Fluids

20 pages, 3520 KiB

Open AccessArticle

Numerical Analysis of the Available Power in an Overtopping Wave Energy Converter Subjected to a Sea State of the Coastal Region of Tramandaí, Brazil

by Lenon A. Cisco, Rafael P. Maciel, Phelype H. Oleinik, Elizaldo D. dos Santos, Mateus N. Gomes, Luiz A. O. Rocha, Liércio A. Isoldi and Bianca N. Machado

Fluids 2022, 7(11), 359; https://doi.org/10.3390/fluids7110359 - 20 Nov 2022

Cited by 2 | Viewed by 1488

Abstract

The present work proposes a numerical study of an overtopping wave energy converter. The goal of this study is to evaluate the theoretical power that can be converted by an overtopping device subjected to sea waves in the coastal region of Tramandaí, Brazil. [...] Read more.

The present work proposes a numerical study of an overtopping wave energy converter. The goal of this study is to evaluate the theoretical power that can be converted by an overtopping device subjected to sea waves in the coastal region of Tramandaí, Brazil. For this, realistic irregular waves were generated using the WaveMIMO methodology, which allows numerical simulation of sea waves through the imposition of transient discrete data as prescribed velocity. For the numerical analysis, a two-dimensional computational model was employed using Fluent, where the device was inserted into a wave channel. The volume of the fluid multiphase model was used for the treatment of the air–water interaction. The results indicated that the free surface elevation obtained using the WaveMIMO methodology, which converts a realistic sea state into a free surface elevation series, was adequately represented. The evaluation of the theoretical power of the overtopping device during around 45 min indicated that 471.28 W was obtained. In addition, a monthly generation projection showed that this device would supply 100% of the electricity demand of a school in the city of Tramandaí. These results demonstrated that the conversion of sea wave energy into electrical energy can contribute to supplying electricity demand, especially for coastal cities. Full article

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Graphical abstract

7 pages, 267 KiB

Open AccessArticle

Sound Propagation in Cigar-Shaped Bose Liquids in the Thomas-Fermi Approximation: A Comparative Study between Gross-Pitaevskii and Logarithmic Models

by Konstantin G. Zloshchastiev

Fluids 2022, 7(11), 358; https://doi.org/10.3390/fluids7110358 - 19 Nov 2022

Cited by 1 | Viewed by 1110

Abstract

A comparative study is conducted of the propagation of sound pulses in elongated Bose liquids and Bose-Einstein condensates in Gross-Pitaevskii and logarithmic models, by means of the Thomas-Fermi approximation. It is demonstrated that in the linear regime the propagation of small density fluctuations [...] Read more.

A comparative study is conducted of the propagation of sound pulses in elongated Bose liquids and Bose-Einstein condensates in Gross-Pitaevskii and logarithmic models, by means of the Thomas-Fermi approximation. It is demonstrated that in the linear regime the propagation of small density fluctuations is essentially one-dimensional in both models, in the direction perpendicular to the cross section of a liquid’s lump. Under these approximations, it is demonstrated that the speed of sound scales as a square root of particle density in the case of the Gross-Pitaevskii liquid/condensate, but it is constant in a case of the homogeneous logarithmic liquid. Full article

(This article belongs to the Section Mathematical and Computational Fluid Mechanics)

8 pages, 2300 KiB

Open AccessArticle

Experimental Study of Cavitation Development and Secondary Circulation Flow between Two Eccentric Cylinders

by Anatoliy Monakhov and Nikolay Bukharin

Fluids 2022, 7(11), 357; https://doi.org/10.3390/fluids7110357 - 19 Nov 2022

Cited by 3 | Viewed by 1404

Abstract

The flow of a hydrophobic fluid in the gap between eccentric cylinders has been experimentally studied. The experimental setup was designed and built for this study. Experimental setup consists of two eccentric cylinders with the ability to rotate and a camera, a microscope, [...] Read more.

