classify an object with known characteristics and determine its coordinates using a technical stereo vision system at a distance up to 5 m from it with appropriate water clarity. The developed program for the system of the technical stereo vision should provide the AUV with the following information: video sequence; a frame with an image of the detected object; previously unknown characteristics of the object if it is possible to detect them (color, size or shape); distance to the object from the technical stereo vision system; and linear coordinates relative to the technical stereo vision system. Testing of the developed software was carried out on the operating module of the stereo vision installed on the AUV in the underbody compartment. The study was carried out in the pool and in open water. The experiments performed have shown the effectiveness of the developed system when used in conjunction with an underwater robot.
      Citation: Fluids
      PubDate: 2023-03-07
      DOI: 10.3390/fluids8030092
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 93: Control of MHD Flow and Heat Transfer of a
           Micropolar Fluid through Porous Media in a Horizontal Channel

    • Authors: Miloš Kocić, Živojin Stamenković, Jelena Petrović, Jasmina Bogdanović-Jovanović
      First page: 93
      Abstract: The problem considered in this paper is a steady micropolar fluid flow in porous media between two plates. This model can be used to describe the flow of some types of fluids with microstructures, such as human and animal blood, muddy water, colloidal fluids, lubricants and chemical suspensions. Fluid flow is a consequence of the constant pressure gradient along the flow, while two parallel plates are fixed and have different constant temperatures during the fluid flow. Perpendicular to the flow, an external magnetic field is applied. General equations of the problem are reduced to ordinary differential equations and solved in the closed form. Solutions for velocity, microrotation and temperature are used to explain the influence of the external magnetic field (Hartmann number), the characteristics of the micropolar fluid (coupling and spin gradient viscosity parameter) and the characteristics of the porous medium (porous parameter) using graphs. The results obtained in the paper show that the increase in the additional viscosity of micropolar fluids emphasizes the microrotation vector. Moreover, the analysis of the effect of the porosity parameter shows how the permeability of a porous medium can influence the fluid flow and heat transfer of a micropolar fluid. Finally, it is shown that the influence of the external magnetic field reduces the characteristics of micropolar fluids and tends to reduce the velocity field and make it uniform along the cross-section of the channel.
      Citation: Fluids
      PubDate: 2023-03-08
      DOI: 10.3390/fluids8030093
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 94: On the Determination of the 3D Velocity Field in
           Terms of Conserved Variables in a Compressible Ocean

    • Authors: Rémi Tailleux
      First page: 94
      Abstract: Explicit expressions of the 3D velocity field in terms of the conserved quantities of ideal fluid thermocline theory, namely the Bernoulli function, density, and potential vorticity, are generalised in this paper to a compressible ocean with a realistic nonlinear equation of state. The most general such expression is the ‘inactive wind’ solution, an exact nonlinear solution of the inviscid compressible Navier–Stokes equation that satisfies the continuity equation as a consequence of Ertel’s potential vorticity theorem. However, due to the non-uniqueness of the choice of the Bernoulli function, such expressions are not unique and primarily differ in the magnitude of their vertical velocity component. Due to the thermobaric nonlinearity of the equation of state, the expression for the 3D velocity field of a compressible ocean is found to resemble its ideal fluid counterpart only if constructed using the available form of the Bernoulli function, the Bernoulli equivalent of Lorenz’s available potential energy (APE). APE theory also naturally defines a quasi-material, approximately neutral density variable known as the Lorenz reference density. This density variable, in turn, defines a potential vorticity variable that is minimally affected by thermobaric production, thus providing all the necessary tools for extending most results of ideal fluid thermocline theory to a compressible ocean.
      Citation: Fluids
      PubDate: 2023-03-08
      DOI: 10.3390/fluids8030094
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 95: Modelling and Simulation of Heat Exchanger with
           Strong Dependence of Oil Viscosity on Temperature

    • Authors: Dinara Kurmanova, Nurbolat Jaichibekov, Anton Karpenko, Konstantin Volkov
      First page: 95
      Abstract: The heating of oil and oil products is widely used to reduce energy losses during transportation. An approach is developed to determine the effective length of the heat exchanger and the temperature of the cold coolant (oil) at its outlet in the case of a strong dependence of oil viscosity on temperature. Oil from the Uzen field (Kazakhstan) is considered as a heated coolant, and water is considered as a heating component. The method of the log–mean temperature difference, modified for the case of variable viscosity, and the methods of computational fluid dynamics (CFD) are used for calculations. The results of the numerical calculations are compared with the data obtained on the basis of a theoretical approach at a constant viscosity. When using a theoretical approach with a constant or variable viscosity, the heat transfer coefficients to cold and hot coolants are found using criterion dependencies. The Reynolds-averaged Navier–Stokes (RANS) and a turbulence model that takes into account the laminar–turbulent transition are applied. In the case of variable oil viscosity, a transition from the laminar flow regime to the turbulent one is manifested, which has a significant effect on the effective length of the heat exchanger. The obtained results of the CFD calculations are of interest for the design of heat exchangers of a new type, for example, helicoid ones.
      Citation: Fluids
      PubDate: 2023-03-08
      DOI: 10.3390/fluids8030095
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 96: Numerical Analysis of Linear Traveling Wave in
           Rotating Rayleigh–Bénard Convection with an Adiabatic

    • Authors: Toshio Tagawa
      First page: 96
      Abstract: In rotating Rayleigh–Bénard problems, convection with traveling waves may occur near the sidewalls. The Rayleigh number, Taylor number and Prandtl number are involved in this phenomenon, and the convection mode is determined depending on their values. We focused on the onset of this convection with traveling waves under the assumption that centrifugal force is neglected. By conducting two-dimensional linear stability analyses assuming periodicity of the flow and temperature fields along the sidewall direction, we investigated the effect of the Taylor number and the Prandtl number on the critical Rayleigh number and also attempted to understand the phenomenon qualitatively through three-dimensional visualizations. It was exhibited that as the Taylor number increases, the wave number, the Rayleigh number and the phase speed are found to increase. On the other hand, as the Prandtl number decreases, the wavenumber and the Rayleigh number decrease, but the phase velocity increases. The present analyses suggest that convection modes localized near the sidewalls are unlikely to emerge for low Prandtl number cases, which are comparable to those of liquid metals.
      Citation: Fluids
      PubDate: 2023-03-08
      DOI: 10.3390/fluids8030096
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 97: Comparison of Mean Properties of Turbulent Pipe
           and Channel Flows at Low-to-Moderate Reynolds Numbers

    • Authors: Carmine Di Nucci, Rafik Absi
      First page: 97
      Abstract: We focus on the fully developed turbulent flow in circular pipes and channels. We provide a comparison of the mean velocity profiles, and we compute the values of the global indicators, such as the skin friction, the mean velocity, the centerline velocity, the displacement thickness, and the momentum thickness. The comparison is done at low-to-moderate Reynolds numbers. For channel flow, we deduced the mean velocity profiles using an indirect turbulent model; for pipe flow, we extracted the needed information from a direct numerical simulation database available in the open literature. A one-to-one comparison of these values at identical Reynolds numbers provides a deep insight into the difference between pipe and channel flows. This line of reasoning allows us to highlight some deviations among the mean velocity profiles extracted from different pipe databases.
      Citation: Fluids
      PubDate: 2023-03-08
      DOI: 10.3390/fluids8030097
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 98: Control of Chemoconvection in a Rectangular Slot
           by Changing Its Spatial Orientation

    • Authors: Elena Mosheva, Ramil Siraev, Dmitry Bratsun
      First page: 98
      Abstract: Recently, we found that a two-layer miscible system placed in a vertical slab reactor shows an occurrence of a density shock-wave-like pattern. This wave resembles a turbulent bore separating immobile fluid and an area of intense mixing. It travels away from the convective core of the system and is highly dependent on the intensity of a gravity-dependent chemoconvection in the cocurrent flow. The novelty of this work is that we demonstrate that the change in angle between gravity and wave direction allows controlling the chemoconvection intensity and, consequently, the rate of a spatially-extended reaction. We study both experimentally and numerically the effect of the spatial orientation of a slab reactor to a gravity field on a flow structure induced by a neutralization reaction. In experiments, we use aqueous mixtures of nitric acid and sodium hydroxide. We apply the Fizeau interferometry to visualize the flow and use the PIV method to measure the fluid velocity. The mathematical model includes reaction–diffusion–convection equations that describe 3D flows. We study the flow modifications with a change in the inclination angle from 0 to 90 degrees. At small angles (up to 30), the cocurrent flow becomes spatially heterogeneous, and the fields of salt and acid are separated. If the inclination exceeds 50 degrees, the wavefront is deformed, and the wave breaks up, resulting in a sharp decrease in the reaction rate.
      Citation: Fluids
      PubDate: 2023-03-09
      DOI: 10.3390/fluids8030098
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 99: Simulation of Particulate Matter Structure
           Detachment from Surfaces of Wall-Flow Filters for Elevated Velocities
           Applying Lattice Boltzmann Methods

    • Authors: Nicolas Hafen, Jan E. Marquardt, Achim Dittler, Mathias J. Krause
      First page: 99
      Abstract: Rearrangement events in wall-flow filters lead to the formation of specific deposition patterns, which affect a filter’s pressure drop, its loading capacity and the separation efficiency. A universal and consistent formulation of probable causes and influence factors does not exist and appropriate calculation models that enable a quantification of respective influence factors are missing. In this work, a previously developed lattice Boltzmann method, which has been used with inflow velocities of up to 2 m s−1, is applied to elevated velocities of up to 60 m s−1. The particle-free flow, a single layer fragment and a deposition layer during break-up are investigated as three different scenarios. One goal of this work is a comprehensive quantification of the stability and accuracy of both particle-free and particle-including flows, considering static, impermeable deposition-layer fragments. A second goal is the determination of the hydrodynamic surface forces and the deduction of the local detachment likelihood of individual layer fragments. Satisfactory stability and accuracy can be shown for fluid velocity, fluid pressure and the hydrodynamic forces. When considering layer fragments, the parameter domain turns out to be limited to inflow velocities of 28 m s−1. It is shown that fragment detachment rather occurs consecutively and regions of no possible detachment are identified. The work contributes to an understanding of rearrangement events and respective deposition pattern predictions and enables potential optimizations in engine performance, fuel consumption and the service life of wall-flow filters.
      Citation: Fluids
      PubDate: 2023-03-10
      DOI: 10.3390/fluids8030099
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 100: A Review of Preconditioning and Artificial
           Compressibility Dual-Time Navier–Stokes Solvers for Multiphase Flows

    • Authors: Van-Tu Nguyen, Warn-Gyu Park
      First page: 100
      Abstract: This review paper aims to summarize recent advancements in time-marching schemes for solving Navier–Stokes (NS) equations in multiphase flow simulations. The focus is on dual-time stepping, local preconditioning, and artificial compressibility methods. These methods have proven to be effective in achieving high time accuracy in simulations, as well as converting the incompressible NS equations into a hyperbolic form that can be solved using compact schemes, thereby accelerating the solution convergence and allowing for the simulation of compressible flows at all Mach numbers. The literature on these methods continues to grow, providing a deeper understanding of the underlying physical processes and supporting technological advancements. This paper also highlights the imposition of dual-time stepping on both incompressible and compressible NS equations. This paper provides an updated overview of advanced methods for the CFD community to continue developing methods and select the most suitable two-phase flow solver for their respective applications.
      Citation: Fluids
      PubDate: 2023-03-16
      DOI: 10.3390/fluids8030100
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 101: Turbulence via Intermolecular Potential:
           Viscosity and Transition Range of the Reynolds Number

    • Authors: Rafail V. Abramov
      First page: 101
      Abstract: Turbulence in fluids is an ubiquitous phenomenon, characterized by spontaneous transition of a smooth, laminar flow to rapidly changing, chaotic dynamics. In 1883, Reynolds experimentally demonstrated that, in an initially laminar flow of water, turbulent motions emerge without any measurable external disturbance. To this day, turbulence remains a major unresolved phenomenon in fluid mechanics; in particular, there is a lack of a mathematical model where turbulent dynamics emerge naturally from a laminar flow. Recently, we proposed a new theory of turbulence in gases, according to which turbulent motions are created in an inertial gas flow by the mean field effect of the intermolecular potential. In the current work, we investigate the effect of viscosity in our turbulence model by numerically simulating the air flow at normal conditions in a straight pipe for different values of the Reynolds number. We find that the transition between laminar and turbulent flow in our model occurs, without any deliberate perturbations, as the Reynolds number increases from 2000 to 4000. As the simulated flow becomes turbulent, the decay rate of the time averaged Fourier spectrum of the kinetic energy in our model approaches Kolmogorov’s inverse five-thirds law. Both results are consistent with experiments and observations.
      Citation: Fluids
      PubDate: 2023-03-18
      DOI: 10.3390/fluids8030101
      Issue No: Vol. 8, No. 3 (2023)
  • Fluids, Vol. 8, Pages 33: Acknowledgment to the Reviewers of Fluids in

    • Authors: Fluids Editorial Office Fluids Editorial Office
      First page: 33
      Abstract: High-quality academic publishing is built on rigorous peer review [...]
      Citation: Fluids
      PubDate: 2023-01-17
      DOI: 10.3390/fluids8020033
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 34: A Generalized Diffusion Equation: Solutions and
           Anomalous Diffusion

    • Authors: Ervin K. Lenzi, Aloisi Somer, Rafael S. Zola, Luciano R. da Silva, Marcelo K. Lenzi
      First page: 34
      Abstract: We investigate the solutions of a generalized diffusion-like equation by considering a spatial and time fractional derivative and the presence of non-local terms, which can be related to reaction or adsorption–desorption processes. We use the Green function approach to obtain solutions and evaluate the spreading of the system to show a rich class of behaviors. We also connect the results obtained with the anomalous diffusion processes.
      Citation: Fluids
      PubDate: 2023-01-17
      DOI: 10.3390/fluids8020034
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 35: Interfacial Dynamics of Miscible Displacement of
           Shear-Thinning Fluid in a Vertical Channel

