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Fluids, Volume 7, Issue 3 (March 2022) – 31 articles

Cover Story (view full-size image): Results from large-eddy simulations of a classical hydraulic jump at inlet Froude number 2 are reported. The computations are performed using OpenFOAM, with the goal to evaluate the influence of modeling parameters on predictive accuracy and establish associated best-practice guidelines. Different modeling parameters are compared: geometric vs. algebraic interface capturing, three mesh resolution levels, four choices of the convective flux interpolation scheme. The results are benchmarked against a new DNS. Geometric interface capturing increases accuracy but deteriorates stability. Interestingly, numerical dissipation systematically improves the results. The conclusion is that sufficient accuracy is found when using a grid half as dense as the benchmark DNS, geometric interface capturing, and a second-order upwind scheme for the convective fluxes. View this paper.
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Article
Towards an Understanding of Multiphase Fluid Dynamics of a Microfluid Jet Polishing Process: A Numerical Analysis
Fluids 2022, 7(3), 119; https://doi.org/10.3390/fluids7030119 - 21 Mar 2022
Cited by 1 | Viewed by 833
Abstract
The microfluid jet polishing (MFJP) process is a manufacturing technology in which small abrasive particles (such as diamond, alumina, and ceria) are premixed with a carrier fluid (typically water) to form a liquid suspension that is pressurized and expelled through a nozzle for [...] Read more.
The microfluid jet polishing (MFJP) process is a manufacturing technology in which small abrasive particles (such as diamond, alumina, and ceria) are premixed with a carrier fluid (typically water) to form a liquid suspension that is pressurized and expelled through a nozzle for material removal. The resulting microjet beam—with a typical nozzle exit diameter in the range from 0.1 to 1.0 mm—impinges the workpiece surface for material removal by erosion and/or abrasion and produces an ultraprecision surface. This work applies a computational fluid dynamics (CFD) model to analyze the key phenomena in the interaction of the liquid suspension and the workpiece surface. The liquid film characteristics (film height, minimum film height, positions of the minimum film height, and hydraulic jump) obtained from the CFD simulations are compared with the results derived from empirical formulations found in the literature. Subsequently, the numerical results are utilized to investigate the impact velocity, pressure distribution, and shear stress caused by the suspension on the workpiece surface. It is observed that the shear stress strongly depends on the injection pressure of the liquid suspension and is weakly dependent on the abrasive suspension concentration (the liquid suspension with different densities, viscosities, and surface tensions). Additionally, the particle behavior is investigated in order to estimate the impact velocity and to identify the impact and erosion zones of the liquid suspension on the workpiece surface. Numerical results indicate that ~50% of total particles are impinging the workpiece surface almost perpendicularly (with a mean impact angle of ~86 degrees) for the first time in the stagnation region, where they are strongly decelerated by the carrier fluid before they reach the workpiece surface. These particles, however, rebound on the surface and are reaccelerated by the carrier fluid, impinging the workpiece surface further in the radial direction. Full article
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Article
Modelling Experimental Measurements of Fluid Flow through Railway Ballast
Fluids 2022, 7(3), 118; https://doi.org/10.3390/fluids7030118 - 21 Mar 2022
Viewed by 717
Abstract
The flooding of railway ballasts can cause extensive damage. This process has been the subject of several experimental investigations. In the present work, a relatively easy to implement approach to modelling this fluid flow is presented. It is shown that good agreement with [...] Read more.
The flooding of railway ballasts can cause extensive damage. This process has been the subject of several experimental investigations. In the present work, a relatively easy to implement approach to modelling this fluid flow is presented. It is shown that good agreement with the experimental results is obtained. The fluid flow is modelled by Darcy’s law, which we extend to the free fluid flowing above the ballast. The main complexity is in determining the free surface position, which is accomplished using an iterative procedure. The equations are solved using the finite element method. The method is illustrated by careful numerical calculations that are carefully compared with the experimental results reported in the literature. The method is then extended to realistic railway ballast, including the effects of ballast fouling. It is shown that when the flow begins to overtop the ballast, the free surface shifts to greatly increase the chance of ballast scouring. Full article
(This article belongs to the Special Issue High Speed Flows)
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Article
Influence of Turbulence Effects on the Runup of Tsunami Waves on the Shore within the Framework of the Navier–Stokes Equations
Fluids 2022, 7(3), 117; https://doi.org/10.3390/fluids7030117 - 20 Mar 2022
Viewed by 727
Abstract
This paper considers turbulence effects on tsunami runup on the shore in tsunami simulations using the system of three-dimensional Navier–Stokes equations. The turbulence effects in tsunami propagation and runup are studied by solving the problem of a wave propagating in a nonuniform-bottom pool [...] Read more.
This paper considers turbulence effects on tsunami runup on the shore in tsunami simulations using the system of three-dimensional Navier–Stokes equations. The turbulence effects in tsunami propagation and runup are studied by solving the problem of a wave propagating in a nonuniform-bottom pool and collapsing with a barrier. To solve this problem, we used the turbulence model, RANS SST (Reynolds-averaged Navier–Stokes shear stress transport). We compared the wave profiles at different times during wave propagation, runup, and collapse. To quantify the turbulence effects, we also compared the forces acting on the basin bottom. We demonstrated that the turbulence had almost no effect on the shape of the wave and the way of its propagation (except collapse). However, turbulence effects during the runup and collapse became noticeable and could boost the flow (increasing the pressure force and the total force) by up to 25 percent. Full article
(This article belongs to the Special Issue Boundary Layer Processes in Geophysical/Environmental Flows)
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Review
Current Trends in Fluid Research in the Era of Artificial Intelligence: A Review
Fluids 2022, 7(3), 116; https://doi.org/10.3390/fluids7030116 - 18 Mar 2022
Cited by 4 | Viewed by 1492
Abstract
Computational methods in fluid research have been progressing during the past few years, driven by the incorporation of massive amounts of data, either in textual or graphical form, generated from multi-scale simulations, laboratory experiments, and real data from the field. Artificial Intelligence (AI) [...] Read more.