The flow of a hydrophobic fluid in the gap between eccentric cylinders has been experimentally studied. The experimental setup was designed and built for this study. Experimental setup consists of two eccentric cylinders with the ability to rotate and a camera, a microscope, and a pressure sensor. The conditions for gaseous cavitation occurrence during the rotation of the outer cylinder was considered in this study. The discreteness of gaseous cavitation occurrence in the form of individual bubbles is shown. When cavitation bubbles merge, the charge is redistributed at the gas–liquid interface, and bubble luminescence is observed. It has been shown that near the surface of the inner cylinder, in the area of flow expansion and compression, reverse flows occur. Full article

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30 pages, 590 KiB

Open AccessArticle

Internal Energy Relaxation Processes and Bulk Viscosities in Fluids

by Domenico Bruno and Vincent Giovangigli

Fluids 2022, 7(11), 356; https://doi.org/10.3390/fluids7110356 - 19 Nov 2022

Cited by 4 | Viewed by 1766

Abstract

Internal energy relaxation processes in fluid models derived from the kinetic theory are revisited, as are related bulk viscosity coefficients. The apparition of bulk viscosity coefficients in relaxation regimes and the links with equilibrium one-temperature bulk viscosity coefficients are discussed. First, a two-temperature [...] Read more.

Internal energy relaxation processes in fluid models derived from the kinetic theory are revisited, as are related bulk viscosity coefficients. The apparition of bulk viscosity coefficients in relaxation regimes and the links with equilibrium one-temperature bulk viscosity coefficients are discussed. First, a two-temperature model with a single internal energy mode is investigated, then a two-temperature model with two internal energy modes and finally a state-to-state model for mixtures of gases. All these models lead to a unique physical interpretation of the apparition of bulk viscosity effects when relaxation characteristic times are smaller than fluid times. Monte Carlo numerical simulations of internal energy relaxation processes in model gases are then performed, and power spectrums of density fluctuations are computed. When the energy relaxation time is smaller than the fluid time, both the two temperature and the single-temperature model including bulk viscosity yield a satisfactory description. When the energy relaxation time is larger than the fluid time, however, only the two-temperature model is in agreement with Boltzmann equation. The quantum population of a He-H

_{2}

mixture is also simulated with detailed He-H

_{2}

cross sections, and the resulting bulk viscosity evaluated from the Green–Kubo formula is in agreement with the theory. The impact of bulk viscosity in fluid mechanics is also addressed, as well as various mathematical aspects of internal energy relaxation and Chapman–Enskog asymptotic expansion for a two-temperature fluid model. Full article

(This article belongs to the Special Issue Bulk Viscosity and Relaxation Processes: Revisited)

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8 pages, 2101 KiB

Open AccessArticle

Drag Reduction in Polymer-Laden Turbulent Pipe Flow

by Francesco Serafini, Francesco Battista, Paolo Gualtieri and Carlo Massimo Casciola

Fluids 2022, 7(11), 355; https://doi.org/10.3390/fluids7110355 - 18 Nov 2022

Cited by 4 | Viewed by 1755

Abstract

The turbulence of a realistic dilute solution of DNA macromolecules is investigated through a hybrid Eulerian–Lagrangian approach that directly solves the incompressible Navier–Stokes equation alongside the evolution of

10^{8}

polymers, modelled as finitely extensible nonlinear elastic (FENE) dumbbells. At a friction Reynolds [...] Read more.

The turbulence of a realistic dilute solution of DNA macromolecules is investigated through a hybrid Eulerian–Lagrangian approach that directly solves the incompressible Navier–Stokes equation alongside the evolution of

10^{8}

polymers, modelled as finitely extensible nonlinear elastic (FENE) dumbbells. At a friction Reynolds number of 320 and a Weissenberg number of

2 \times 10^{4}

, the drag reduction is equal to 26%, which is similar to the one obtained at the lower Reynolds number of 180. The polymers induce an increase in the flow rate and the turbulent kinetic energy, whose axial contribution is predominantly augmented. The stress balance is analysed to investigate the causes of the drag reduction and eventually the effect of the friction Reynolds number on the probability distribution of the polymer configuration. Near the wall, the majority of the polymers are fully stretched and aligned along the streamwise direction, inducing an increase in the turbulence anisotropy. Full article