    • Authors: Yao Zhang, Andrianifaliana H. Rabenjafimanantsoa, Hans Joakim Skadsem
      First page: 35
      Abstract: The displacement of a shear-thinning fluid by a denser and less viscous Newtonian fluid in a vertical duct is investigated using experiments and numerical simulations. We study how shear-thinning and increased viscosity contrast between the fluids affect the displacement. Our results show that the degree of shear-thinning significantly influences the development of interfacial patterns and the growth of perturbations. In the weakly shear-thinning regime, the displacement progresses as a stable displacement with no visible instabilities. Increasing the viscosity of the displaced fluids result in a Saffman–Taylor type instability with several finger-shaped channels carved across the width of the duct. In the strongly shear-thinning regime, a unique viscous finger with an uneven interface is formed in the middle of the displaced fluid. This finger eventually breaks through at the outlet, leaving behind considerably stagnant wall layers at the duct side walls. We link the onset of viscous fingering instability to the viscosity contrast between the fluids, and the stabilizing density difference, as expressed through a modified, unperturbed pressure gradient for the two fluids. Numerical simulations are performed with both an initial flat interface, and with a perturbed interface, and we find good qualitative agreement between experimental observations and computations.
      Citation: Fluids
      PubDate: 2023-01-18
      DOI: 10.3390/fluids8020035
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 36: Instability Problems and Density-Driven
           Convection in Saturated Porous Media Linking to Hydrogeology: A Review

    • Authors: Elena Soboleva
      First page: 36
      Abstract: Investigations of fluid instability and density-driven convection in soils and rocks are motivated by both natural phenomena giving rise to ecological problems, and human activities. Knowledge about the admixture transportation by underground fluid flows driven by the gravity force is relevant, for example, to succeed in preventing degradation of soil quality or to improve the efficiency of carbon capture and sequestration technologies. We focus on fully saturated porous media containing two-component miscible fluid systems and consider the dynamic processes, which can be reduced to one of three principal problems, namely one-sided convection, two-sided convection, or convection caused by evaporation. This work reviews the main achievements in the field with more attention to the recent literature. Dependence of the convection onset on perturbations of physical parameters, asymmetric development of the Rayleigh–Taylor instability, appearance of salt drops under the evaporation surface, and other important findings are reported in the review.
      Citation: Fluids
      PubDate: 2023-01-18
      DOI: 10.3390/fluids8020036
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 37: Valorization of Lignocellulosic Wastes Material
           for Efficient Adsorption of a Cationic Azo Dye and Sludge Recycling as a
           Reinforcement of Thermoplastic Composite

    • Authors: Maria Jabri, Younes Dehmani, Ilyasse Loulidi, Abderahim Kali, Abdelouahed Amar, Hassane Lgaz, Chaimaa Hadey, Fatima Boukhlifi
      First page: 37
      Abstract: This work explored the adsorption of Malachite Green (MG) dye by Acorn Pericarp (AP) in the context of biomass valorization. The Acorn Pericarp was analyzed by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction. The adsorption capacity under optimal experimental conditions was studied at different adsorbent doses, the initial concentration times of the dye and pH. The results presented in this work on the adsorption kinetics of MG showed that the pseudo-first-order model (R2 = 0.9971) better described the adsorption kinetics at 10−5 M. The experimental isotherms showed that Acorn Pericarp adsorption followed the Langmuir isotherm model (R2 = 0.9889). The thermodynamic study showed that MG adsorption is endothermic (ΔH° > 0) and spontaneous (ΔG° < 0). For a sustainable industry, the sludge was converted into reinforcement of polystyrene using in-situ polymerization with 10% by weight of filler. A morphological and structural analysis was performed using SEM and FTIR, the results of characterization showed that the AP sludge was incorporated well into the PS matrix.
      Citation: Fluids
      PubDate: 2023-01-18
      DOI: 10.3390/fluids8020037
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 38: Intensification of Droplet Disintegration for
           Liquid–Liquid Systems in a Pulsating Flow Type Apparatus by Adding
           an Inert Gas

    • Authors: Maksim P. Vasilev, Rufat Sh. Abiev
      First page: 38
      Abstract: Experimental studies have revealed that the introduction of a small amount (0.5% by volume) of permanent and chemically inert gas bubbles leads to the intensification of droplets disintegration in a liquid–liquid system (emulsification) in a pulsating flow type apparatus. The liquids used were water (continuous phase) and oil (dispersed phase) at room temperature, and nitrogen was used as a gas. The gas hold-up was varied in the range of 0% to 4%. The volume fraction of the dispersed phase (oil) was 1% with respect to the continuous phase. The size of the oil droplets was determined by microphotographs; at least 600 drops were photographed in each experiment. The optimal gas hold-up in terms of the highest interfacial area (for the studied conditions) was found to be 0.5%, at which value the droplets’ Sauter mean diameter d32 decreased 1.88 times, and the maximum droplet size decreased 1.3 times, compared with the case without gas input. The effect of decreasing the average droplet size d32 upon the injection of an inert gas in the continuous phase disappears at φin » 2%. The pressure loss at φin ≤ 2% within the measurement error remained constant, while at 4%, it increases by only 5.4%. The role of an inert gas is explained by several factors: (i) a redistribution of momentum over the volume of liquid; (ii) the occurrence of microflows near bubbles and drops, which leads to an increase in shear stresses on the surface of the drops; and (iii) gas bubbles act as pseudocavitation bubbles, whereby when they collapse, they break up adjacent droplets.
      Citation: Fluids
      PubDate: 2023-01-19
      DOI: 10.3390/fluids8020038
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 39: Fully Coupled Fluid–Structure Interaction
           with Heat Transfer Effects in an Adaptive NACA Airfoil

    • Authors: Paolo Caccavale, Benedetto Mele, Marco Brandizzi, Gianpaolo Ruocco
      First page: 39
      Abstract: In the framework of innovative aerodynamics, active airfoils can be developed and exploited based on the integration of shape memory metal alloys (SMAs), allowing for surface adaptation, i.e., shape changes in response to operative thermal inputs, depending on the desired aerodynamic behavior. The purpose of thermally activated shape-changing (TASC) airfoils’ improved capabilities is to offer benefits in terms of aircraft performance and fuel consumption rate. TASC airfoil design hinges upon three intertwined and nonlinear phenomena, namely the solid–fluid–thermal interactions. In this paper, in order to approach the definition of appropriate design parameters, the space of operating variables is explored for the first time by devising a finite element method simulation encompassing the equations of structural motion, energy, and turbulent Reynolds-averaged Navier–Stokes. Such a fully coupled model is then tested by implementing a sensitivity analysis for a preliminary design of a TASC/NACA airfoil. Temperature and velocity distributions are presented and discussed, including new metrics leading to aerodynamic lift calculations. When the efficiency is computed as the lift-to-drag ratio, it is found to vary nonlinearly in the 0–45 range, with the activating power feed in the 0–1000 W range.
      Citation: Fluids
      PubDate: 2023-01-20
      DOI: 10.3390/fluids8020039
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 40: Parametric Study of the Ground-Air Heat
           Exchanger (GAHE): Effect of Burial Depth and Insulation Length

    • Authors: Alfredo Aranda-Arizmendi, Martín Rodríguez-Vázquez, Carlos Miguel Jiménez-Xamán, Rosenberg J. Romero, Moisés Montiel-González
      First page: 40
      Abstract: A parametric study of a ground-to-air heat exchanger (GAHE) using numerical models based on computational fluid dynamics with the finite volume method to evaluate the thermal potential of GAHE is presented. After the validation of the numerical code developed with published experimental data, it is proceeded to the study of the geometric parameters to define those that have the greatest impact on the application potential of GAHE. Climatological variables such as relative humidity, air flow velocity, and inlet air temperature are analyzed, as well as the increase in the thermal conductivity of the soil due to its humidity content. In addition, a study of the optimal installation depth as well as the length of the thermal insulation in the outlet pipe of the GAHE is presented. The results reveal that there is a higher heat exchange potential in the GAHE for an optimal burial depth of 4 m and a length of pipe of 15 m, 30%soil moisture content for heating and 32% for cooling, and a pipe diameter of 0.15 m. The use of thermal insulating is recommended only for the last 2 m of length in the outlet pipe of the GAHE.
      Citation: Fluids
      PubDate: 2023-01-21
      DOI: 10.3390/fluids8020040
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 41: Combination of Riprap and Submerged Vane as an
           Abutment Scour Countermeasure

    • Authors: Abazar Fathi, S. M. Ali Zomorodian, Masih Zolghadr, Aaron Chadee, Yee-Meng Chiew, Bimlesh Kumar, Hector Martin
      First page: 41
      Abstract: Scour is one of the main causes of hydraulic structural failures. The present experimental study examines the use of riprap, submerged vanes, and a combination of these for scour reduction around vertical walls and spill-through abutments under clear-water conditions. Specifically, the influence of placing riprap stones with different apron shapes (geometry) and/or a group of submerged vanes of constant height and length on abutment scour was examined. The main aim is to propose the optimum apron geometry and placement of submerged vanes to (1) reduce edge failure at vertical walls and spill-through abutments; and (2) prevent shear failure at the spill-through abutment (no shear failure is observed around the vertical wall abutment). The results show that using ripraps for scour protection is more effective than submerged vanes. However, the highest reduction in scour depth was achieved when a combination of riprap and submerged vanes was used together. This arrangement can reduce the maximum clear-water scour depth by up to 54% and 39% with vertical walls and spill-through abutments, respectively. Furthermore, selecting appropriate apron scale ratios reduces the required riprap volume by up to 46% and 31% for the vertical wall and spill-through abutment, respectively. In addition, the installation of vanes increased the riprap stability and reduced edge failure in both abutments tested. Finally, using riprap aprons with proper scales ratios at the downstream side of the spill-through abutment also prevents shear failure in this zone.
      Citation: Fluids
      PubDate: 2023-01-21
      DOI: 10.3390/fluids8020041
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 42: Dynamics of Nonmagnetic and Magnetic Emulsions
           in Microchannels of Various Materials

    • Authors: Kalyuzhnaya, Sokolov, Vasilyeva, Sutarina, Ryapolov
      First page: 42
      Abstract: The formation of droplets in microchannels (droplet microfluidics) has a large number of applications, such as in micro-dosing and gas meters. This paper considers the dynamics of direct and inverse emulsions based on water, polydimethylsiloxane, and synthetic and mineral oil in microfluidic chips based on two technologies: glass–parafilm–glass sandwich structures and removable scaffold in a silicone compound. It is shown that wettability, roughness and chip wall material; channel thickness; magnetic fluid flow rate; and magnetic field strength affect the size of emulsion droplets formed in a microfluidic chip. The addition of another mechanism for regulating the hydrodynamics of emulsions using a magnetic field opens up new possibilities for the development of promising devices.
      Citation: Fluids
      PubDate: 2023-01-25
      DOI: 10.3390/fluids8020042
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 43: Turbulence Modeling for Physics-Informed Neural
           Networks: Comparison of Different RANS Models for the Backward-Facing Step

    • Authors: Fabian Pioch, Jan Hauke Harmening, Andreas Maximilian Müller, Franz-Josef Peitzmann, Dieter Schramm, Ould el Moctar
      First page: 43
      Abstract: Physics-informed neural networks (PINN) can be used to predict flow fields with a minimum of simulated or measured training data. As most technical flows are turbulent, PINNs based on the Reynolds-averaged Navier–Stokes (RANS) equations incorporating a turbulence model are needed. Several studies demonstrated the capability of PINNs to solve the Naver–Stokes equations for laminar flows. However, little work has been published concerning the application of PINNs to solve the RANS equations for turbulent flows. This study applied a RANS-based PINN approach to a backward-facing step flow at a Reynolds number of 5100. The standard k-ω model, the mixing length model, an equation-free νt and an equation-free pseudo-Reynolds stress model were applied. The results compared favorably to DNS data when provided with three vertical lines of labeled training data. For five lines of training data, all models predicted the separated shear layer and the associated vortex more accurately.
      Citation: Fluids
      PubDate: 2023-01-26
      DOI: 10.3390/fluids8020043
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 44: High-Speed Digital Photography of Vapor
           Cavitation in a Narrow Gap Flow

    • Authors: Reinke, Beckmann, Ahlers, Ahlrichs, Hammou, Schmidt
      First page: 44
      Abstract: Digital photography of cavitation in narrow gap flows, e.g., lubrication films in journal bearings or squeeze film dampers, demands a high time-resolution and a solution to approaching the particular spatial restrictions. Typically, the lubrication film thickness is in the range of a few microns and the characteristic time for vapor bubble generation and collapse is about one millisecond, respectively. The authors have developed a Journal Bearing Model Experiment, which is designed according to similarity laws providing fully similar flow conditions to real journal flows while offering ideal access to the flow by means of optical measurement equipment. Compared with other methods, e.g., pulsed laser, electrical discharge, tube arrest, applied to produce vapor bubbles, the work on hand applies a dynamic variation of the minimum film thickness to produce suction cavitation, which proves the applicability of this novel approach to study vapor cavitation in fluid films similar to lubricant flows. The results are obtained by means of digital high-speed photography of vapor bubbles from inception to implosion triggered by the dynamic variation of the minimum film thickness of a narrow gap flow. Moreover, the results are set in relation to a general overview of cavitation processes.
      Citation: Fluids
      PubDate: 2023-01-26
      DOI: 10.3390/fluids8020044
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 45: Experimental Study of Wave Processes in Main Gas
           Pipelines under Normal Operating Conditions