Computational methods in fluid research have been progressing during the past few years, driven by the incorporation of massive amounts of data, either in textual or graphical form, generated from multi-scale simulations, laboratory experiments, and real data from the field. Artificial Intelligence (AI) and its adjacent field, Machine Learning (ML), are about to reach standardization in most fields of computational science and engineering, as they provide multiple ways for extracting information from data that turn into knowledge, with the aid of portable software implementations that are easy to adopt. There is ample information on the historical and mathematical background of all aspects of AI/ML in the literature. Thus, this review article focuses mainly on their impact on fluid research at present, highlighting advances and opportunities, recognizing techniques and methods having been proposed, tabulating, and testing some of the most popular algorithms that have shown significant accuracy and performance on fluid applications. We also investigate algorithmic accuracy on several fluid datasets that correspond to simulation results for the transport properties of fluids and suggest that non-linear, decision tree-based methods have shown remarkable performance on reproducing fluid properties. Full article
(This article belongs to the Section Mathematical and Computational Fluid Mechanics)
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Article
Effects of Numerical Forcing on the Two-Time Correlation of Fluid Velocity Differences in Stationary Isotropic Turbulence
Fluids 2022, 7(3), 115; https://doi.org/10.3390/fluids7030115 - 18 Mar 2022
Viewed by 653
Abstract
In direct numerical simulations (DNS) of homogeneous isotropic turbulence, numerical forcing is needed to achieve statistically stationary velocity fields. The Eulerian two-time correlation tensor of the fluid velocity difference field, [...] Read more.
In direct numerical simulations (DNS) of homogeneous isotropic turbulence, numerical forcing is needed to achieve statistically stationary velocity fields. The Eulerian two-time correlation tensor of the fluid velocity difference field, Δu(r,t)=u(x+r,t)u(x,t), characterizes the temporal evolution of turbulent eddies whose sizes scale with separation r=|r|. In this study, we investigate the effects of two spectral forcing schemes on the temporal decay of the Eulerian two-time correlation of fluid velocity differences Δu(r,t)Δu(r,t). Accordingly, DNS of homogeneous isotropic turbulence were performed for two grid sizes, 1283 and 5123, corresponding to the Taylor micro-scale Reynolds numbers Reλ80 and 210, respectively. Statistical stationarity was achieved by employing deterministic and stochastic spectral forcing schemes. In the stochastic scheme, one needs to specify the time scale, Tf, of the Uhlenbeck–Ornstein (UO) processes that constitute the forcing. We considered four values of the UO time scale (Tf=TE/4,TE,2TE, and 4TE) for each Reλ, where TE is the large-eddy time scale obtained from the DNS run with deterministic forcing at the same Reλ. It is seen that the correlations Δu(r,t)Δu(r,t) obtained from the deterministic-forcing DNS runs decay more slowly than those from stochastic-forcing DNS runs of all four Tf values. The slower decay of correlations in deterministic DNS runs is more pronounced at larger separations and for higher Reλ. Full article
(This article belongs to the Collection Complex Fluids)
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Article
On the Prediction of Boundary Layer Quantities at High Reynolds Numbers
Fluids 2022, 7(3), 114; https://doi.org/10.3390/fluids7030114 - 17 Mar 2022
Viewed by 693
Abstract
In the current paper, a new formula for calculating boundary layer quantities—such as the boundary layer thickness, friction coefficients, and the boundary layer profile—for a flat plate is presented. The formula is based on the power-law approach and represents a generalisation of the [...] Read more.
In the current paper, a new formula for calculating boundary layer quantities—such as the boundary layer thickness, friction coefficients, and the boundary layer profile—for a flat plate is presented. The formula is based on the power-law approach and represents a generalisation of the 1/7 power-law to a more extensive Reynolds number range. In addition to the derivation and the theoretical background, the main focus is on the comparison with various experimental data from the literature. The good agreement of the data shows that this approach allows for precise predictions of boundary layer quantities for a flat plate with zero-pressure gradients. Especially for estimating boundary layers along with large vehicles such as trains, ships, or aeroplanes, the formula offers added value in terms of accuracy compared to previously existing approaches, such as the 1/7 power-law. Full article
(This article belongs to the Special Issue Aerodynamics of Road Vehicles and Trains)
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Article
Thermal Effect on the Bioconvection Dynamics of Gravitactic Microorganisms in a Rectangular Cavity
Fluids 2022, 7(3), 113; https://doi.org/10.3390/fluids7030113 - 17 Mar 2022
Viewed by 703
Abstract
In this work, the dynamics of the bioconvection process of gravitactic microorganisms enclosed in a rectangular cavity, is analyzed. The dimensionless cell and energy conservation equations are coupled with the vorticity-stream function formulation. Then, the effects of the bioconvection Rayleigh number and the [...] Read more.
In this work, the dynamics of the bioconvection process of gravitactic microorganisms enclosed in a rectangular cavity, is analyzed. The dimensionless cell and energy conservation equations are coupled with the vorticity-stream function formulation. Then, the effects of the bioconvection Rayleigh number and the heating source on the dynamics of microorganisms are discussed. The results based in streamlines, concentration and temperature contours are obtained through numerical simulations considering eight different configurations of symmetrical and asymmetrical heat sources. It is concluded that microorganisms accumulate in the warmer regions and swim through the cooler regions to reach the surface. They form cells for each heat source, but at high concentrations, they form a single stable cell. The results presented here can be applied to control and to understand the dynamics of microorganisms with discrete heat sources. Full article
(This article belongs to the Special Issue Computational Biofluiddynamics: Advances and Applications)
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Article
Simplified CFD Model for Perforated Tile with Distorted Outflow
Fluids 2022, 7(3), 112; https://doi.org/10.3390/fluids7030112 - 17 Mar 2022
Viewed by 643
Abstract
Most of the perforated tile flow CFD models in the literature so far assume the air velocity coming out of the tile openings (pores) is uniform. However, in typical applications, such as data centers and indoor environments, perforated tile or diffuser outflow can [...] Read more.