(This article belongs to the Special Issue Drag Reduction in Turbulent Flows)

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24 pages, 7881 KiB

Open AccessArticle

On the Rarefied Thermally-Driven Flows in Cavities and Bends

by Mostafa Mousivand and Ehsan Roohi

Fluids 2022, 7(11), 354; https://doi.org/10.3390/fluids7110354 - 18 Nov 2022

Cited by 2 | Viewed by 1650

Abstract

This study examined rarefied thermally-driven flow in a square cavity (Case 1) and rectangular bend (Case 2), with various uniform wall temperatures in two dimensions. We employed the direct simulation Monte Carlo (DSMC) to solve problems with a wide range of Knudsen numbers [...] Read more.

This study examined rarefied thermally-driven flow in a square cavity (Case 1) and rectangular bend (Case 2), with various uniform wall temperatures in two dimensions. We employed the direct simulation Monte Carlo (DSMC) to solve problems with a wide range of Knudsen numbers Kn = 0.01 to 10, and the discrete unified gas kinetic scheme (DUGKS) solver was used at Kn = 0.01. The scenario was that, in case 1, the bottom side and its opposite were set hot, and the other sides were set cold. Diffuse reflector boundary conditions were set for all walls. The imposed temperature differences created four primary vortices. The results of the continuum set of equations of the slow non-isothermal flow (SNIT) solver proved that the primary vortices in the square cavity were caused by nonlinear thermal stress effects, and other smaller vortices appearing at Kn = 0.01, 0.1 were brought about by thermal creep processes. As the Kn increased, vortices generated by thermal creep disappeared, and eddies created by nonlinear thermal stress occupied the cavity. In case 2, i.e., a rectangular bend, two sides were set cold, and the others were hot. Two primary vortices were formed, which were caused by nonlinear thermal stress effects. The direction of streamlines in the two main vortices was opposite, from the warm to the cold zone, as some eddies on the left were counterclockwise, and others were clockwise. Full article

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17 pages, 2828 KiB

Open AccessArticle

Aerodynamic Shape Optimization of a Symmetric Airfoil from Subsonic to Hypersonic Flight Regimes

by Bernardo Leite, Frederico Afonso and Afzal Suleman

Fluids 2022, 7(11), 353; https://doi.org/10.3390/fluids7110353 - 15 Nov 2022

Viewed by 5735

Abstract

Hypersonic flight has been the subject of numerous research studies during the last eight decades. This work aims to optimize the aerodynamic performance of a two-dimensional baseline airfoil (NACA0012) at distinct flight regimes from subsonic to hypersonic speeds. A mission profile has been [...] Read more.

Hypersonic flight has been the subject of numerous research studies during the last eight decades. This work aims to optimize the aerodynamic performance of a two-dimensional baseline airfoil (NACA0012) at distinct flight regimes from subsonic to hypersonic speeds. A mission profile has been defined, where four points representing the subsonic, transonic, supersonic, and hypersonic flow conditions have been selected. A framework has been implemented based on high-fidelity RANS computational fluid dynamics simulations. Gradient-based optimizations have been conducted with the objective of minimizing the drag. The optimization results show an overall improvement in aerodynamic performance, including a decrease in the drag coefficient of up to 79.2% when compared to the baseline airfoil. In the end, a morphing strategy has been laid out based on the optimal shapes produced by the optimization. Full article

(This article belongs to the Special Issue Recent Advances in Aerodynamics and Aeroacoustics: Towards Greener Aviation)

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Graphical abstract

17 pages, 1329 KiB

Open AccessArticle

Oblique Long Wave Scattering by an Array of Bottom-Standing Non-Smooth Breakwaters

by Prakash Kar, Harekrushna Behera and Trilochan Sahoo

Fluids 2022, 7(11), 352; https://doi.org/10.3390/fluids7110352 - 15 Nov 2022

Viewed by 1665

Abstract

Bragg scattering of surface gravity waves by an array of submerged bottom-standing non-smooth breakwaters is studied under the assumption of linearized long wave theory. The closed-form long-wave analytical solutions are derived and validated by comparing them with the results available in the literature. [...] Read more.