    • Authors: Mikhail Sukharev, Olga Kochueva, Anna Zhaglova
      First page: 45
      Abstract: The article presents the results of processing non-emergency pressure measurements that are transmitted via the SCADA hardware and software system to the control center of the main gas pipeline. SCADA is widely used for real-time control of the modes of gas transmission systems. However, the data provided by SCADA contain some information that is not fully analyzed. The performed processing of pressure measurements is a passive experiment aimed at a deeper understanding of the flow processes that occur during the flow of gas in industrial pipelines of high power. As a result of the experiment, four types of wave phenomena were found: single waves of (a) compression and (b) decompression that were both damped as they moved along the pipe, (c) oscillations with an amplitude and frequency practically unchanged in a fixed section within a period of about 2 h, and (d) wave phenomena with a sarply changing amplitude and frequency within a period of several minutes. The characteristics of wave processes, such as the speed of movement and the decreasing or attenuating amplitude of oscillations, were evaluated. For evaluation, models were built that take into account the specifics of information transfer procedures, namely, SCADA’s representation of continuous functions in a discrete form. The results obtained can be used as an additional tool for searching for leaks, such as in fistulas and unauthorized tie-ins, allowing us to more accurately separate the useful signal from the noise. The question is raised about the adequacy of the one-dimensional model for describing the flow processes in the zones of large gradients of regime parameters. The reasons for possible inadequacy and ways to overcome it are indicated.
      Citation: Fluids
      PubDate: 2023-01-26
      DOI: 10.3390/fluids8020045
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 46: Modeling of 3D Blood Flows with Physics-Informed
           Neural Networks: Comparison of Network Architectures

    • Authors: Philipp Moser, Wolfgang Fenz, Stefan Thumfart, Isabell Ganitzer, Michael Giretzlehner
      First page: 46
      Abstract: Machine learning-based modeling of physical systems has attracted significant interest in recent years. Based solely on the underlying physical equations and initial and boundary conditions, these new approaches allow to approximate, for example, the complex flow of blood in the case of fluid dynamics. Physics-informed neural networks offer certain advantages compared to conventional computational fluid dynamics methods as they avoid the need for discretized meshes and allow to readily solve inverse problems and integrate additional data into the algorithms. Today, the majority of published reports on learning-based flow modeling relies on fully-connected neural networks. However, many different network architectures are introduced into deep learning each year, each with specific benefits for certain applications. In this paper, we present the first comprehensive comparison of various state-of-the-art networks and evaluate their performance in terms of computational cost and accuracy relative to numerical references. We found that while fully-connected networks offer an attractive balance between training time and accuracy, more elaborate architectures (e.g., Deep Galerkin Method) generated superior results. Moreover, we observed high accuracy in simple cylindrical geometries, but slightly poorer estimates in complex aneurysms. This paper provides quantitative guidance for practitioners interested in complex flow modeling using physics-based deep learning.
      Citation: Fluids
      PubDate: 2023-01-27
      DOI: 10.3390/fluids8020046
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 47: Dynamics of Magnetic Fluids and Bidisperse
           Magnetic Systems under Oscillatory Shear

    • Authors: Elena Shel’deshova, Alexander Churaev, Petr Ryapolov
      First page: 47
      Abstract: This article presents the results of a study on the dynamics of a volume of magnetic fluid levitating in a uniform magnetic field of an electromagnet experiencing an oscillatory shift. Samples with different physical parameters were considered, and the dependence of the magnetoviscous effect was studied. It showed that the greatest influence on the dynamics of a magnetic fluid that experiences vibrational-shear and magnetic-viscosity effects is exerted by the sample microstructure and the presence of large magnetic particles. The results of this work can be used in the development of a technique for magnetic fluid samples express testing, as well as in the development of acceleration and vibration sensors based on magnetic fluids
      Citation: Fluids
      PubDate: 2023-01-28
      DOI: 10.3390/fluids8020047
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 48: Continuum Models for Bulk Viscosity and
           Relaxation in Polyatomic Gases

    • Authors: Elena Kustova, Mariia Mekhonoshina, Anna Bechina, Semen Lagutin, Yulia Voroshilova
      First page: 48
      Abstract: Bulk viscosity and acoustic wave propagation in polyatomic gases and their mixtures are studied in the frame of one-temperature and multi-temperature continuum models developed using the generalized Chapman–Enskog method. Governing equations and constitutive relations for both models are written, and the dispersion equations are derived. In the vibrationally nonequilibrium multi-component gas mixture, wave attenuation mechanisms include viscosity, thermal conductivity, bulk viscosity, diffusion, thermal diffusion, and vibrational relaxation; in the proposed approach these mechanisms are fully coupled contrarily to commonly used models based on the separation of classical Stokes–Kirchhoff attenuation and relaxation. Contributions of rotational and vibrational modes to the bulk viscosity coefficient are evaluated. In the one-temperature approach, artificial separation of rotational and vibrational modes causes great overestimation of bulk viscosity whereas using the effective internal energy relaxation time yields good agreement with experimental data and molecular-dynamic simulations. In the multi-temperature approach, the bulk viscosity is specified only by rotational modes. The developed two-temperature model provides excellent agreement of theoretical and experimental attenuation coefficients in polyatomic gases; both the location and the value of its maximum are predicted correctly. One-temperature dispersion relations do not reproduce the non-monotonic behavior of the attenuation coefficient; large bulk viscosity improves its accuracy only in the very limited frequency range. It is emphasized that implementing large bulk viscosity in the one-temperature Navier–Stokes–Fourier equations may lead to unphysical results.
      Citation: Fluids
      PubDate: 2023-01-31
      DOI: 10.3390/fluids8020048
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 49: Motion of a Light Free Sphere and Liquid in a
           Rotating Vertical Cylinder of Finite Length

    • Authors: Victor Kozlov, Ekaterina Zvyagintseva, Ekaterina Kudymova, Vlada Romanetz
      First page: 49
      Abstract: The paper is devoted to an experimental study of the fluid motion excited by a light spherical body floating along the axis of a rotating vertical cylinder. The experiments are performed with fast rotation. The high-speed video recording examines the behavior of the body depending on the rotation rate and liquid viscosity. PIV-method is used to investigate the velocity fields of liquid. In the cavity frame, the body excites the motion liquid in the form of a Taylor–Proudman column, the diameter of which is consistent with the body diameter. In the upper column, the liquid performs a retrograde differential rotation, and in the lower, a prograde one. Outside the columns, the differential rotation is practically absent. It is found that the intensity of the retrograde azimuthal motion in the frontal column increases as the body goes up, while the intensity of the prograde rotation in the rear column decreases. As a result, the free body simultaneously with motion along the axis performs differential rotation: in the lower part of the cavity it is prograde, while in the upper one it is retrograde. The intensity of the body differential rotation varies with the longitudinal coordinate linearly and decreases with the dimensionless rotation velocity.
      Citation: Fluids
      PubDate: 2023-02-01
      DOI: 10.3390/fluids8020049
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 50: Aerodynamic Interaction of Migratory Birds in
           Gliding Flight

    • Authors: Fabien Beaumont, Sébastien Murer, Fabien Bogard, Guillaume Polidori
      First page: 50
      Abstract: (1) Background: Many studies suggest that migratory bird groups fly in a V-formation to improve their aerodynamic efficiency, the goal being to reduce their energy expenditure to fly longer distances. To further validate this hypothesis, we numerically simulated the aerodynamic interaction of two gliding migratory birds and evaluated the aerodynamic forces as a function of the bird spacing. (2) Methods: Computational Fluid Dynamics (CFD) was used to model the flow pattern in and around the wake of Canada geese flying at an altitude of 1000 m and a speed of 13.9 m/s. (3) Results: The post-processing of the 3D results revealed a complex flow structure composed of two contra-rotating vortices developing at the wing tip. (4) Conclusions: In a plane perpendicular to the main flow direction, we showed that the bird’s wake could be broken down into two distinct zones: the downwash zone and the upwash zone, the latter being used by birds flying in formation to reduce their energy expenditure. The results of our study suggested an optimal wingtip spacing of -26cm to maximize the lift/drag ratio that characterizes aerodynamic efficiency.
      Citation: Fluids
      PubDate: 2023-02-01
      DOI: 10.3390/fluids8020050
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 51: Understanding Atmospheric Convection Using Large
           Eddy Simulation

    • Authors: Gaurav Dogra, Anupam Dewan, Sandeep Sahany
      First page: 51
      Abstract: Cloud formation is based on the fundamental principle of atmospheric convection, which involves the vertical transport of heat and moisture into an unstable environment. Convective transfer of moisture and heat in the form of turbulent fluxes over the Bay of Bengal (BoB) has not been explored much and is not resolved in global and regional climate models (GCMs and RCMs) due to the coarser grid resolutions used. Therefore, the present study is an attempt to understand the convection phenomenon over the BoB using a high-resolution cloud-resolving large eddy simulation. Due to the lack of observational data over the BoB, initial and boundary conditions were generated using reanalysis data. We found that the LES successfully captured the cloud formation and convection phenomenon. The turbulence in the convection was analyzed by using Reynolds averaging to obtain variances and covariances. The presence of turbulence over the region was observed. The cloud characteristics were verified by conditionally averaging the output fields. The present study paves a pathway to perform various simulations at different atmospheric conditions over the region in order to create a library of high-resolution simulations.
      Citation: Fluids
      PubDate: 2023-02-02
      DOI: 10.3390/fluids8020051
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 52: Evolution of Water Wave Groups in the Forced
           Benney–Roskes System

    • Authors: Montri Maleewong, Roger H. J. Grimshaw
      First page: 52
      Abstract: For weakly nonlinear waves in one space dimension, the nonlinear Schrödinger Equation is widely accepted as a canonical model for the evolution of wave groups described by modulation instability and its soliton and breather solutions. When there is forcing such as that due to wind blowing over the water surface, this can be supplemented with a linear growth term representing linear instability leading to the forced nonlinear Schrödinger Equation. For water waves in two horizontal space dimensions, this is replaced by a forced Benney–Roskes system. This is a two-dimensional nonlinear Schrödinger Equation with a nonlocal nonlinear term. In deep water, this becomes a local nonlinear term, and it reduces to a two-dimensional nonlinear Schrödinger Equation. In this paper, we numerically explore the evolution of wave groups in the forced Benney–Roskes system using four cases of initial conditions. In the one-dimensional unforced nonlinear Schrödinger equa tion, the first case would lead to a Peregrine breather and the second case to a line soliton; the third case is a long-wave perturbation, and the fourth case is designed to stimulate modulation instability. In deep water and for finite depth, when there is modulation instability in the one-dimensional nonlinear Schdrödinger Equation, the two-dimensional simulations show a similar pattern. However, in shallow water where there is no one-dimensional modulation instability, the extra horizontal dimension is significant in producing wave growth through modulation instability.
      Citation: Fluids
      PubDate: 2023-02-02
      DOI: 10.3390/fluids8020052
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 53: A Meshless Algorithm for Modeling the
           Gas-Dynamic Interaction between High-Inertia Particles and a Shock Layer

    • Authors: Andrey Sposobin, Dmitry Reviznikov
      First page: 53
      Abstract: This paper is devoted to numerical modeling of a supersonic flow around a blunt body by a viscous gas with an admixture of relatively large high-inertia particles that, after reflection from the surface, may go beyond the shock layer and change the flow structure dramatically. To calculate the gas-dynamic interaction of moving particles with the shock layer, it is important to take into account the large difference in scales of the flow around the particles and around the body. To make the computations effective, we use a meshless method to solve non-stationary Navier–Stokes equations. The algorithm is based on the approximation of partial derivatives by the least squares method on a set of nodes distributed in the calculation area. Each moving particle is surrounded by a cloud of calculation nodes belonging to its domain and moving with it in space. The algorithm has been tested on the problem of the motion of a single particle and a pair of particles in a supersonic flow around a sphere.
      Citation: Fluids
      PubDate: 2023-02-02
      DOI: 10.3390/fluids8020053
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 54: Thermal Convection of an Ellis Fluid Saturating
           a Porous Layer with Constant Heat Flux Boundary Conditions

    • Authors: Pedro Vayssière Brandão, Michele Celli, Antonio Barletta, Stefano Lazzari
      First page: 54
      Abstract: The present work analyzes the thermal instability of mixed convection in a horizontal porous channel that is saturated by a shear-thinning fluid following Ellis’ rheology. The fluid layer is heated from below by a constant heat flux and cooled from above by the same heat flux. The instability of such a system is investigated by means of a small-disturbances analysis and the resulting eigenvalue problem is solved numerically by means of a shooting method. It is demonstrated that the most unstable modes on the instability threshold are those with infinite wavelength and an analytical expression for such conditions is derived from an asymptotic analysis. Results show that the non-Newtonian character of the fluid has a destabilizing role.
      Citation: Fluids
      PubDate: 2023-02-02
      DOI: 10.3390/fluids8020054
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 55: Numerical Study of Flow Downstream a Step with a
           Cylinder Part 1: Validation of the Numerical Simulations

    • Authors: Milad Abdollahpour, Paola Gualtieri, David F. Vetsch, Carlo Gualtieri
      First page: 55
      Abstract: The backward-facing step flow (BFSF) is a classical problem in fluid mechanics, hydraulic engineering, and environmental hydraulics. The nature of this flow, consisting of separation and reattachment, makes it a problem worthy of investigation. In this study, divided into two parts, the effect of a cylinder placed downstream of the step on the 2D flow structure was investigated. In Part 1, the classical 2D BFSF was validated by using OpenFOAM. The BFSF characteristics (reattachment, recirculation zone, velocity profile, skin friction coefficient, and pressure coefficient) were validated for a step-height Reynolds number in the range from 75 to 9000, covering both laminar and turbulent flow. The numerical results at different Reynolds numbers of laminar flow and four RANS turbulence models (standard k-ε, RNG k-ε, standard k-ω, and SST k-ω) were found to be in good agreement with the literature data. In laminar flow, the average error between the numerical results and experimental data for velocity profiles and reattachment lengths and the skin friction coefficient were lower than 8.1, 18, and 20%, respectively. In turbulent flow, the standard k-ε was the most accurate model in predicting pressure coefficients, skin friction coefficient, and reattachment length with an average error lower than 20.5, 17.5, and 6%, respectively. In Part 2, the effect on the 2D flow structure of a cylinder placed at different horizontal and vertical locations downstream of the step was investigated.
      Citation: Fluids
      PubDate: 2023-02-03
      DOI: 10.3390/fluids8020055
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 56: Non-Singular Burton–Miller Boundary
           Element Method for Acoustics