Most of the perforated tile flow CFD models in the literature so far assume the air velocity coming out of the tile openings (pores) is uniform. However, in typical applications, such as data centers and indoor environments, perforated tile or diffuser outflow can be highly non-uniform due to many reasons (e.g., spatial variation of plenum pressure or varying local tile geometrical patterns). For an ideal (uniform) tile flow velocity that has the same flow rate as the non-uniform tile flow velocity, the tile flow momentum of the latter will always be greater because momentum scales with velocity squared. To illustrate the effect of tile flow velocity distortion, two generic CFD cases (one with uniform velocity and the other with non-uniform velocity) with multiple openings model are presented here. Their CFD results are compared to the momentum source model results and validated against previously published data of an isolated tile flow measurement. The momentum source model is one of the simplest/most practical CFD models that uses a body-force value for correcting the momentum deficit between perforated and fully open areas. Initially, the momentum source model results show good agreement with results of the uniform velocity case only. Thus, due to velocity distortion (non-uniform velocity) in the tile flow, the CFD results presented in this paper show a potential reason to modify the body-force value in the momentum source model with an adjustment factor (C). Several values of C factor are numerically investigated for the present distorted tile flow CFD case, and the best match is found to be at C, equaling approximately 1.6. Full article
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Article
Calibration of the k-ω SST Turbulence Model for Free Surface Flows on Mountain Slopes Using an Experiment
Fluids 2022, 7(3), 111; https://doi.org/10.3390/fluids7030111 - 17 Mar 2022
Viewed by 696
Abstract
We calibrate the k-ωSST turbulence model for free surface flows in the channel or on the slope using machine learning techniques. To calibrate the turbulence model, an experiment is carried out in an inclined rectangular research chute. In [...] Read more.
We calibrate the k-ωSST turbulence model for free surface flows in the channel or on the slope using machine learning techniques. To calibrate the turbulence model, an experiment is carried out in an inclined rectangular research chute. In the experiment, the pressure values in the flow are measured at different distances from the bottom; after transforming data, the flow velocity profile is obtained. The k-ωSST turbulence model is calibrated based on experimental data using the Nelder-Mead optimization algorithm. The calibrated turbulence model is then used to calculate the glacial lake Maliy Azau outburst flood on the Elbrus (Central Caucasus). Full article
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Article
Efficient Reduced Order Modeling of Large Data Sets Obtained from CFD Simulations
Fluids 2022, 7(3), 110; https://doi.org/10.3390/fluids7030110 - 17 Mar 2022
Cited by 1 | Viewed by 774
Abstract
The ever-increasing computational power has shifted direct numerical simulations towards higher Reynolds numbers and large eddy simulations towards industrially-relevant flow scales. However, this increase in both temporal and spatial resolution has severely increased the computational cost of model order reduction techniques. Reducing the [...] Read more.
The ever-increasing computational power has shifted direct numerical simulations towards higher Reynolds numbers and large eddy simulations towards industrially-relevant flow scales. However, this increase in both temporal and spatial resolution has severely increased the computational cost of model order reduction techniques. Reducing the full data set to a smaller subset in order to perform reduced-order modeling (ROM) may be an interesting method to keep the computational effort reasonable. Moreover, non-tomographic particle image velocimetry measurements obtain a 2D data set of a 3D flow field and an interesting research question would be to quantify the difference between this 2D ROM compared to the 3D ROM of the full flow field. To provide an answer to both issues, the aim of this study was to test a new method for obtaining POD basis functions from a small subset of data initially and using them afterwards in the ROM of either the complete data set or the reduced data set. Hence, no new method of ROM is presented, but we demonstrate a procedure to significantly reduce the computational effort required for the ROM of very large data sets and a quantification of the error introduced by reducing the size of those data sets. The method applies eigenvalue decomposition on a small subset of data extracted from a full 3D simulation and the obtained temporal coefficients are projected back on the 3D velocity fields to obtain the 3D spatial modes. To test the method, an annular jet was chosen as a flow topology due to its simple geometry and the rich dynamical content of its flow field. First, a smaller data set is extracted from the 2D cross-sectional planes and ROM is performed on that data set. Secondly, the full 3D spatial structures are reconstructed by projecting the temporal coefficients back on the 3D velocity fields and the 2D spatial structures by projecting the temporal coefficients back on the 2D velocity fields. It is shown that two perpendicular lateral planes are sufficient to capture the relevant large-scale structures. As such, the total processing time can be reduced by a factor of 136 and up to 22 times less RAM is needed to complete the ROM processing. Full article
(This article belongs to the Special Issue Reduced Order Models for Computational Fluid Dynamics)
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Article
A Multi-Node Lumped Parameter Model Including Gravity and Real Gas Effects for Steady and Transient Analysis of Heat Pipes
Fluids 2022, 7(3), 109; https://doi.org/10.3390/fluids7030109 - 16 Mar 2022
Viewed by 681
Abstract
This work presents a multi-node lumped parameter model able to predict the steady and transient behavior of capillary heat pipes, taking into account the effects of gravity (orientation angle) and the real gas effects in the vapor modeling. The model was validated against [...] Read more.