Bragg scattering of surface gravity waves by an array of submerged bottom-standing non-smooth breakwaters is studied under the assumption of linearized long wave theory. The closed-form long-wave analytical solutions are derived and validated by comparing them with the results available in the literature. The role of various physical parameters such as breakwaters friction coefficient, depth, width and gap between the adjacent breakwaters are investigated by analyzing the reflection and transmission coefficients. Further, the time-domain simulation for the scattering of long gravity waves over multiple breakwaters is analysed for different values of parameters of breakwaters. The results reveal that the rough surface of the breakwater plays a vital role in reducing wave reflection and transmission. Moreover, it is observed that the transmitted wave dissipates completely for larger values of friction parameters. For certain critical angles, change in wave dissipation becomes maximum due to the variation of phase of the incident wave. Various findings can be considered as benchmark results for the design of the non-smooth structures to attenuate the waves based on the Bragg reflection. Full article

(This article belongs to the Special Issue Fluid Dynamics: Wave–Structure Interactions)

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24 pages, 661 KiB

Open AccessArticle

A Framework for Generating Radial and Surface-Oriented Regularized Stokeslets

by Nicholas G. Chisholm and Sarah D. Olson

Fluids 2022, 7(11), 351; https://doi.org/10.3390/fluids7110351 - 14 Nov 2022

Cited by 1 | Viewed by 1872

Abstract

Error in the method of regularized Stokeslets is highly dependent on the choice of the blob or regularization function that is utilized to handle singularities in the flow. In this work, we develop a general framework to choose regularizations at the level of [...] Read more.

Error in the method of regularized Stokeslets is highly dependent on the choice of the blob or regularization function that is utilized to handle singularities in the flow. In this work, we develop a general framework to choose regularizations at the level of the vector potential via smoothing factors. We detail the derivation for radial smoothing factors and specify properties which ensure that the solution is a regularized flow satisfying the incompressible Stokes equations. Error analysis is completed for both the far-field flow (away from the location of the forces) as well as at the location of the forces, relating our newly derived smoothing factors to commonly used blob functions and moment conditions. When forces are on a surface, we extend the radial smoothing factor case to the case of non-radial regularizations that are surface-oriented. We illustrate the utility of this framework by computing the forward and inverse problems of a translating sphere using radial and surface-oriented regularizations. Full article

(This article belongs to the Special Issue 20 Years of Regularized Stokeslets: Applications, Computation, and Theory)

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9 pages, 2858 KiB

Open AccessArticle

Dynamics of Laser-Induced Shock Waves in Supercritical CO₂

by Nika Asharchuk and Evgenii Mareev

Fluids 2022, 7(11), 350; https://doi.org/10.3390/fluids7110350 - 10 Nov 2022

Cited by 1 | Viewed by 1824

Abstract

We studied the dynamics of laser-induced shock waves in supercritical CO₂ (scCO₂) for different pressures and temperatures under nanosecond optical breakdown. We estimated the shock wave pressure and energy, including their evolution during shock wave propagation. The maximal shock wave [...] Read more.

We studied the dynamics of laser-induced shock waves in supercritical CO₂ (scCO₂) for different pressures and temperatures under nanosecond optical breakdown. We estimated the shock wave pressure and energy, including their evolution during shock wave propagation. The maximal shock wave pressure ~0.5 GPa was obtained in liquid-like scCO₂ (155 bar 55 °C), where the fluid density is greater. However, the maximal shock wave energy ~25 μJ was achieved in sub-critical conditions (67 bar, 55 °C) due to a more homogeneous microstructure of fluid in comparison with supercritical fluid. The minimal pressure and energy of the shock wave are observed in the Widom delta (a delta-like region in the vicinity of the critical point) due to the clusterization of scCO₂, which strongly affects the energy transfer from the nanosecond laser pulse to the shock wave. Full article

(This article belongs to the Special Issue Frontiers in Supercritical Fluids)

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29 pages, 12987 KiB

Open AccessArticle

Magnetohydrodynamics Simulation of the Nonlinear Behavior of Single Rising Bubbles in Liquid Metals in the Presence of a Horizontal Magnetic Field

by Marino Corrado and Yohei Sato

Fluids 2022, 7(11), 349; https://doi.org/10.3390/fluids7110349 - 10 Nov 2022

Cited by 2 | Viewed by 1972

Abstract

Rising bubbles in liquid metals in the presence of magnetic fields is an important phenomenon in many engineering processes. The nonlinear behavior of the terminal rise velocities of the bubbles as a function of increasing field strength has been observed experimentally, but it [...] Read more.