    • Authors: Qiang Sun, Evert Klaseboer
      First page: 56
      Abstract: The problem of non-unique solutions at fictitious frequencies that can appear in the boundary element method for external acoustic phenomena described by the Helmholtz equation is studied. We propose a method to fully desingularise in an analytical way the otherwise hyper-singular Burton–Miller framework, where the original boundary element method and its normal derivative are combined. The method considerably simplifies the use of higher-order elements, for example, quadratic curved surface elements. The concept is validated using the example of scattering on a rigid sphere and a rigid cube, and its robustness and effectiveness for external sound-wave problems are confirmed.
      Citation: Fluids
      PubDate: 2023-02-05
      DOI: 10.3390/fluids8020056
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 57: A New Rheological Model for Phosphate Slurry

    • Authors: Zeineb Ghoudi, Souhail Maazioui, Fayssal Benkhaldoun, Noureddine Hajjaji
      First page: 57
      Abstract: In this paper, a new rheological model for the flow of phosphate-water suspensions is proposed. The model’s ability to replicate the rheological characteristics of phosphate-water suspensions under different shear rate conditions is evaluated using rheometric tests, and it is found to be in good agreement with experimental data. A comprehensive methodology for obtaining the model parameters is presented. The proposed model is then incorporated into the OpenFoam numerical code. The results demonstrate that the model is capable of reproducing the rheological behavior of phosphate suspensions at both low and high concentrations by comparing it with suitable models for modeling the rheological behavior of phosphate suspensions. The proposed model can be applied to simulate and monitor phosphate slurry flows in industrial applications.
      Citation: Fluids
      PubDate: 2023-02-08
      DOI: 10.3390/fluids8020057
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 58: Ideal Reactors as an Illustration of Solving
           Transport Phenomena Problems in Engineering

    • Authors: Santiago Laín, Mario A. Gandini
      First page: 58
      Abstract: This contribution aims at emphasizing the importance of ideal reactors in the field of environmental engineering and in the education of the corresponding engineers. The exposition presents the mass flow governing equations of the ideal reactors (batch, completely mixed flow, and plug flow reactors) as particular cases derived from the integral version of the conservation of mass of a chemical/biological species. In the case of transient problems and simple kinetics, such expressions result in first-order ordinary differential equations amenable to be solved analytically when they are linear. In this article, it is shown that when they are non-linear, due to the presence of a second-order kinetics reaction, an analytical solution is also possible, a situation not dealt with in the textbooks. Finally, the previous findings are integrated into a teaching proposal addressed to help undergraduate students to solve more efficiently ideal reactor problems.
      Citation: Fluids
      PubDate: 2023-02-08
      DOI: 10.3390/fluids8020058
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 59: The Aerodynamic Effects of a 3D Streamlined Tail
           on the Windsor Body

    • Authors: Howell, Varney, Passmore, Butcher
      First page: 59
      Abstract: The aerodynamic drag reduction of road vehicles is of continuing interest. The drag arising from the rear surfaces is usually the dominant component, but this can be alleviated by the tapering of the rear body. The effects on the aerodynamic characteristics of a simple body from adding an elongated tapered tail have been investigated in a wind tunnel experiment. The streamlined tail consists of a constant rear body side taper added to a constant upper body taper. The results have been compared with an earlier study of the same body with upper body tapering only. The effects of truncating the long tail are explored. Adding the planform tapering reduces the impact of the slant edge vortices, and drag and lift are substantially reduced. The lateral aerodynamic characteristics are largely unaffected.
      Citation: Fluids
      PubDate: 2023-02-08
      DOI: 10.3390/fluids8020059
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 60: Numerical Study of Flow Downstream a Step with a
           Cylinder Part 2: Effect of a Cylinder on the Flow over the Step

    • Authors: Milad Abdollahpour, Paola Gualtieri, David F. Vetsch, Carlo Gualtieri
      First page: 60
      Abstract: In this study, divided into two parts, the effect on a two-dimensional backward-facing step flow (BFSF) of a cylinder placed downstream of the step was numerically investigated. While in Part 1, the numerical simulations carried out without the cylinder were validated using the available literature data, in Part 2 the effect of the cylinder was investigated. In the laminar regime, different Reynolds numbers were considered. In the turbulent regime, the effects on the flow structure of a cylinder placed at different horizontal and vertical locations downstream of the step were comparatively studied. When the cylinder was positioned below the step edge mid-plane, flow over the step was not altered by a cylinder. However, in other locations of a cylinder, the added cylinder modified the structure of flow, increasing the skin friction coefficient in the recirculation zone. Furthermore, the pressure coefficient of the bottom wall increased immediately downstream of the cylinder and farther downstream of the reattachment point and remained stable in the flow recovery process. Moreover, the presence of the step significantly influenced the dynamics of the vortex generation and shedding leading to an asymmetric wake distribution.
      Citation: Fluids
      PubDate: 2023-02-10
      DOI: 10.3390/fluids8020060
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 61: Investigation of Flow-Induced Instabilities in a
           Francis Turbine Operating in Non-Cavitating and Cavitating Part-Load

    • Authors: Mohammad Hossein Arabnejad, Håkan Nilsson, Rickard E. Bensow
      First page: 61
      Abstract: The integration of intermittent renewable energy resources to the grid system requires that hydro turbines regularly operate at part-load conditions. Reliable operation of hydro turbines at these conditions is typically limited by the formation of a Rotating Vortex Rope (RVR) in the draft tube. In this paper, we investigate the formation of this vortex using the scale-resolving methods SST-SAS, wall-modeled LES (WMLES), and zonal WMLES. The numerical results are first validated against the available experimental data, and then analyzed to explain the effect of using different scale-resolving methods in detail. It is revealed that although all methods can capture the main features of the RVRs, the WMLES method provides the best quantitative agreement between the simulation results and experiment. Furthermore, cavitating simulations are performed using WMLES method to study the effect of cavitation on the flow in the turbine. These effects of cavitation are shown to be highly dependent on the amount of vapor in the RVR. If the amount of vapor is small, cavitation induces broadband high-frequency fluctuations in the pressure and forces exerted on the turbine. As the amount of cavitation increases, these fluctuations tend to have a distinct dominant frequency which is different from the frequency of the RVR.
      Citation: Fluids
      PubDate: 2023-02-10
      DOI: 10.3390/fluids8020061
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 62: Optimization of the Circular Experimental
           Channel for PIV Measurements of Internal Aerodynamic Cases

    • Authors: Jaroslav Pulec, Petra Dančová, Jan Novosád
      First page: 62
      Abstract: This paper deals with the design of an experimental circular channel model specially adapted for particle image velocimetry (PIV) measurements of airflow. The goal is to find a simple, fast, and functional approach for creating experimental models. The issue of the required PIV signal quality is defined, and the limits of acquiring relevant PIV data in the interior of circular channels are described, primarily as reflections of direct and scattered laser light caused by light passage through the cylindrical wall. As part of the experiment, measurements of reflections were made on differently coated surfaces of a plexiglass plate. Samples of various combinations of black matt spray, Rhodamine 6G coating, and roughened surface were created. From the presented results, a combination of black matt spray with a layer of Rhodamine 6G spray paint was chosen. The selected modification was further used for the internal modification of the experimental channel surface. Furthermore, a geometry modification to prevent light spread through the tube material is described. Data obtained from measurements before and after channel modification are presented and explained.
      Citation: Fluids
      PubDate: 2023-02-10
      DOI: 10.3390/fluids8020062
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 63: Modeling and Experimental Study on Drying
           Characteristics of Corn Particles with Hot Air in Downward Moving Bed

    • Authors: Hairui Wang, Shuangming Zhang, Haodong Fan, Man Zhang, Nan Hu, Hairui Yang
      First page: 63
      Abstract: With regard to drying fresh grain prior to storage, the drying tower with a downward moving bed with hot air is often used, which always has high energy consumption during operation. To optimize the operation, according to the actual operating parameters of a corn drying tower with hot air, a heat balance model was established, and the heat transfer between the hot air and corn flow in a downward moving bed was analyzed. Since the downward moving time is short, the heat absorbed by corn significantly depends on the heat transfer coefficient, mainly the convective heat transfer, between the hot air and corn surface. To determine the convective heat transfer coefficient, a hot air drying experimental system for corn grains was established, and the effects of hot air temperature and wind speed on the central temperature and moisture content of corn grains were analyzed. Utilizing the heat balance model, the convective heat transfer coefficients between corn particles and hot air were calculated. The total convective heat transfer coefficients are in the range of 39.4–53.8 W/m2 · K. With an average value of 46.7 W/m2 · K, drying energy efficiencies in different drying zones in the drying tower were calculated, and the accuracy of the model was verified by the operation data. Due to the high inlet temperature of hot air, the maximum energy efficiency of the first zone is 60.15%, whereas when the temperature of hot air in the second drying tower is 140 °C, the energy efficiency is only 41.97%. Therefore, under the premise of ensuring the drying rate, the temperature of hot air of the second zone should be appropriately reduced to improve the whole drying energy efficiency.
      Citation: Fluids
      PubDate: 2023-02-10
      DOI: 10.3390/fluids8020063
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 64: Experimental and Numerical Investigation of the
           Aerodynamic Ventilation Drag of Heavy-Duty Vehicle Wheels

    • Authors: Carlos Peiró Frasquet, Daniel Stoll, Timo Kuthada, Andreas Wagner
      First page: 64
      Abstract: Due to current EU regulations, constant-speed testing on test tracks is used for aerodynamic certification of heavy-duty vehicles (HDV). However, the aerodynamic development of HDVs is performed using wind tunnels and computational fluid dynamics (CFD). Both techniques commonly neglect the rotational aerodynamic losses of the wheels—the so-called ventilation drag—that are present when driving on the road. This is due to the fact that there is no full-scale wind tunnel for this type of vehicle with a suitable belt system for the simulation of the wheel rotation. Furthermore, the ventilation drag of HDV wheels has been neglected in CFD due to their almost completely closed rim design. These constraints lead to an underprediction of the aerodynamic forces in comparison to the results under on-road conditions when performing constant-speed tests. In order to investigate the ventilation drag of HDV wheels, measurements were carried out on a 1:4.5 scale generic tractor-trailer model in the Model Scale Wind Tunnel of the University of Stuttgart. The measured aerodynamic forces as well as the measured flow field data provide the basis for the definition and validation of a procedure for analyzing the ventilation drag in CFD. Accordingly, the ventilation drag of a full scale HDV was investigated in CFD. The results show that the tire treading and rim geometry have a significant influence on ventilation drag that contributes to the total aerodynamic drag of the HDV. The present work shows that the ventilation drag has a relevant impact on the total aerodynamic drag of HDVs and should therefore not be neglected. The presented CFD approach thus allows to assess the aerodynamic drag under real on-road conditions in an early stage of the vehicle development.
      Citation: Fluids
      PubDate: 2023-02-10
      DOI: 10.3390/fluids8020064
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 65: Stability and Resolution Analysis of the Wavelet
           Collocation Upwind Schemes for Hyperbolic Conservation Laws

    • Authors: Bing Yang, Jizeng Wang, Xiaojing Liu, Youhe Zhou
      First page: 65
      Abstract: The numerical solution of hyperbolic conservation laws requires algorithms with upwind characteristics. Conventional methods such as the numerical difference method can realize this characteristic by constructing special distributions of nodes. However, there are still no reports on how to construct algorithms with upwind characteristics through wavelet theory. To solve this problem, a system of high-order and stable wavelet collocation upwind schemes was successfully proposed by constructing interpolation wavelets with specific symmetry and smoothness. The effects of the characteristics of the scaling functions on the schemes were explored based on numerical tests and Fourier analysis. The numerical results revealed that the stability of the constructed scheme is affected by the smoothness order, N, and the asymmetry of the scaling function. The dissipation analysis suggested that schemes with N ∈ even have negative dissipation coefficients, leading to unstable behaviors. Only scaling functions with N ∈ odd and a bias magnitude of 1 can be used to construct stable upwind schemes due to the non-negative dissipation coefficients. Typical numerical examples verified the effectiveness of the proposed method, which is proved to have high accuracy and efficiency in solving high-speed flow problems with multi-scale smooth structures and discontinuities.
      Citation: Fluids
      PubDate: 2023-02-13
      DOI: 10.3390/fluids8020065
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 66: Heat Transfer Correlations for Smooth and Rough

    • Authors: Sepehr Samadani, François Morency
      First page: 66
      Abstract: Low-fidelity methods such as the Blade Element Momentum Theory frequently provide rotor aerodynamic performances. However, these methods must be coupled to databases or correlations to compute heat transfer. The literature lacks correlations to compute the average heat transfer around airfoil. The present study develops correlations for an average heat transfer over smooth and rough airfoil. The correlation coefficients were obtained from a CFD database using RANS equations and the Spalart–Allmaras turbulent model. This work studies the NACA 0009, NACA 0012, and NACA 0015 with and without the leading roughness representative of a small ice accretion. The numerical results are validated against lift and drag coefficients from the literature. The heat transfer at the stagnation point compares well with the experimental results. The database indicates a negligible dependency on airfoil thickness. The work presents two correlations from the database analysis: one for the smooth airfoils and one for the rough airfoils. For the zero lift coefficient, the average Nusselt number is maximum. This increases with Re0.636 for the smooth surface and with Re0.85 for the rough surface. As the lift increases, the average Nusselt is reduced by values proportional to the square of the lift coefficient for the smooth surface, while it is reduced by values proportional to Re and the square of the lift coefficient for the rough surface.
      Citation: Fluids
      PubDate: 2023-02-13
      DOI: 10.3390/fluids8020066
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 67: Unsteady Coupled Heat Transfer in the Air and
           Surrounding Rock Mass for Mine Excavations with Distributed Heat Sources

    • Authors: Artem Zaitsev, Andrey Shalimov, Dmitriy Borodavkin
      First page: 67
      Abstract: This paper presents an unsteady coupled heat transfer model in mine air and surrounding rock mass in the presence of distributed heat sources. The case of distributed heat sources is typical when analyzing the temperature distribution in mine excavations equipped with conveyor systems. For this case, the asymptotic value of the air temperature at the end of the mine excavation is determined not only by the heat exchange between the air and surrounding rock mass but also by the thermal power of distributed heat sources and the total airflow. This conclusion is confirmed by the experimental data presented in the paper for a longwall in a potash mine. We formulate the mathematical model and calculate the distribution of air parameters along the length of an excavation, considering heat release from the conveyor and surrounding rock mass. The results show that a distributed heat release is necessary for correctly calculating the air temperature in working areas. The numerical simulations allow us to recommend a redistribution of air between the haulage and conveyor roadways in the presence of distributed heat sources.
      Citation: Fluids
      PubDate: 2023-02-14
      DOI: 10.3390/fluids8020067
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 68: The LNG Flow Simulation in Stationary Conditions
           through a Pipeline with Various Types of Insulating Coating