This work presents a multi-node lumped parameter model able to predict the steady and transient behavior of capillary heat pipes, taking into account the effects of gravity (orientation angle) and the real gas effects in the vapor modeling. The model was validated against experimental results acquired by Leonardo S.p.A., which were obtained by simulating the behavior of a heat pipe embedded in a chassis cover, subject to seven cycles of transient thermal loading. After the validation, the analysis is focused on the model accuracy when using the ideal and real gas assumptions, using different working fluids (water, ammonia, acetone, HFC134a). The results showed that when using water or ammonia as working fluid, the error in modeling the vapor as an ideal instead of as real gas is negligible, both for the vapor temperatures and pressures predictions. On the contrary, when using acetone or HFC134a as working fluid, modeling the vapor as a real gas leads to a significant increase in the accuracy of the vapor pressure predictions. Full article
(This article belongs to the Special Issue Fluids in Space)
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Article
A Wavelet-Based Time-Frequency Analysis on the Supersonic Jet Noise Features with Chevrons
Fluids 2022, 7(3), 108; https://doi.org/10.3390/fluids7030108 - 16 Mar 2022
Cited by 3 | Viewed by 703
Abstract
A detailed investigation of the statistical properties of the near-field pressure fluctuations induced by an under-expanded jet, by varying the nozzle exit shapes has been presented. Experiments using different convergent Chevron nozzles were carried out in the anechoic chamber at the University of [...] Read more.
A detailed investigation of the statistical properties of the near-field pressure fluctuations induced by an under-expanded jet, by varying the nozzle exit shapes has been presented. Experiments using different convergent Chevron nozzles were carried out in the anechoic chamber at the University of Bristol to assess the importance of the Chevron shape on the near pressure field emitted by a single stream under-expanded jet. Measurements were carried out through an axial microphone array traversed radially to various positions for jet in an under-expanded condition at Mach number M = 1.3. The intermittent behavior is investigated considering the standard statistical indicators and a wavelet-based conditional approach, including the phase angle. The intermittent degree of various features related to different scales, such as Screech tones and broadband shock associate noise were estimated. A series of recently developed wavelet-based tools were assessed as a viable approach to investigate the noise emitted by under-expanded jets. Full article
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Article
The Functional Relationship of Sediment Transport under Various Simulated Rainfall Conditions
Fluids 2022, 7(3), 107; https://doi.org/10.3390/fluids7030107 - 15 Mar 2022
Viewed by 695
Abstract
Sediment removed in the detachment process is transported by overland flow. Previous experimental and field works studied that sediment transport is influenced by hydraulic properties of flow, physical properties of soil, and surface characteristics. Several equations in predicting sediment transport have been developed [...] Read more.
Sediment removed in the detachment process is transported by overland flow. Previous experimental and field works studied that sediment transport is influenced by hydraulic properties of flow, physical properties of soil, and surface characteristics. Several equations in predicting sediment transport have been developed from previous research. The objective of this paper was to establish the selected parameters that contribute to the sediment transport capacity in overland flow conditions under different rainfall pattern conditions and to evaluate their significance. The establishment of independent variables was performed using the dimensional analysis approach that is Buckingham’s π theorem. The final results obtained are a series of independent parameters; the Reynolds number (Re), dimensionless rainfall parameter iLν, hydraulic characteristics QLν that related to the dependent parameters; and dimensionless sediment transport qsρv. The relationship indicates that 63.6% to 72.44% of the variance in the independent parameters is in relation to the dependent parameter. From the iteration method, the estimation of constant and regression coefficient values is presented in the form of the general formula for linear and nonlinear model equations. The linear and nonlinear model equations have the highest model accuracy of 93.1% and 81.5%, respectively. However, the nonlinear model equation has the higher discrepancy ratio of 54.9%. Full article
(This article belongs to the Collection Complex Fluids)
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Article
Extension of Spectral/hp Element Methods towards Robust Large-Eddy Simulation of Industrial Automotive Geometries
Fluids 2022, 7(3), 106; https://doi.org/10.3390/fluids7030106 - 14 Mar 2022
Viewed by 948
Abstract
A spectral/hp element methodology is utilised to investigate the SAE Notchback geometry with 20 backlight and 3 diffuser at Re=2.3×106. The study presented here considered two different mesh approaches: one focusing on classical h-type [...] Read more.
A spectral/hp element methodology is utilised to investigate the SAE Notchback geometry with 20 backlight and 3 diffuser at Re=2.3×106. The study presented here considered two different mesh approaches: one focusing on classical h-type refinement with standard solution polynomial order (HFP3) and a second case considering relatively coarse mesh combined with high solution polynomial order (HCP5). For the same targeted number of degrees of freedom in both meshes, the results show significant differences in vorticity, flow structures and surface pressure. The first guidelines for hp refinement strategy are deduced for complex industrial cases. Further work on investigating the requirements for these hybrid techniques is required in order to maximize the benefits of the solution and mesh refinements in spectral/hp element method simulations. Full article
(This article belongs to the Special Issue Aerodynamics of Road Vehicles and Trains)
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Article
Stochastic Modeling of Particle Transport in Confined Geometries: Problems and Peculiarities
Fluids 2022, 7(3), 105; https://doi.org/10.3390/fluids7030105 - 12 Mar 2022
Viewed by 693
Abstract
The equivalence between parabolic transport equations for solute concentrations and stochastic dynamics for solute particle motion represents one of the most fertile correspondences in statistical physics originating from the work by Einstein on Brownian motion. In this article, we analyze the problems and [...] Read more.
The equivalence between parabolic transport equations for solute concentrations and stochastic dynamics for solute particle motion represents one of the most fertile correspondences in statistical physics originating from the work by Einstein on Brownian motion. In this article, we analyze the problems and the peculiarities of the stochastic equations of motion in microfluidic confined systems. The presence of solid boundaries leads to tensorial hydrodynamic coefficients (hydrodynamic resistance matrix) that depend also on the particle position. Singularity issues, originating from the non-integrable divergence of the entries of the resistance matrix near a solid no-slip boundary, determine some mass-transport paradoxes whenever surface phenomena, such as surface chemical reactions at the walls, are considered. These problems can be overcome by considering the occurrence of non vanishing slippage. Added-mass effects and the influence of fluid inertia in confined geometries are also briefly addressed. Full article
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Article
Validation and Enhancement of a Supermesh Strategy for the CFD Simulation of Four-Stroke Internal Combustion Engines
Fluids 2022, 7(3), 104; https://doi.org/10.3390/fluids7030104 - 11 Mar 2022
Viewed by 814
Abstract
The present paper describes and validates an efficient CFD implementation to replicate the working fluid-dynamics of a real four-stroke internal combustion engine. To do this, experimental data obtained on a single-cylinder engine are used to validate the proposed computational approach. The engine domain [...] Read more.