    • Authors: Ildar Shammazov, Ekaterina Karyakina
      First page: 68
      Abstract: Liquefied natural gas (LNG) is one of the most promising fuels for energy supply because it has a favorable combination of environmental and economic properties in connection with new trends aimed at the development of ecological and sustainable consumption of natural resources, which ensure a constant growth in LNG consumption. The article presents an analytical review of the main technical solutions for the construction of cryogenic pipelines and insulating coating structures. The ANSYS Fluent software was used for simulation of the LNG flow in a pipeline section 10 m long with an outer diameter of 108 mm for three types of insulating coating (polyurethane (PU) foam, aerogel, and vacuum-insulated pipe (VIP)). In addition, an assessment was made of the insulating effect on the LNG temperature distribution along the length of the pipeline. The largest increase in temperature from 113 K to 113.61 K occurs in PU foam-insulated pipes; the smallest was observed in VIP. Further, as an alternative to steel, the use of ultra-high molecular weight polyethylene (UHMWPE) for pipeline material was considered. The optimal result in terms of temperature distributions was obtained while simulating the flow of an LNG pipeline with PU foam by increasing the thickness of the insulating coating to 0.05 m.
      Citation: Fluids
      PubDate: 2023-02-14
      DOI: 10.3390/fluids8020068
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 69: Numerical Study of the Effects of Asymmetric
           Velocity Profiles in a Curvilinear Channel on Migration of Neutral Buoyant

    • Authors: Ryo Naito, Tomohiro Fukui
      First page: 69
      Abstract: The microstructure and suspended particle behavior should be considered when studying the flow properties exhibited by particle suspension. In addition, particle migration, also known as Segré–Silberberg effects, alters the microstructure of the suspension and significantly affects the viscosity properties of the suspension. Therefore, particle behavior with respect to the changes in mechanical factors should be considered to better understand suspension. In this study, we investigated the particle behavior in asymmetric velocity profiles with respect to the channel center numerically using the lattice Boltzmann method and a two-way coupling scheme. Our findings confirmed that the final equilibrium position of particles in asymmetric velocity profiles converged differently between the outer and inner wall sides with respect to the channel center. This indicates that the mechanical equilibrium position of particles can be changed by asymmetric velocity profiles. In addition, centrifugal force acting on the particles is also important in the study of equilibrium position. These results suggest that the microstructure and viscosity characteristics of a suspension in a pipe could be handled by changes in velocity profiles.
      Citation: Fluids
      PubDate: 2023-02-16
      DOI: 10.3390/fluids8020069
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 70: High Power Output Augmented Vertical Axis Wind

    • Authors: Hayder Salem, Adel Mohammedredha, Abdullah Alawadhi
      First page: 70
      Abstract: Nowadays, wind energy is one of the most cost-effective and environmentally friendly energies in high demand due to shortages in fossil fuels and the necessity to reduce global carbon footprint. One of the main goals of wind turbine development is to increase the power output of the turbine either by increasing the turbine blade swept area or increasing the velocity of the wind. In this article, a proprietary augmentation system was introduced to increase the power output of vertical axis wind turbines (VAWT) by increasing the free stream velocity to more than two folds. The system comprises two identical airfoiled casings within which the turbine/turbines are seated. The results showed that the velocity slightly increases when decreasing the gap between the casing. It was also found that changing the angle of attack of the housing has more impact on the augmented airspeed. CFD technique was used to assess the velocity and flow of air around the system.
      Citation: Fluids
      PubDate: 2023-02-16
      DOI: 10.3390/fluids8020070
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 71: Hemodynamic Assessment of the Pathological Left
           Ventricle Function under Rest and Exercise Conditions

    • Authors: Jana Korte, Thomas Rauwolf, Jan-Niklas Thiel, Andreas Mitrasch, Paulina Groschopp, Michael Neidlin, Alexander Schmeißer, Rüdiger Braun-Dullaeus, Philipp Berg
      First page: 71
      Abstract: Purpose: The analysis of pathological human left ventricular hemodynamics using high-resolved image-based blood flow simulations shows a major potential for examining mitral valve insufficiency (MI) under exercise conditions. Since capturing and simulating the patient-specific movement of the left ventricle (LV) during rest and exercise is challenging, this study aims to propose a workflow to analyze the hemodynamics within the pathologically moving LV. Methods: Patient-specific ultrasound (US) data of ten patients with MI in different stages were captured with three-dimensional real-time echocardiography. US measurements were performed while patients were resting and while doing handgrip exercise (2–4 min work). Patient-specific hemodynamic simulations were carried out based on the captured ventricular wall movement. Velocity and kinetic energy were analyzed for rest and exercise and for the different MI stages. Results: The results reveal a dependency of the kinetic energy over time in the ventricular volume curves. Concerning the comparison between rest and exercise, the left ventricular function reveals lower systolic kinetic energy under exercise (kinetic energy normalized by EDV; mean ± standard deviation: rest = 0.16 ± 0.14; exercise = 0.06 ± 0.05; p-value = 0.04). Comparing patients with non-limiting (MI I) and mild/moderate (MI II/III) MI, lower velocities (mean ± standard deviation: non-limiting = 0.10 ± 0.03; mild/moderate = 0.06 ± 0.02; p-value = 0.01) and lower diastolic kinetic energy (kinetic energy normalized by EDV; mean ± standard deviation: non-limiting = 0.45 ± 0.30; mild/moderate = 0.20 ± 0.19; p-value = 0.03) were found for the latter. Conclusion: With the proposed workflow, the hemodynamics within LVs with MI can be analyzed under rest and exercise. The results reveal the importance of the patient-specific wall movement when analyzing intraventricular hemodynamics. These findings can be further used within patient-specific simulations, based on varying the imaging and segmentation methods.
      Citation: Fluids
      PubDate: 2023-02-16
      DOI: 10.3390/fluids8020071
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 72: Calculation of Thermodynamic Characteristics and
           Sound Velocity for Two-Dimensional Yukawa Fluids Based on a Two-Step
           Approximation for the Radial Distribution Function

    • Authors: Ilnaz I. Fairushin, Anatolii V. Mokshin
      First page: 72
      Abstract: We propose a simple two-step approximation for the radial distribution function of a one-component two-dimensional Yukawa fluid. This approximation is specified by the key parameters of the system: coupling parameter and screening parameter. On the basis of this approximation, analytical expressions are obtained for the same thermodynamic quantities as internal energy, internal pressure, excess entropy in the two-particle approximation, and also longitudinal sound velocity. The theoretical results show an agreement with the results obtained in the case of a true radial distribution function.
      Citation: Fluids
      PubDate: 2023-02-17
      DOI: 10.3390/fluids8020072
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 73: Investigating the Linear Dynamics of the
           Near-Field of a Turbulent High-Speed Jet Using Dual-Particle Image
           Velocimetry (PIV) and Dynamic Mode Decomposition (DMD)

    • Authors: Vishal Chaugule, Alexis Duddridge, Tushar Sikroria, Callum Atkinson, Julio Soria
      First page: 73
      Abstract: The quest for the physical mechanisms underlying turbulent high-speed jet flows is underpinned by the extraction of spatio-temporal coherent structures from their flow fields. Experimental measurements to enable data decomposition need to comprise time-resolved velocity fields with a high-spatial resolution—qualities which current particle image velocimetry hardware are incapable of providing. This paper demonstrates a novel approach that addresses this challenge through the implementation of an experimental high-spatial resolution dual-particle image velocimetry methodology coupled with dynamic mode decomposition. This new approach is exemplified by its application in studying the dynamics of the near-field region of a turbulent high-speed jet, enabling the spatio-temporal structure to be investigated by the identification of the spatial structure of the dominant dynamic modes and their temporal dynamics. The spatial amplification of these modes is compared with that predicted by classical linear stability theory, showing close agreement, which demonstrates the powerful capability of this technique to identify the dominant frequencies and their associated spatial structures in high-speed turbulent flows.
      Citation: Fluids
      PubDate: 2023-02-17
      DOI: 10.3390/fluids8020073
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 74: Kinetic Features of Cd and Zn Cathodic
           Formations in the Membrane Electrolysis Process

    • Authors: Vasyl Serdiuk, Ivan Pavlenko, Svitlana Bolshanina, Vsevolod Sklabinskyi, Sylwia Włodarczak, Andżelika Krupińska, Magdalena Matuszak, Zdzisław Bielecki, Marek Ochowiak
      First page: 74
      Abstract: Chromate and dichromate solutions used for the activation and passivation of cadmium and zinc galvanic coatings of metal products are widely used due to their ability to form corrosion-protective films. Therefore, in this article, we examined the kinetic features of the cathodic deposition of Cd and Zn during membrane electrolysis. As a result of comprehensive experimental and theoretical studies, the features of Cd and Zn cathodic depositions were analyzed under different hydrodynamic conditions in a submembrane zone of an anolyte. Experimental physicochemical methods such as the experimental analysis of solutions, analytical modeling, and a statistical analysis were used during the research. A regression dependence for evaluating a reaction rate constant was assessed based on the least-square approximation of the proposed model. As a result, the peculiarities of the cathodic formations for Cd and Zn during the membrane electrolysis process were analyzed. The effect of mechanical mixing with different values of the Reynolds number on the deposition of Cd and Zn on a cathode was evaluated. A change in Cd2+ and Zn2+ ion concentrations was also considered during the research. Overall, the obtained results increased the Cd deposition rate by 2.2 times using an active hydrodynamic environment with the anolyte.
      Citation: Fluids
      PubDate: 2023-02-17
      DOI: 10.3390/fluids8020074
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 75: Effect of Internal Waves on Moving Small Vessels
           in the Sea

    • Authors: Andrey Serebryany
      First page: 75
      Abstract: Internal waves are responsible for many important processes in the ocean environment (ocean ventilation, energy transfer from large-scale processes to turbulence, etc.). Our goal is to draw attention to the relatively little studied effect of internal waves on ships at sea. We encountered this effect many times while working on the study of internal waves in the shelf zone. The work was carried out from a yacht equipped with the “Rio Grande 600 kHz” ADCP, which makes it possible to measure both the parameters of internal waves and other important parameters of the medium. Two typical examples of the impact of internal waves on a yacht are given. One, when the yacht was at anchor and a train of soliton-like internal waves passed under it. The second, when the yacht was moving and passing over a train of internal waves. Internal waves passing under the moored yacht shifted its position synchronously with the periods of passing waves. A uniformly moving yacht, passing over a package of internal waves synchronously with the period of waves, alternately increased and decreased the speed of its movement. The described effect is explained by the impact on the vessel of the orbital currents of internal waves. Under our conditions, at heights of internal waves reaching 10–15 m, the ship’s speed fluctuations reached 0.30 m/s, which was more than 10% of the ship’s speed.
      Citation: Fluids
      PubDate: 2023-02-18
      DOI: 10.3390/fluids8020075
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 76: The Use of Uncertainty Quantification and
           Numerical Optimization to Support the Design and Operation Management of
           Air-Staging Gas Recirculation Strategies in Glass Furnaces

    • Authors: Carlo Cravero, Davide De Domenico, Davide Marsano
      First page: 76
      Abstract: The reduction in energy consumption and the increasingly demanding emissions regulations have become strategic challenges for every industrial sector. In this context, the glass industry would be one of the most affected sectors due to its high energy demand and emissions productions, especially in terms of NOx. For this reason, various emission abatement systems have been developed in this field and one of the most used is the air staging system. It consists in injecting air into the upper part of the regenerative chamber on the exhaust gases side in order to create the conditions for combustion that reduces NOx emissions. In this work, the combined use of CFD with data analysis techniques offers a tool for the design and management of a hybrid air staging system. Surrogate models of the bypass mass flow rate and uniformity index in the regenerative chamber have been obtained starting from DoE based on different simulations by varying the air mass flow rate of the two injectors located in a bypass duct that connects the two regenerative chambers. This model allows a UQ analysis to verify how the uncertainty of the air injectors can affect the bypass mass flow rate. Finally, an optimization procedure has identified the optimal condition for the best bypass mass flow rates and uniformity of the oxygen concentration in the chamber. High values of the mass flow rate of the pros injector and medium-low values for the cons injectors are identified as operating parameters for best conditions.
      Citation: Fluids
      PubDate: 2023-02-18
      DOI: 10.3390/fluids8020076
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 77: Molecular Dynamics of Nanodroplet Coalescence in
           Quasi-Saturated Vapor

    • Authors: Dmitry Beloborodov, Aleksey Vishnyakov
      First page: 77
      Abstract: The dynamics of coalescence of small Lennard–Jones droplets as a function of droplet size and temperature is explored with molecular simulations. Droplet sizes vary from several hundred to several thousand molecules, and three different temperatures are explored. As the droplets establish contact, a liquid-like bridge between them forms and grows, ultimately leading to a complete coalescence. The dynamics of the bridge growth are consistent with the “collective molecular jumps” mechanism reported in the literature rather than with the continuous interpretation of the coalescence process in terms of capillary and viscous forces. The effective coalescence time shows a linear growth with the droplet sizes. The influence of the larger droplet size is weaker but non-negligible. Surprisingly, practically no dependence of the coalescence time on the temperature is observed. Comparison of the coalescence times with the droplet lifespan in a suspension shows that for reasonably dense suspensions and small droplet sizes, the coalescence time becomes significant and should be accounted for in the theoretical models of aggregation.
      Citation: Fluids
      PubDate: 2023-02-20
      DOI: 10.3390/fluids8020077
      Issue No: Vol. 8, No. 2 (2023)
  • Fluids, Vol. 8, Pages 16: The Role of Inertia in the Onset of Turbulence
           in a Vortex Filament

    • Authors: Jean-Paul Caltagirone
      First page: 16
      Abstract: The decay of the kinetic energy of a turbulent flow with time is not necessarily monotonic. This is revealed by simulations performed in the framework of discrete mechanics, where the kinetic energy can be transformed into pressure energy or vice versa; this persistent phenomenon is also observed for inviscid fluids. Different types of viscous vortex filaments generated by initial velocity conditions show that vortex stretching phenomena precede an abrupt onset of vortex bursting in high-shear regions. In all cases, the kinetic energy starts to grow by borrowing energy from the pressure before the transfer phase to the small turbulent structures. The result observed on the vortex filament is also found for the Taylor–Green vortex, which significantly differs from the previous results on this same case simulated from the Navier–Stokes equations. This disagreement is attributed to the physical model used, that of discrete mechanics, where the formulation is based on the conservation of acceleration. The reasons for this divergence are analyzed in depth; however, a spectral analysis allows finding the established laws on the decay of kinetic energy as a function of the wave number.
      Citation: Fluids
      PubDate: 2023-01-02
      DOI: 10.3390/fluids8010016
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 17: Energy and Information Fluxes at Upper Ocean
           Density Fronts

    • Authors: Pablo Cornejo, Adolfo Bahamonde
      First page: 17
      Abstract: We present large eddy simulations of a midlatitude open ocean front using a modified state-of-the-art computational fluid dynamics code. We investigate the energy and information fluxes at the submesoscale/small-scale range in the absence of any atmospheric forcing. We find submesoscale conditions (Ro∼1, Ri∼1) near the surface within baroclinic structures, related to partially imbalanced frontogenetic activity. Near the surface, the simulations show a significant scale coupling on scales larger than ∼103 (m). This is manifested as a strong direct energy cascade and intense mutual communication between scales, where the latter is evaluated using an estimator based on Mutual Information Theory. At scales smaller than ∼103 (m), the results show near-zero energy flux; however, at this scale range, the estimator of mutual communication still shows values corresponding with a significant level of communication between them. This fact motivates investigation into the nature of the self-organized turbulent motion at this scale range with weak energetic coupling but where communication between scales is still significant and to inquire into the existence of synchronization or functional relationships between scales, with emphasis on the eventual underlying nonlocal processes.
      Citation: Fluids
      PubDate: 2023-01-02
      DOI: 10.3390/fluids8010017
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 18: CFD Simulation of a Hybrid Solar/Electric
           Reactor for Hydrogen and Carbon Production from Methane Cracking

    • Authors: Malek Msheik, Sylvain Rodat, Stéphane Abanades
      First page: 18
      Abstract: Methane pyrolysis is a transitional technology for environmentally benign hydrogen production with zero greenhouse gas emissions, especially when concentrated solar energy is the heating source for supplying high-temperature process heat. This study is focused on solar methane pyrolysis as an attractive decarbonization process to produce both hydrogen gas and solid carbon with zero CO2 emissions. Direct normal irradiance (DNI) variations arising from inherent solar resource variability (clouds, fog, day-night cycle, etc.) generally hinder continuity and stability of the solar process. Therefore, a novel hybrid solar/electric reactor was designed at PROMES-CNRS laboratory to cope with DNI variations. Such a design features electric heating when the DNI is low and can potentially boost the thermochemical performance of the process when coupled solar/electric heating is applied thanks to an enlarged heated zone. Computational fluid dynamics (CFD) simulations through ANSYS Fluent were performed to investigate the performance of this reactor under different operating conditions. More particularly, the influence of various process parameters including temperature, gas residence time, methane dilution, and hybridization on the methane conversion was assessed. The model combined fluid flow hydrodynamics and heat and mass transfer coupled with gas-phase pyrolysis reactions. Increasing the heating temperature was found to boost methane conversion (91% at 1473 K against ~100% at 1573 K for a coupled solar-electric heating). The increase of inlet gas flow rate Q0 lowered methane conversion since it affected the gas space-time (91% at Q0 = 0.42 NL/min vs. 67% at Q0 = 0.84 NL/min). A coupled heating also resulted in significantly better performance than with only electric heating, because it broadened the hot zone (91% vs. 75% methane conversion for coupled heating and only electric heating, respectively). The model was further validated with experimental results of methane pyrolysis. This study demonstrates the potential of the hybrid reactor for solar-driven methane pyrolysis as a promising route toward clean hydrogen and carbon production and further highlights the role of key parameters to improve the process performance.
      Citation: Fluids
      PubDate: 2023-01-02
      DOI: 10.3390/fluids8010018
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 19: Detection of Partial Blockages in Pressurized
           Pipes by Transient Tests: A Review of the Physical Experiments

    • Authors: Bruno Brunone, Filomena Maietta, Caterina Capponi, Huan-Feng Duan, Silvia Meniconi
      First page: 19
      Abstract: Pressure waves, while traveling along pressurized pipes, collect precious information about possible faults (e.g., leaks and partial blockages). In fact, the characteristics of the pressure wave reflected by the fault are strongly related to it. To encourage the use of the transient test-based technologies (TTBTs) for partial blockage (PB) detection in pressurized pipe systems, it can be of interest to critically analyze the available experimental results and to point out the aspects that need to be investigated in more detail, since no review has been executed so far. Such a deficiency has two negative consequences. The first one is that TTBTs are still relegated to limbo by technicians. The second one is that not enough material is available for refining tools to extract all the information contained in the acquired pressure signals and then to pursue an effective PB detection. As main results of the executed analysis, the following issues can be counted: (i) the lack of tests carried out in large diameter and concrete pipes; (ii) the absence of tests carried out in complex pipe systems (e.g., looped networks); and (iii) the extreme need for considering real pipe systems. The fulfillment of the last issue will greatly contribute to the solutions of the other ones.
      Citation: Fluids
      PubDate: 2023-01-03
      DOI: 10.3390/fluids8010019
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 20: CFD Study of Thermal Stratification in a
           Scaled-Down, Toroidal Suppression Pool of Fukushima Daiichi Type BWR

    • Authors: Sampath Bharadwaj Kota, Seik Mansoor Ali, Sreenivas Jayanti
      First page: 20
      Abstract: During the 2011 nuclear catastrophe at Fukushima Daiichi, Unit 3 had a sharper increase in containment pressure than Unit 2, with thermal stratification of the suppression pool cited as one of the contributing factors. In the present work, the buoyancy-induced circulation consequent to steam condensation in a large, toroidal pool of water is studied using computational fluid dynamics (CFD) simulations with a view to understanding the role of important design parameters of the suppression pool system. The tunnelling phenomenon observed in the development of the thermal stratification process is delineated in terms of the establishment of a thermocline. The effects of the number of steam injection points and the cross-section of the pool on thermal stratification characteristics have been investigated through a number of case studies. In all the cases, the surface temperature, which is responsible for over-pressurization of the containment, is found to be significantly higher than the bulk pool temperature. Multiple injection points with the same overall steam flow rate are found to lead to higher surface temperatures due to a shortened circulation path. For the same volume of pool water, the simulations show that a deeper and narrower pool gives rise to significantly higher temperatures than a wider and shallower pool. This is attributed to the relatively deeper penetration of the buoyancy-induced circulation into the pool.
      Citation: Fluids
      PubDate: 2023-01-04
      DOI: 10.3390/fluids8010020
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 21: The Hindered Settling Velocity of Particles of
           Any Shape in Low Reynolds Number Flow

    • Authors: Yuri Mendez
      First page: 21
      Abstract: This article takes insights from a previously derived mathematical framework for the free settling velocity of particles of any shape to model analytical constructs to solve the hindered settling velocity of hard particles of any shape. Because the geometry of the physical environment and continuity can be strictly enforced in the construct model, the relative velocity of the fluid front pumped upward by the settling particles can be found, thus allowing for calculation by subtracting the front velocity from the calculated velocity.
      Citation: Fluids
      PubDate: 2023-01-06
      DOI: 10.3390/fluids8010021
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 22: Numerical Simulation of Carbon
           Dioxide–Nitrogen Mixture Dissolution in Water-Saturated Porous
           Media: Considering Cross-Diffusion Effects

    • Authors: Saeed Mahmoodpour, Mrityunjay Singh, Ramin Mahyapour, Sina Omrani, Ingo Sass
      First page: 22
      Abstract: The possibility of impure carbon dioxide (CO2) sequestration can reduce the cost of these projects and facilitate their widespread adoption. Despite this, there are a limited number of studies that address impure CO2 sequestration aspects. In this study, we examine the convection–diffusion process of the CO2–nitrogen (N2) mixture dissolution in water-saturated porous media through numerical simulations. Cross-diffusion values, as the missing parameters in previous studies, are considered here to see the impact of N2 impurity on dissolution trapping in more realistic conditions. Homogeneous porous media are used to examine this impact without side effects from the heterogeneity, and then simulations are extended to heterogeneous porous media, which are a good representative of the real fields. Heterogeneity in the permeability field is generated with sequential Gaussian simulation. Using the averaged dissolved CO2 and dissolution fluxes for each case, we could determine the onset of different dissolution regimes and behaviors of dissolution fluxes in CO2–N2 mixture dissolution processes. The results show that there is a notable difference between the pure cases and impure cases. Additionally, a failure to recognize the changes in the diffusion matrix and cross-diffusion effects can result in significant errors in the dissolution process. At lower temperatures, the N2 impurity decreases the amount and flux of CO2 dissolution; however, at higher temperatures, sequestrating the CO2–N2 mixture would be a more reasonable choice due to enhancing the dissolution behavior and lowering the project costs. The results of the heterogeneous cases indicate that heterogeneity, in most cases, reduces the averaged dissolved CO2, and dissolution flux and impedes the onset of convection. We believe that the results of this study set a basis for future studies regarding the CO2–N2 mixture sequestration in saline aquifers.
      Citation: Fluids
      PubDate: 2023-01-06
      DOI: 10.3390/fluids8010022
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 23: Assessment of a RANS Transition Model with
           Flapping Foils at Moderate Reynolds Numbers

    • Authors: Luca Alberti, Emanuele Carnevali, Andrea Crivellini
      First page: 23
      Abstract: Numerical simulations based on a high-order discontinuous Galerkin solver were performed to investigate two-dimensional flapping foils at moderate Reynolds numbers, moving with different prescribed harmonic motion laws. A Spalart–Allmaras RANS model with and without an algebraic local transition modification was employed for the resolution of multiple kinematic configurations, considering both moderate-frequency large-amplitude flapping and high-frequency small-amplitude pure heaving. The propulsive performance of the airfoils with the two modelling approaches were tested by referring to experimental and (scale-resolving) numerical data available in the literature. The results show an increase in effectiveness in predicting loads when applying the transition model. This is particularly true at low Strouhal numbers when, after laminar separation at the leading edge, vorticity dynamics appears to have a strong effect on the forces exerted on the profile. Specifically, the transition model more accurately predicts the wake topology emerging in the flow field, which is the primary influence on thrust/drag generation.
      Citation: Fluids
      PubDate: 2023-01-08
      DOI: 10.3390/fluids8010023
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 24: Gas Dynamics of Micro- and Nanofluidic Systems

    • Authors: Oleg Sazhin
      First page: 24
      Abstract: The size of micro- and nanofluidic devices accounts for their operation in modes that differ significantly from those for the corresponding macroscopic counterparts. Deep understanding of gas-dynamic processes occurring in micro- and nanofluidic systems opens new opportunities for the practical use of molecular transport at the micro- and nanoscale. Models and simulation methods with high reliability are described. The article also outlines the important flow parameters which must be considered in the first place to correctly simulate gas-dynamic processes in micro- and nanofluidic systems. The review will be useful as a reference for researchers interested in implementing preliminary analysis in the development and optimization of micro- and nanofluid devices.
      Citation: Fluids
      PubDate: 2023-01-09
      DOI: 10.3390/fluids8010024
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 25: Shear Flows of Dilatant Fluids with Limited
           Shear Rates: Analytical Results and Linear Stability Analysis

    • Authors: Lorenzo Fusi
      First page: 25
      Abstract: In this paper, we study the simple shear flows of a class of dilatant fluids with a limited shear rate. This class of fluids is characterized by shear thickening behavior in which the apparent viscosity tends to infinity as the modulus of the stress approaches a finite threshold. The apparent viscosity function is a logarithmic type with two material parameters. We considered this specific form because it fits very well with the flow curves of some granular suspensions for specific values of the material parameters. Despite the nonlinearity of the constitutive law, it is possible to determine explicit steady-state solutions for a simple shear flow, namely (i) the channel flow; (ii) the flow between coaxial cylinders, and (iii) the flow down an inclined plane. We performed a two-dimensional linear stability analysis to investigate the onset of possible instabilities of the steady basic flow, putting into evidence the dependency of the critical Reynolds number on the material parameters.
      Citation: Fluids
      PubDate: 2023-01-09
      DOI: 10.3390/fluids8010025
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 26: Research on Location Selection of Personnel Door
           and Anemometer Based on FLUENT

    • Authors: Tao Qin, Teng Zhang, Yanwei Duan, Yongli Liu
      First page: 26
      Abstract: The structural design of ventilation structures and the arrangement of anemometers in the main ventilation roadway of an underground mine play an important role in the accurate measurement of air speed. It is one of the important tasks of mine ventilation management and intelligent-ventilation-system construction to determine the position of anemometers. In this paper, the CFD numerical simulation method is used to determine the position of the personnel door in the automatic air door by FLUENT software simulating and analyzing the air-speed cloud diagram and air-pressure cloud diagram in the two-dimensional roadway model. Under the same air speed, comparing the air-speed distribution of different cross-sections in the three-dimensional roadway model when the wide door and the personnel door are opened, the anemometer is set at the 25 m cross-section behind the air door, and the air-speed distribution of the cross-section at different air speeds is simulated. The average air-speed line and the specific installation position of the anemometer on the line are obtained by Origin software. The result shows that the position of the personnel door is 400 mm from the middle line of the roadway, and the measurement error of the anemometer is small on the left side of the roadway (0.41, 2.45) and the right side of the roadway (4.59, 2.43) at 25 m behind the air door, which provides a theoretical basis for the measurement of air speed in a coal mine ventilation roadway.
      Citation: Fluids
      PubDate: 2023-01-10
      DOI: 10.3390/fluids8010026
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 27: Hydrodynamics of an OWC Device in Irregular
           Incident Waves Using RANS Model