The present paper describes and validates an efficient CFD implementation to replicate the working fluid-dynamics of a real four-stroke internal combustion engine. To do this, experimental data obtained on a single-cylinder engine are used to validate the proposed computational approach. The engine domain is divided into regions according to each moving zone, and these are coupled using a pseudo-supermesh interface presented in a previous work by the authors. In this work, the original pseudo-supermesh strategy is enhanced by introducing the dual-boundary concept to model the valve opening/closing events to increase the accuracy and simplicity of the simulation procedure. The results produced by the proposed software tool show a good correlation to the experimental measurements of the complete engine cycle. Macroscopic quantities of the in-cylinder flow are accurately replicated as well as the instantaneous evolution of the in-cylinder and intake manifold pressure. Furthermore, the present work shows that the computational efficiency and scalability of the enhanced pseudo-supermesh approach are preserved even when applied to more complex real problems. In this sense, this work contributes to a new engineering tool promoting the enhanced pseudo-supermeshes as an effective tool for the design, development, and optimization of internal combustion engines. Full article
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Article
Simulations of Graphene Oxide Dispersions as Discotic Nematic Liquid Crystals in Couette Flow Using Ericksen-Leslie (EL) Theory
Fluids 2022, 7(3), 103; https://doi.org/10.3390/fluids7030103 - 10 Mar 2022
Viewed by 788
Abstract
The objective of this study was to simulate the flow of graphene oxide (GO) dispersions, a discotic nematic liquid crystal (DNLC), using the Ericksen-Leslie (EL) theory. GO aqueous suspension, as a lubricant, effectively reduces the friction between solid surfaces. The geometry considered in [...] Read more.
The objective of this study was to simulate the flow of graphene oxide (GO) dispersions, a discotic nematic liquid crystal (DNLC), using the Ericksen-Leslie (EL) theory. GO aqueous suspension, as a lubricant, effectively reduces the friction between solid surfaces. The geometry considered in this study was two cylinders with a small gap size, which is the preliminary geometry for journal bearings. The Leslie viscosity coefficients calculated in our previous study were used to calculate the stress tensor in the EL theory. The behavior of GO dispersions in the concentration range of 15 mg/mL to 30 mg/mL, shown in our recent experiments to be in the nematic phase, was investigated to obtain the orientation and the viscosity profile. The viscosities of GO dispersions obtained from numerical simulations were compared with those from our recent experimental study, and we observed that the values are within the range of experimental uncertainty. In addition, the alignment angles of GO dispersions at different concentrations were calculated numerically using EL theory and compared with the respective theoretical values, which were within 1% error. The anchoring angles corresponding to viscosity values closest to the experimental results were between 114 and 118 degrees. Moreover, a sensitivity analysis was performed to determine the effects of different ratios of the elasticity coefficients in EL theory. Using this procedure, the same study could be extended for other DNLCs in different geometries. Full article
(This article belongs to the Collection Complex Fluids)
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Article
High-Speed Imaging of Water Hammer Cavitation in Oil–Hydraulic Pipe Flow
Fluids 2022, 7(3), 102; https://doi.org/10.3390/fluids7030102 - 09 Mar 2022
Viewed by 861
Abstract
A pipe water hammer with column separation was studied in a range of flow rates (Re=465 to 2239) in a test rig with an acrylic glass observation section. Pressure transients were measured with piezoresistive pressure sensors, while the gas [...] Read more.
A pipe water hammer with column separation was studied in a range of flow rates (Re=465 to 2239) in a test rig with an acrylic glass observation section. Pressure transients were measured with piezoresistive pressure sensors, while the gas evaporation and condensation were captured by high-speed recording with a Photron SA-Z at a frame rate of 75,000 fps. Separation lengths were estimated by a threshold value in the images. The results did not show a sharp gas–oil interface but consisted of small, dispersed bubbles mixed with larger vapor structures, where the bubbles seemed to become smaller after each collapse. These findings differ from the transient cavitating characteristics commonly reported in nonhydraulic piping systems governed by different fluid properties and time scales. Good repeatability, both in terms of pressure transients and bubble distribution, was observed. The column separation was quantified as a metric of separation length, which was consistent between the tests. Combined with pressure measurements, these results may assist in obtaining a better understanding of the transient cavitation dynamics within oil–hydraulic systems as well as be used to improve modelling strategies towards more accurate cavitation erosion predictions. Full article
(This article belongs to the Special Issue Unsteady Flows in Pipes)
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Article
Harmonic Response Analysis of Tank Design Effect on Ultrasonic Cleaning Process
Fluids 2022, 7(3), 99; https://doi.org/10.3390/fluids7030099 - 07 Mar 2022
Viewed by 765
Abstract
Several ultrasonic cleaning tanks (UCTs) had a problem: a manufacturer complained that there were damages to cleaning objects, they were unclarified, and it needed to be abruptly solved. To investigate and solve the problem, a small UCT filled with 3.92 L of water, [...] Read more.
Several ultrasonic cleaning tanks (UCTs) had a problem: a manufacturer complained that there were damages to cleaning objects, they were unclarified, and it needed to be abruptly solved. To investigate and solve the problem, a small UCT filled with 3.92 L of water, with a frequency of 28 kHz, two horn transducers, and a total power of 100 W was built for simulation and experiment. A built tank body material of UCT can be adjustable to acrylic, glass, and stainless steel. Since the cavitation causing the cleaning relates to acoustic pressure, harmonic response analysis (HRA) in ANSYS software was employed to calculate the acoustic pressure inside the UCT for different designs such as mentioned materials, power, thickness, volume, and frequency. The HRA results revealed uneven acoustic pressure depending on the tank designs, consistent with foil corrosion and power concentration experiments. Furthermore, using the tank body material with acrylic, glass, and stainless steel provided the highest, moderate, and lowest acoustic pressure levels, respectively. The uneven acoustic pressure resulted from the differences in material transmission coefficients. In addition, the damage occurred because of improper tank design, resulting in excessive acoustic pressure. Therefore, the tank design is indispensable in designing high-efficiency UCTs to reduce damage and meet customer requirements. Full article
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Article
Flow Characteristics at a River Diversion Juncture and Implications for Juvenile Salmon Entrainment
Fluids 2022, 7(3), 98; https://doi.org/10.3390/fluids7030098 - 07 Mar 2022
Viewed by 662
Abstract
Flow structures at a river diversion juncture are complex and have been studied extensively. Their impact on the juvenile salmon entrainment into the side channel, however, is less investigated, and based mostly on empiricism. In this study, a Eulerian fish tracking model is [...] Read more.
Flow structures at a river diversion juncture are complex and have been studied extensively. Their impact on the juvenile salmon entrainment into the side channel, however, is less investigated, and based mostly on empiricism. In this study, a Eulerian fish tracking model is developed and used in conjunction with a 3D flow solver to quantitatively evaluate the implications of complex flow characteristics at typical junctures on fish entrainment. First, the flow model is validated with the available experimental data, key flow structures are examined using the results, and their implications for fish entrainment are discussed. Next, the numerical fish tracking model is used to show that the cross-sectional fish distribution immediately upstream of a juncture is an important factor that controls fish entrainment efficiency. Fish entrainment efficiency curves are developed for different flow diversion ratios and fish distribution patterns and used to shed light on the reasons behind some field-observed fish entrainment patterns. Further, the model is used to show that the secondary flow in a river bend may have a significant impact on fish entrainment at flow junctures, in agreement with field observations. Finally, a submerged vane is demonstrated to be a potential management option to locally generate secondary flows upstream of a juncture to achieve the desired fish entrainment property. Full article
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Article
Large-Eddy Simulation of a Classical Hydraulic Jump: Influence of Modelling Parameters on the Predictive Accuracy
Fluids 2022, 7(3), 101; https://doi.org/10.3390/fluids7030101 - 07 Mar 2022
Viewed by 924
Abstract
Results from large-eddy simulations of a classical hydraulic jump at inlet Froude number two are reported. The computations were performed using the general-purpose finite-volume-based code OpenFOAM®, and the primary goal was to evaluate the influence of the modelling parameters on the [...] Read more.
Results from large-eddy simulations of a classical hydraulic jump at inlet Froude number two are reported. The computations were performed using the general-purpose finite-volume-based code OpenFOAM®, and the primary goal was to evaluate the influence of the modelling parameters on the predictive accuracy, as well as establish the associated best-practice guidelines. A benchmark simulation was conducted on a grid with a 1 mm-cell-edge length to validate the solver and provide a reference solution for the parameter influence study. The remaining simulations covered different selections of the modelling parameters: geometric vs. algebraic interface capturing, three mesh resolution levels, and four choices of the convective flux interpolation scheme. Geometric interface capturing led to better accuracy, but deteriorated the numerical stability and increased the simulation times. Interestingly, numerical dissipation was shown to systematically improve the results, both in terms of accuracy and stability. Strong sensitivity to the grid resolution was observed directly downstream of the toe of the jump. Full article
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Article
Fluid Flow and Structural Numerical Analysis of a Cerebral Aneurysm Model
Fluids 2022, 7(3), 100; https://doi.org/10.3390/fluids7030100 - 07 Mar 2022
Viewed by 848
Abstract
Intracranial aneurysms (IA) are dilations of the cerebral arteries and, in most cases, have no symptoms. However, it is a very serious pathology, with a high mortality rate after rupture. Several studies have been focused only on the hemodynamics of the flow within [...] Read more.
Intracranial aneurysms (IA) are dilations of the cerebral arteries and, in most cases, have no symptoms. However, it is a very serious pathology, with a high mortality rate after rupture. Several studies have been focused only on the hemodynamics of the flow within the IA. However, besides the effect of the flow, the development and rupture of the IA are also associated with a combination of other factors such as the wall mechanical behavior. Thus, the objective of this work was to analyze, in addition to the flow behavior, the biomechanical behavior of the aneurysm wall. For this, CFD simulations were performed for different Reynolds numbers (1, 100, 500 and 1000) and for two different rheological models (Newtonian and Carreau). Subsequently, the pressure values of the fluid simulations were exported to the structural simulations in order to qualitatively observe the deformations, strains, normal stresses and shear stress generated in the channel wall. For the structural simulations, a hyperelastic constitutive model (5-parameter Mooney–Rivlin) was used. The results show that with the increase in the Reynolds number (Re), the recirculation phenomenon is more pronounced, which is not seen for Re = 1. The higher the Re, the higher the strain, displacement, normal and shear stresses values. Full article
(This article belongs to the Special Issue Image-Based Computational and Experimental Biomedical Flows)
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Article
Subject-Specific Computational Fluid-Structure Interaction Modeling of Rabbit Vocal Fold Vibration
Fluids 2022, 7(3), 97; https://doi.org/10.3390/fluids7030097 - 06 Mar 2022
Viewed by 985
Abstract
A full three-dimensional (3D) fluid-structure interaction (FSI) study of subject-specific vocal fold vibration is carried out based on the previously reconstructed vocal fold models of rabbit larynges. Our primary focuses are the vibration characteristics of the vocal fold, the unsteady 3D flow field, [...] Read more.