    • Authors: Kshma Trivedi, Amya Ranjan Ray, Parothidil Anjusree Krishnan, Santanu Koley, Trilochan Sahoo
      First page: 27
      Abstract: This research examines the hydrodynamic performance of an oscillating water column device placed over a sloping seabed under the influence of irregular incident waves. The numerical model is based on the Reynolds-veraged Navier–Stokes (RANS) equations with a modified k−ω turbulence model and uses the volume-of-fluid (VOF) approach to monitor the air–water interface. To explore the hydrodynamic performance of the OWC device in actual ocean conditions, the Pierson–Moskowitz (P-M) spectrum was used as the incident wave spectrum, together with the four distinct sea states which occur most often along the western coast of Portugal. The numerical simulation offers a comprehensive velocity vector and streamline profiles inside the OWC device’s chamber during an entire cycle of pressure fluctuation. In addition, the impact of the irregular wave conditions on the free-surface elevation at various places, the pressure drop between the chamber and the outside, and the airflow rate via the orifice per unit width of the OWC device are investigated in detail. The results demonstrate that the amplitudes of the inward and outward velocities via the orifice, free-surface elevations, and flow characteristics are greater for more significant wave heights. Further, it is noticed that the power generation and capture efficiency are higher for a seabed having moderate slopes.
      Citation: Fluids
      PubDate: 2023-01-11
      DOI: 10.3390/fluids8010027
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 28: Bulk Viscosity of Dilute Gases and Their

    • Authors: Bhanuday Sharma, Rakesh Kumar, Savitha Pareek
      First page: 28
      Abstract: In this work, we use the Green–Kubo method to study the bulk viscosity of various dilute gases and their mixtures. First, we study the effects of the atomic mass on the bulk viscosity of dilute diatomic gas by estimating the bulk viscosity of four different isotopes of nitrogen gas. We then study the effects of addition of noble gas on the bulk viscosity of dilute nitrogen gas. We consider mixtures of nitrogen with three noble gases, viz., neon, argon, and krypton at eight different compositions between pure nitrogen to pure noble gas. It is followed by an estimation of bulk viscosity of pure oxygen and mixtures of nitrogen and oxygen for various compositions. In this case, three different composition are considered, viz., 25% N2 + 75% O2, 50% N2 + 50% O2, and 78% N2 + 22% O2. The last composition is aimed to represent the dry air. A brief review of works that study the effects of incorporation of bulk viscosity in analysis of various flow situations has also been provided.
      Citation: Fluids
      PubDate: 2023-01-12
      DOI: 10.3390/fluids8010028
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 29: Self-Propelled Swimming of a Flexible Propulsor
           Actuated by a Distributed Active Moment

    • Authors: Changhong Han, Zhiyu Zhang, Xing Zhang
      First page: 29
      Abstract: The self-propelled swimming of a flexible propulsor is numerically investigated by using fluid-structure interaction simulations. A distributed active moment mimicking the muscle actuation in fish is used to drive the self-propulsion. The active moment imposed on the body of the swimmer takes the form of a traveling wave. The influences of some key parameters, such as the wavenumber, the amplitude of moment density and the Reynolds number, on the performance of straight-line swimming are explored. The influence of the ground effect on speed and efficiency is investigated through the simulation of near-wall swimming. The turning maneuver is also successfully performed by adopting a simple evolution law for the leading-edge deflection angle. The results of the present study are expected to be helpful to the design of bio-inspired autonomous underwater vehicles.
      Citation: Fluids
      PubDate: 2023-01-13
      DOI: 10.3390/fluids8010029
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 30: The Effects of Buoyancy on Laminar Heat Transfer
           Rates to Supercritical CO2 in Vertical Upward Flows

    • Authors: Krishnamoorthy Viswanathan, Gautham Krishnamoorthy
      First page: 30
      Abstract: Buoyancy effects in vertical, upward laminar flows can result in an augmentation in heat transfer rates to supercritical CO2 (sCO2) near its pseudocritical temperature (TPC). This is in contrast to corresponding flows in the turbulent regime, or laminar sCO2 flows (with minimum buoyancy effects), where a deterioration in heat transfer near TPC, followed by a recovery phase, have been observed. To exploit these sCO2 heat transfer enhancement characteristics and improve heat exchange efficiencies, the location of the TPC pinch point and the variables controlling these buoyancy effects need to be identified. To fill this void, numerical simulations of sCO2 (at inlet: 8.2 MPa, 265 K) in vertical circular tubes of diameters (D) 0.2–2 mm, heated with constant wall heat fluxes (Q) of 1–4 kW/m2) and inlet Reynolds numbers (Re) of 100, 400, were carried out. The tube lengths were varied to maintain an exit temperature of 320 K (TPC ~ 309 K). The results indicated that buoyancy-augmented laminar heat transfer rates may be expected when Gr/Re2.7 > 10−4 (Gr = Grashof number). A modified Nusselt number correlation in terms of (Gr/Re) is proposed and is observed to fit the observed variations within a mean absolute percentage error < 15%, in most regions.
      Citation: Fluids
      PubDate: 2023-01-14
      DOI: 10.3390/fluids8010030
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 31: Implementation of Flux Limiters in Simulation of
           External Aerodynamic Problems on Unstructured Meshes

    • Authors: A. V. Struchkov, A. S. Kozelkov, R. N. Zhuchkov, K. N. Volkov, D. Yu. Strelets
      First page: 31
      Abstract: The study is dedicated to the peculiarities of implementing the flux limiter of the flow quantity gradient when solving 3D aerodynamic problems using the system of Navier–Stokes equations on unstructured meshes. The paper describes discretisation of the system of Navier–Stokes equations on a finite-volume method and a mathematical model including Spalart–Allmaras turbulence model and the Advection Upstream Splitting Method (AUSM+) computational scheme for convective fluxes that use a second-order approximation scheme for reconstruction of the solution on a facet. A solution of problems with shock wave structures is considered, where, to prevent oscillations at discontinuous solutions, the order of accuracy is reduced due to the implementation of the limiter function of the gradient. In particular, the Venkatakrishnan limiter was chosen. The study analyses this limiter as it impacts the accuracy of the results and monotonicity of the solution. It is shown that, when the limiter is used in a classical formulation, when the operation threshold is based on the characteristic size of the cell of the mesh, it facilitates suppression of non-physical oscillations in the solution and the upgrade of its monotonicity. However, when computing on unstructured meshes, the Venkatakrishnan limiter in this setup can result in the occurrence of the areas of its accidental activation, and that influences the accuracy of the produced result. The Venkatakrishnan limiter is proposed for unstructured meshes, where the formulation of the operation threshold is proposed based on the gas dynamics parameters of the flow. The proposed option of the function is characterized by the absence of parasite regions of accidental activation and ensures its operation only in the region of high gradients. Monotonicity properties, as compared to the classical formulation, are preserved. Constants of operation thresholds are compared for both options using the example of numerical solution of the problem with shock wave processes on different meshes. Recommendations regarding optimum values of these quantities are provided. Problems with a supersonic flow in a channel with a wedge and transonic flow over NACA0012 airfoil were selected for the examination of the limiter functions applicability. The computation was carried out using unstructured meshes consisting of tetrahedrons, truncated hexahedrons, and polyhedrons. The region of accidental activation of the Venkatakrishnan limiter in a classical formulation, and the absence of such regions in case a modified option of the limiter function, is implemented. The analysis of the flow field around a NACA0012 indicates that the proposed improved implementation of the Venkatakrishnan limiter enables an increase in the accuracy of the solution.
      Citation: Fluids
      PubDate: 2023-01-15
      DOI: 10.3390/fluids8010031
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 32: Experimental Observations on Flow
           Characteristics around a Low-Aspect-Ratio Wall-Mounted Circular and Square

    • Authors: Seyed M. Hajimirzaie
      First page: 32
      Abstract: The mean wake structures of a cube (square cylinder) and circular cylinder of height-to-width aspect ratio 1.0, at a Reynolds number of 1.78 × 104 based on the obstacle width, were investigated experimentally. The boundary-layer thickness was 0.14 of the obstacle height. The study was performed using thermal anemometry and two-dimensional digital particle image velocimetry (DPIV). Streamwise structures observed in the mean wake for both cylinders included well-known tip- and horseshoe (HS)-,vortex pairs as well as additional structures akin to the base vortices. In addition to tip-, base-, and HS-vortices, in the near wake of the cube, two more counter-rotating pairs of streamwise structures, including upper and inboard vortices, were observed. The existence of base vortices formed in the near wake for both obstacles is a unique observation and has not been previously reported for such low-aspect-ratio obstacles in thin boundary-layers. A model of arch-vortex evolution was proposed, in which arch structures were deformed by the external shear-flow to explain the observed base-vortices in the cylinder wake. A weak dominant-frequency of St = f0D/U∞ = 0.114 was observed across the height for the cube, while no discernible spectral peaks were apparent in the wake of the cylinder. Cross-spectral analysis revealed the shedding to be symmetric (in-phase) arch-type for the cylinder and predominantly anti-symmetric (out-of-phase) Karman-type for the cube. The study makes fundamental contributions to the understanding of the flow-field surrounding low-aspect-ratio cylinders.
      Citation: Fluids
      PubDate: 2023-01-15
      DOI: 10.3390/fluids8010032
      Issue No: Vol. 8, No. 1 (2023)
  • Fluids, Vol. 8, Pages 2: Formation and Behaviour of Active Droplets and
           Bubbles in a Magnetic Fluid in an Inhomogeneous Magnetic Field

    • Authors: Sokolov, Kaluzhnaya, Shel’deshova, Ryapolov
      First page: 2
      Abstract: This work proposes a new technique for creating active bubbles and droplets with a non-magnetic core and a coating formed by a magnetic fluid. The procedure consists of the injection of a non-magnetic phase into a magnetic one that is supported by the presence of an inhomogeneous magnetic field from the source, which combines an annular magnet and an electromagnet. We explored various modes leading to different active bubbles and drops as well as the influence of the magnetic field on the size, velocity, and acceleration of the formed active droplets. It is shown that active bubbles change their trajectory under the action of a constant magnetic field and also disintegrate under the action of a pulsed one. This provides a new mechanism for controlling the absorption of droplets and bubbles using a magnetic field. Therefore, these results can be applied to create droplet-based microfluidics systems, in which an inhomogeneous magnetic field can be used for focusing droplet and bubble flows in a magnetic fluid.
      Citation: Fluids
      PubDate: 2022-12-21
      DOI: 10.3390/fluids8010002
      Issue No: Vol. 8, No. 1 (2022)
  • Fluids, Vol. 8, Pages 3: Local Scour Patterns around a Bridge Pier with

    • Authors: Valentine Muhawenimana, Nadine Foad, Pablo Ouro, Catherine A. M. E. Wilson
      First page: 3
      Abstract: The performance of cable flow-altering bed scour countermeasures was experimentally evaluated based on the scour reduction, bed morphology, and the effects on the flow field. An unprotected 40 mm diameter pier was compared to piers protected with spiral cables (2, 4, 6, 8 and 10 mm diameters) wrapped at a 15-degree angle for two-bed sediment sizes with median grain sizes of 0.86 and 1.83 mm, for a cylinder Reynolds number of 7120. The scour depth was reduced by the cables by up to 52 percent compared to the unprotected pier case, a reduction that increased with increasing cable diameter for both sediment beds. Scour depth and sediment deposition varied by sediment size, where the scour hole was up to 45 percent deeper for the finer sediment bed than that of the coarser bed. Velocity and turbulence statistics showed that cables attenuated the flow within the scour hole by diminishing the downflow and horseshoe vortex, whereas in the case of finer sediment, spatially averaged turbulent kinetic energy and Reynolds shear stresses were respectively up to 1.4 and 1.8 times higher for the unprotected pier than the protected pier, resulting in scour depth reduction. The presence of the cable also reduced the vortex shedding frequency in the pier wake as indicated by a Strouhal number of around 0.175. The results demonstrate the potential of cable threading as a flow-altering scour countermeasure to reduce bridge pier scour.
      Citation: Fluids
      PubDate: 2022-12-21
      DOI: 10.3390/fluids8010003
      Issue No: Vol. 8, No. 1 (2022)
  • Fluids, Vol. 8, Pages 4: Numerical Analysis Related to the ROCOM
           Pressurized Thermal Shock Benchmark

    • Authors: Thomas Höhne, Sören Kliem
      First page: 4
      Abstract: The development, verification, and validation of Computational Fluid Dynamics (CFD) codes in reference to nuclear power plant (NPP) safety has been a focus of many research organizations over the last few decades. Therefore, a collection of Rossendorf Coolant Mixing Test Facility (ROCOM) CFD-grade experiments was made obtainable to line up a global International Atomic Energy Agency (IAEA) benchmark regarding Pressurized Thermal Shock (PTS) situations. The benchmark experiment describes the complicated flow structures in mixed convection zones of the RPV during PTS events. The experiments were utilized to validate CFD codes. Additionally, an experiment with no buoyancy forces was elite to point out the influence of density variations. Compared to earlier studies, the turbulence models of the CFD code improved a lot. The turbulence modeling approach shows a respectable agreement with the experimental data.
      Citation: Fluids
      PubDate: 2022-12-22
      DOI: 10.3390/fluids8010004
      Issue No: Vol. 8, No. 1 (2022)
  • Fluids, Vol. 8, Pages 5: Navier–Stokes Equations and Bulk Viscosity
           for a Polyatomic Gas with Temperature-Dependent Specific Heats

    • Authors: Shingo Kosuge, Kazuo Aoki
      First page: 5
      Abstract: A system of Navier–Stokes-type equations with two temperatures is derived, for a polyatomic gas with temperature-dependent specific heats (thermally perfect gas), from the ellipsoidal statistical (ES) model of the Boltzmann equation extended to such a gas. Subsequently, the system is applied to the problem of shock-wave structure for a gas with large bulk viscosity (or, equivalently, with slow relaxation of the internal modes), and the numerical results are compared with those based on the ordinary Navier–Stokes equations. It is shown that the latter equations fail to describe the double-layer structure of shock profiles for a gas with large bulk viscosity.
      Citation: Fluids
      PubDate: 2022-12-22
      DOI: 10.3390/fluids8010005
      Issue No: Vol. 8, No. 1 (2022)
  • Fluids, Vol. 8, Pages 6: Evaluation of SPH and FVM Models of Kinematically
           Prescribed Peristalsis-like Flow in a Tube