A full three-dimensional (3D) fluid-structure interaction (FSI) study of subject-specific vocal fold vibration is carried out based on the previously reconstructed vocal fold models of rabbit larynges. Our primary focuses are the vibration characteristics of the vocal fold, the unsteady 3D flow field, and comparison with a recently developed 1D glottal flow model that incorporates machine learning. The 3D FSI model applies strong coupling between the finite-element model for the vocal fold tissue and the incompressible Navier-Stokes equation for the flow. Five different samples of the rabbit larynx, reconstructed from the magnetic resonance imaging (MRI) scans after the in vivo phonation experiments, are used in the FSI simulation. These samples have distinct geometries and a different inlet pressure measured in the experiment. Furthermore, the material properties of the vocal fold tissue were determined previously for each individual sample. The results demonstrate that the vibration and the intraglottal pressure from the 3D flow simulation agree well with those from the 1D flow model based simulation. Further 3D analyses show that the inferior and supraglottal geometries play significant roles in the FSI process. Similarity of the flow pattern with the human vocal fold is discussed. This study supports the effective usage of rabbit larynges to understand human phonation and will help guide our future computational studies that address vocal fold disorders. Full article
(This article belongs to the Special Issue Computational Biofluiddynamics: Advances and Applications)
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Article
Numerical Study of Particle Margination in a Square Channel Flow with Red Blood Cells
Fluids 2022, 7(3), 96; https://doi.org/10.3390/fluids7030096 - 06 Mar 2022
Viewed by 865
Abstract
Red blood cells flow near the axis in a small vessel, known as axial accumulation. This causes a region called the cell-free layer, which does not contain red blood cells near the wall. Then, small particles such as platelets come out to the [...] Read more.
Red blood cells flow near the axis in a small vessel, known as axial accumulation. This causes a region called the cell-free layer, which does not contain red blood cells near the wall. Then, small particles such as platelets come out to the cell-free layer. This phenomenon is called platelet margination. In this study, related to this phenomenon, direct numerical simulations were conducted using the immersed boundary method. The effects of the shear rate, channel size, and hematocrit value were investigated on the pressure-driven flow in a straight tube with a square cross-section. The simulation results indicated that the margination rate, which is the ratio of the distance traveled in the flow direction to the margination distance in the wall direction, is independent of the shear rate. The effect of the channel size on platelet margination was found to be well scaled by introducing a dimensionless parameter, which included the shear rate and effective area of the particle movement. It was also found that the margination rate varied nonlinearly with the tube hematocrit. This was due to the volume exclusion effect of red blood cells, which facilitated or hindered the motion of particles depending on the hematocrit. The relationship between the stable position of the particles near the corner and the width of the cell-free layer was also found. Furthermore, velocity fluctuations normalized by wall shear rate in a cross-section collapsed to one curve in the presented simulations. This indicates that the lateral force acting on the particles increases linearly with the shear rate. Full article
(This article belongs to the Special Issue Computational Biofluiddynamics: Advances and Applications)
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Article
Particle Transport Velocity in Vertical Transmission with an Airlift Pump
Fluids 2022, 7(3), 95; https://doi.org/10.3390/fluids7030095 - 05 Mar 2022
Viewed by 843
Abstract
This paper presents the optimal conditions for fast transfer of solid particle with an airlift pump. The experimental examinations were carried out in an airlift pump with a length of 5.64 m and an inner diameter of 0.102 m in order to determine [...] Read more.
This paper presents the optimal conditions for fast transfer of solid particle with an airlift pump. The experimental examinations were carried out in an airlift pump with a length of 5.64 m and an inner diameter of 0.102 m in order to determine the impact of submergence ratio, air flow, and physical particle properties, such as shape, size, and density, on the vertical velocity of the particle in detail. The results showed that with the same air flow, the maximum particle velocity was achieved when the churn flow regime is established with a submergence ratio close to 0.89. However, in bubble and slug flow, it is not possible to carry a large particle in the dimensions of centimeters. Furthermore, in a churn flow, the velocity of the particle exceeds the velocity of pumped water; hence, water is not the only particle carrier in a vertical three-phase flow. Full article
(This article belongs to the Special Issue Multiphase Flows in Engineering Applications)
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Review
Computational Methods for Fluid-Structure Interaction Simulation of Heart Valves in Patient-Specific Left Heart Anatomies
Fluids 2022, 7(3), 94; https://doi.org/10.3390/fluids7030094 - 04 Mar 2022
Cited by 2 | Viewed by 1107
Abstract
Given the complexity of human left heart anatomy and valvular structures, the fluid–structure interaction (FSI) simulation of native and prosthetic valves poses a significant challenge for numerical methods. In this review, recent numerical advancements for both fluid and structural solvers for heart valves [...] Read more.
Given the complexity of human left heart anatomy and valvular structures, the fluid–structure interaction (FSI) simulation of native and prosthetic valves poses a significant challenge for numerical methods. In this review, recent numerical advancements for both fluid and structural solvers for heart valves in patient-specific left hearts are systematically considered, emphasizing the numerical treatments of blood flow and valve surfaces, which are the most critical aspects for accurate simulations. Numerical methods for hemodynamics are considered under both the continuum and discrete (particle) approaches. The numerical treatments for the structural dynamics of aortic/mitral valves and FSI coupling methods between the solid Ωs and fluid domain Ωf are also reviewed. Future work toward more advanced patient-specific simulations is also discussed, including the fusion of high-fidelity simulation within vivo measurements and physics-based digital twining based on data analytics and machine learning techniques. Full article
(This article belongs to the Special Issue Computational Biofluiddynamics: Advances and Applications)
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Article
Thermal Hydraulics and Thermochemical Design of Fatty Acid Methyl Ester (Biodiesel) Esterification Reactor by Heating with High Boiling Point Phenyl-Naphthalene Liquid
Fluids 2022, 7(3), 93; https://doi.org/10.3390/fluids7030093 - 04 Mar 2022
Viewed by 1347
Abstract
FAME (biodiesel) is an alternative fuel that can be produced from vegetable oils. There is growing interest in the research and development of renewable energy sources. A possible solution is a biofuel usable in compression-ignition engines (diesel engines) produced from biomass rich in [...] Read more.