    • Authors: Xinying Liu, Simon M. Harrison, Paul W. Cleary, David F. Fletcher
      First page: 6
      Abstract: Peristaltic flow is important in many biological processes, including digestion, and forms an important component of any in silico model of the stomach. There is a clear need to verify the simulations of such flows. An analytical solution was identified that can be used for model verification, which gives an equation for the net volumetric flow over a cycle for an applied sinusoidal wall motion. Both a smooth particle hydrodynamics (SPH) code (from the CSIRO), which is being used to develop a stomach model that includes wall motion, buoyancy, acid secretion and food breakdown, and the Ansys Fluent Finite Volume Method (FVM) solver, that is widely used in industry for complex engineering flows, are used in this exercise. Both give excellent agreement with the analytic solution for the net flow over a cycle for a range of occlusion ratios of 0.1–0.6. Very similar velocity fields are obtained with the two methods. The impact of parameters affecting solution stability and accuracy are described and investigated. Having validated the moving wall capability of the SPH model it can be used with confidence in stomach simulations that include wall motion.
      Citation: Fluids
      PubDate: 2022-12-23
      DOI: 10.3390/fluids8010006
      Issue No: Vol. 8, No. 1 (2022)
  • Fluids, Vol. 8, Pages 7: Cancer Exosomes: An Overview and the Applications
           of Flow

    • Authors: Parker Bryant, Vassilios I. Sikavitsas
      First page: 7
      Abstract: Cancer is one of the most prevalent and disruptive diseases affecting the population, and as such, is the subject of major research efforts. Recently, these efforts have been put towards understanding the role that exosomes can play in the progression of cancer. Exosomes are small extracellular vesicles ranging from 40–150 nm in size that carry bioactive molecules like proteins, DNA, RNA, miRNA, and surface receptors. One of the most important features of exosomes is their ability to easily travel throughout the body, extending the reach of parent cell’s signaling capabilities. Cancer derived exosomes (CDEs) carry dangerous cargo that can aid in the metastasis, and disease progression through angiogenesis, promoting epithelial to mesenchymal transition, and immune suppression. Exosomes can transport these molecules to cells in the tumor environment as well as distant premetastatic locations making them an extremely versatile tool in the toolbelt of cancer. This review aims to compile the present knowledge and understanding of the involvement of exosomes in the progression of cancer as well as current production, isolation, and purification methods, with particular interest on flow perfusion bioreactor and microfluidics systems, which allow for accurate modeling and production of exosomes.
      Citation: Fluids
      PubDate: 2022-12-24
      DOI: 10.3390/fluids8010007
      Issue No: Vol. 8, No. 1 (2022)
  • Fluids, Vol. 8, Pages 8: Detailed Investigation of the Droplet Dynamics
           Parameters Produced by Artificially Induced Bag-Breakup Fragmentation

    • Authors: Daniil Sergeev, Alexander Kandaurov, Maksim Vdovin, Yuliya Troitskaya
      First page: 8
      Abstract: This paper presents the results of detailed studies of the processes of droplet formation and its characteristics under conditions of artificially induction of a bag-breakup fragmentation event. A shadow imaging method was used in combination with the high-speed video filming of the side-view fragmentation process. Trajectories and ejection velocity characteristics of the formed droplets are determined by identifying particles in consecutive frames with combined use of Particle Imaging Velocimetry (PIV) and Particle Tracking Velocimetry (PTV). Based on the results of trajectory processing, the distributions of droplet velocities for the selected regions are obtained, and estimates of the ejection velocities at various heights are proposed.
      Citation: Fluids
      PubDate: 2022-12-24
      DOI: 10.3390/fluids8010008
      Issue No: Vol. 8, No. 1 (2022)
  • Fluids, Vol. 8, Pages 9: CFD Analysis of a Hydrostatic Pressure Machine
           with an Open Source Solver

    • Authors: Rodolfo Pienika, José Cataldo, Helena M. Ramos
      First page: 9
      Abstract: The open source and freely available fluid flow solver named caffa3d was adapted to simulate the performance of a medium-scale Hydrostatic Pressure Machine (HPM) with straight radial blades, which had been previously tested in a laboratory facility. A fully detailed explanation of the code caffa3d is not intended to be included in this paper, but some of its main characteristics are mentioned for completeness. In addition, convergence and grid sensitivity analysis were performed in order to assess the adequacy of the model. Evolution of instantaneous power over a few turns of the HPM shows typical blade pass frequency for all operating discharges and another oscillatory phenomena at rotating frequency for higher discharges. The Power—Discharge and Efficiency—Discharge curves obtained from the simulations present a good correlation with the experimental curves, up to the discharge value corresponding to the maximum power of the HPM. The comparison error in power and efficiency remained below 6% for discharge lower than 97.8 L/s. For higher discharge, the flow through the HPM becomes very unsteady, with big eddy structures, underfilling of the buckets and recirculation down to the entry of the channel, significantly reducing the generated power. This behaviour was also observed during the previous experiments. In the present work, the foundations for the study of other types of turbines with caffa3d are laid.
      Citation: Fluids
      PubDate: 2022-12-26
      DOI: 10.3390/fluids8010009
      Issue No: Vol. 8, No. 1 (2022)
  • Fluids, Vol. 8, Pages 10: Wall-Modeled and Hybrid Large-Eddy Simulations
           of the Flow over Roughness Strips

    • Authors: Teresa Salomone, Ugo Piomelli, Giuliano De Stefano
      First page: 10
      Abstract: The flow over alternating roughness strips oriented normally to the mean stream is studied using wall-modeled large-eddy simulations (WMLES) and improved delayed detached-eddy simulations (IDDES) (a hybrid method solving the Reynolds-averaged Navier–Stokes (RANS) equations near the wall and switching to large-eddy simulations (LES) in the core of the flow). The calculations are performed in an open-channel configuration. Various approaches are used to account for roughness by either modifying the wall boundary condition for WMLES or the model itself for IDDES or by adding a drag forcing term to the momentum equations. By comparing the numerical results with the experimental data, both methods with both roughness modifications are shown to reproduce the non-equilibrium effects, but noticeable differences are observed. The WMLES, although affected by the underlying equilibrium assumption, predicts the return to equilibrium of the skin friction in good agreement with the experiments. The velocity predicted by the IDDES does not have memory of the upstream conditions and recovers to the equilibrium conditions faster. Memory of the upstream conditions appears to be a critical factor for the accurate computational modeling of this flow.
      Citation: Fluids
      PubDate: 2022-12-27
      DOI: 10.3390/fluids8010010
      Issue No: Vol. 8, No. 1 (2022)
  • Fluids, Vol. 8, Pages 11: Combined Surface Heating by Laser Beam and
           Subsonic Nitrogen Plasma Jet

    • Authors: Aleksey Chaplygin, Mikhail Kotov, Mikhail Yakimov, Ilya Lukomskii, Semen Galkin, Anatoly Kolesnikov, Andrey Shemyakin, Nikolay Solovyov
      First page: 11
      Abstract: The paper describes new combined heating capability of the IPMech RAS inductively coupled plasma facility VGU-4. A 200 W ytterbium laser was added to the facility as a source of radiative heating. The cylindrical specimen made of the Buran orbital vehicle’s heat-shielding tile material with a black low catalytic coating was exposed to subsonic pure nitrogen plasma jet and laser radiation. The specimen surface temperature reached 1325 ∘C during combined radiative and convective heating. The maximum heat flux obtained in the combined mode for a laser incident power of 47 W and a VGU-4 HF-generator anode power of 22 kW was 32.1 W/cm2. The convective heat flux from the nitrogen plasma jet at the same anode power was 12.6 W/cm2. Adding a laser to an existing inductively coupled plasma facility gives the future opportunity to better simulate entry into the atmospheres of Mars, Venus, the outer planets and their moons.
      Citation: Fluids
      PubDate: 2022-12-28
      DOI: 10.3390/fluids8010011
      Issue No: Vol. 8, No. 1 (2022)
  • Fluids, Vol. 8, Pages 12: Patterning Behavior of Hybrid Buoyancy-Marangoni
           Convection in Inclined Layers Heated from Below

    • Authors: Wasim Waris, Marcello Lappa
      First page: 12
      Abstract: Alongside classical effects driven by gravity or surface tension in non-isothermal fluids, the present experimental study concentrates on other exotic (poorly known) modes of convection, which are enabled in a fluid layer delimited from below by a hot plate and unbounded from above when its container is inclined to the horizontal direction. By means of a concerted approach based on the application of a thermographic visualization technique, multiple temperature measurements at different points and a posteriori computer-based reconstruction of the spatial distribution of wavelengths, it is shown that this fluid-dynamic system is prone to develop a rich set of patterns. These include (but are not limited to), spatially localized (compact) cells, longitudinal wavy rolls, various defects produced by other instabilities and finger-like structures resulting from an interesting roll pinching mechanism (by which a single longitudinal roll can be split into two neighboring rolls with smaller wavelength). Through parametric variation of the tilt angle, the imposed temperature difference and the volume of liquid employed, it is inferred that the observable dynamics are driven by the ability of gravity-induced shear flow to break the in-plane isotropy of the system, the relative importance of surface-tension-driven and buoyancy effects, and the spatially varying depth of the layer. Some effort is provided to identify universality classes and similarities with other out-of-equilibrium thermal systems, which have attracted significant attention in the literature.
      Citation: Fluids
      PubDate: 2022-12-29
      DOI: 10.3390/fluids8010012
      Issue No: Vol. 8, No. 1 (2022)
  • Fluids, Vol. 8, Pages 13: Phase Resolved Simulation of the
           Landau–Alber Stability Bifurcation

    • Authors: Agissilaos G. Athanassoulis
      First page: 13
      Abstract: It has long been known that plane wave solutions of the cubic nonlinear Schrödinger Equation (NLS) are linearly unstable. This fact is widely known as modulation instability (MI), and sometimes referred to as Benjamin–Feir instability in the context of water waves. In 1978, I.E. Alber introduced a methodology to perform an analogous linear stability analysis around a sea state with a known power spectrum, instead of around a plane wave. This analysis applies to second moments, and yields a stability criterion for power spectra. Asymptotically, it predicts that sufficiently narrow and high-intensity spectra are unstable, while sufficiently broad and low-intensity spectra are stable, which is consistent with empirical observations. The bifurcation between unstable and stable behaviour has no counterpart in the classical MI (where all plane waves are unstable), and we call it Landau–Alber bifurcation because the stable regime has been shown to be a case of Landau damping. In this paper, we work with the realistic power spectra of ocean waves, and for the first time, we produce clear, direct evidence for an abrupt bifurcation as the spectrum becomes narrow/intense enough. A fundamental ingredient of this work was to look directly at the nonlinear evolution of small, localised inhomogeneities, and whether these can grow dramatically. Indeed, one of the issues affecting previous investigations of this bifurcation seem to have been that they mostly looked for the indirect evidence of instability, such as an increase in overall extreme events. It is also found that a sufficiently large computational domain is crucial for the bifurcation to manifest.
      Citation: Fluids
      PubDate: 2022-12-30
      DOI: 10.3390/fluids8010013
      Issue No: Vol. 8, No. 1 (2022)
  • Fluids, Vol. 8, Pages 14: Express Method for Assessing Optimality of
           Industrial Heat Exchangers for Adsorption Heat Transformation

    • Authors: Alexandra Grekova, Irina Krivosheeva, Marina Solovyeva, Mikhail Tokarev
      First page: 14
      Abstract: In this work, four radiators with different core geometries were tested using a wind tunnel. The values of the global heat transfer coefficient (UA = 5 ÷ 65 W/K) were measured depending on the flow of air and water. The obtained UA values correlate well with the data of sorption experiments described in the literature. The found correlations between the Nusselt and Reynolds numbers made it possible to propose an algorithm for ranging commercial air radiators for the use in adsorption heat transformers. It is shown that the use of a wind tunnel can serve as an effective tool for express assessment of the prospects of using air radiators for adsorption heat conversion without destroying radiators or their direct testing in a complex adsorption installation requiring vacuum maintenance.
      Citation: Fluids
      PubDate: 2022-12-30
      DOI: 10.3390/fluids8010014
      Issue No: Vol. 8, No. 1 (2022)
  • Fluids, Vol. 8, Pages 15: Prediction of Self-Sustained Oscillations of an
           Isothermal Impinging Slot Jet

    • Authors: Bruno A. C. Barata, Jorge E. P. Navalho, José C. F. Pereira
      First page: 15
      Abstract: The present results are focused on the self-sustained oscillations of a confined impinging slot jet and their role in the flow structure and modeling requirements. Unsteady laminar, large-eddy simulation (LES), and Reynolds-averaged Navier–Stokes (RANS) predictions of an isothermal confined impinging jet were validated for several nozzle-to-plate ratios (H/B=4–15) and for laminar (Re=340 and 480) and turbulent (Re=104–2.7×104) conditions. The impinging flow structure was found to be highly influenced by the H/B ratio. For high ratios (H/B>5), the studied steady RANS turbulence models could not satisfactorily predict the high diffusion reported experimentally in the jet-impinging influence zone. The failure of these models has been attributed to the modeling issues of turbulence closures. However, for H/B=8, unsteady laminar 3D and LES calculations were verified, and a sinuous oscillation mode was developed, revealing self-sustained oscillations and the display of periodic flapping of the impinging jet in good agreement with the experiments. The predicted flapping oscillation is one of the reasons for the higher diffusion near the impingement wall, which was verified in several time-averaged experimental studies. The presence of jet flapping matters for clarifying the already long discussion on the RANS model’s validation in predicting impinging jets with high H/B ratios, adding justification to the failure of these turbulence models. This unsteady behavior is correctly computed through LES.
      Citation: Fluids
      PubDate: 2022-12-31
      DOI: 10.3390/fluids8010015
      Issue No: Vol. 8, No. 1 (2022)
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