FAME (biodiesel) is an alternative fuel that can be produced from vegetable oils. There is growing interest in the research and development of renewable energy sources. A possible solution is a biofuel usable in compression-ignition engines (diesel engines) produced from biomass rich in fats and oils. This paper contains a new and safer design of an esterification reactor for producing FAME (biodiesel) by utilizing high boiling point fluid (called phenyl-naphthalene). CFD simulation of biodiesel production by using methyl imidazolium hydrogen sulfate ionic liquid has been carried out. Ionic liquids (ILs) are composed of anions and cations that exist as liquids at relatively low temperatures. They have many advantages, such as chemical and thermal stability, low flammability, and low vapor pressures. In this work, the ionic liquids have been applied in organic reactions as solvents and catalysts of the esterification reaction. The great qualities of high boiling temperature fluids, along with advances in the oil and gas industries, make the organic concept more suitable and safer (water coming into contact with liquid metal may cause a steam explosion hazard) for heating the esterification reactor. The COMSOL Multiphysics code has been employed and simultaneously solves the continuity, fluid flow, heat transfer, and diffusion with chemical reaction kinetics equations. It was shown that the heat flux could provide the necessary heat flux required for maintaining the esterification process. It was found that the mass fractions of methanol and oleic acid decrease along the reactor axis. The FAME mass fraction increased along the reactor axis. The maximal biodiesel yield obtained in the esterification reactor was 86%. This value is very similar to the experimental results obtained by Elsheikh et al. Full article
(This article belongs to the Special Issue Thermodynamic Properties of Liquid Mixtures)
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Article
Volume Transport by a 3D Quasigeostrophic Heton
Fluids 2022, 7(3), 92; https://doi.org/10.3390/fluids7030092 - 02 Mar 2022
Viewed by 680
Abstract
Oceanic flows self-organize into coherent vortices, which strongly influence their transport and mixing properties. Counter-rotating vortex pairs can travel long distances and carry trapped fluid as they move. These structures are often modeled as hetons, viz. counter-rotating quasigeostrophic point vortex pairs with equal [...] Read more.
Oceanic flows self-organize into coherent vortices, which strongly influence their transport and mixing properties. Counter-rotating vortex pairs can travel long distances and carry trapped fluid as they move. These structures are often modeled as hetons, viz. counter-rotating quasigeostrophic point vortex pairs with equal circulations. Here, we investigate the structure of the transport induced by a single three-dimensional heton. The transport is determined by the Hamiltonian structure of the velocity field induced by the heton’s component vortices. The dynamics display a sequence of bifurcations as one moves through the heton-induced velocity field in height. These bifurcations create and destroy unstable fixed points whose associated invariant manifolds bound the trapped volume. Heton configurations fall into three categories. Vertically aligned hetons, which are parallel to the vertical axis and have zero horizontal separation, do not move and do not transport fluid. Horizontally aligned hetons, which lie on the horizontal plane and have zero vertical separation, have a single parameter, the horizontal vortex half-separation Y, and simple scaling shows the dimensional trapped volume scales as Y3. Tilted hetons are described by two parameters, Y and the vertical vortex half-separation Z, rendering the scaling analysis more complex. A scaling theory is developed for the trapped volume of tilted hetons, showing that it scales as Z4/Y for large Z. Numerical calculations illustrate the structure of the trapped volume and verify the scaling theory. Full article
(This article belongs to the Special Issue Instabilities and Nonlinear Dynamics in Oceanic Flows)
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Editorial
Editorial for the Special Issue on “Fluidic Oscillators—Devices and Applications”
Fluids 2022, 7(3), 91; https://doi.org/10.3390/fluids7030091 - 01 Mar 2022
Viewed by 722
Abstract
Fluidic oscillators are devices that produce a temporally and/or spatially oscillating output of fluid flow without requiring any moving parts [...] Full article
(This article belongs to the Special Issue Fluidic Oscillators-Devices and Applications)
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Article
Physical Model of a Single Bubble Growth during Nucleate Pool Boiling
Fluids 2022, 7(3), 90; https://doi.org/10.3390/fluids7030090 - 27 Feb 2022
Viewed by 1019
Abstract
A simplified physical model of a single bubble growth during nucleate pool boiling was developed. The model was able to correlate the experimentally observed data of the bubble’s growth time and its radius evolution with the use of the appropriate input parameters. The [...] Read more.
A simplified physical model of a single bubble growth during nucleate pool boiling was developed. The model was able to correlate the experimentally observed data of the bubble’s growth time and its radius evolution with the use of the appropriate input parameters. The calculated values of separated heat fluxes from the heater wall, thermal boundary layer, and to the bulk liquid gave us a new insight into the complex mechanisms of the nucleate pool boiling process. The thermal boundary layer was found to supply the majority of the heat to the growing bubble. The heat flux from the thermal boundary layer to the bubble was found to be close to the Zuber’s critical heat flux limit (890 kW/m2). This heat flux was substantially larger than the input heater wall heat flux of 50 kW/m2. The thermal boundary layer acts as a reservoir of energy to be released to the growing bubble, which is filled during the waiting time of the bubble growth cycle. Therefore, the thickness of the thermal boundary layer was found to have a major effect on the bubble’s growth time. Full article
(This article belongs to the Section Heat and Mass transfer)
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