Rising bubbles in liquid metals in the presence of magnetic fields is an important phenomenon in many engineering processes. The nonlinear behavior of the terminal rise velocities of the bubbles as a function of increasing field strength has been observed experimentally, but it remains poorly understood. We offer an explanation of the phenomenon through numerical calculations. A single rising bubble in stagnant liquid metal in the presence of an applied horizontal magnetic field is simulated. The observed nonlinear behavior is successfully reproduced; the terminal velocity increases with the increase in the magnetic field strength in the lower magnetic field regions but decreases in higher regions. It is shown that, in the lower region, the increase in the average bubble rise velocity results from the suppression of the fluctuations in the bubble trajectory in the vertical plane perpendicular to the magnetic field, as a consequence of the Lorentz force resulting from the component of induced electric current due to the magnetic field, which (approximately) acts in the opposite direction to that of the flow velocity. For higher magnetic field strengths, the Lorentz force induces a broadened wake in the vertical plane parallel to the applied magnetic field, resulting in a decrease in the rise velocity. Full article

(This article belongs to the Topic Computational Fluid Dynamics (CFD) and Its Applications)

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19 pages, 7201 KiB

Open AccessArticle

Enhanced Energy Storage Using Pin-Fins in a Thermohydraulic System in the Presence of Phase Change Material

by Mohamad Ziad Saghir

Fluids 2022, 7(11), 348; https://doi.org/10.3390/fluids7110348 - 9 Nov 2022

Cited by 5 | Viewed by 1491

Abstract

Energy storage has been an essential topic in thermal management. With the low conductivity of phase change material, the effort is to propose the best mechanism for heat transfer. In the present paper, pin-fins are used in the hydraulic system to transfer the [...] Read more.

Energy storage has been an essential topic in thermal management. With the low conductivity of phase change material, the effort is to propose the best mechanism for heat transfer. In the present paper, pin-fins are used in the hydraulic system to transfer the heat coming from wastewater management into phase change material. Different flow rates have been tested, and it was found that pin-fins can create mixing in the flow chamber allowing a large convective heat flux to move heat into the phase change material. In the present design, it was found that natural convection assists in heat transfer. Additional findings suggested that the pin-fins height influence the heat transfer process. In the current configuration, 5 mm in height pin-fins demonstrated the best heat transfer when compared to pin-fins varying from 1 mm to 6 mm, respectively. Full article

(This article belongs to the Section Heat and Mass Transfer)

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12 pages, 2069 KiB

Open AccessArticle

Superficial Velocity in Heterogeneous Two-Phase Systems with Truncated Fractal Distribution of Particle Concentration

by Jianting Zhu

Fluids 2022, 7(11), 347; https://doi.org/10.3390/fluids7110347 - 9 Nov 2022

Cited by 1 | Viewed by 1667

Abstract

The interplay of particles in a heterogeneous multiparticle two-phase system and its effect on superficial velocity have not been well quantified. In this study, a new model is developed to examine the superficial velocity in a heterogeneous multiparticle two-phase system. To examine the [...] Read more.

The interplay of particles in a heterogeneous multiparticle two-phase system and its effect on superficial velocity have not been well quantified. In this study, a new model is developed to examine the superficial velocity in a heterogeneous multiparticle two-phase system. To examine the heterogeneous effects to the potentially maximum extent, the particle concentration is assumed to follow a truncated fractal distribution, which is integrated into the free surface cell model. In a statistical sense, the multiparticle two-phase system is stationary, so the mean of spatial heterogeneity can be replaced by the ensemble mean. Since the underlying physical concept is rooted in the free surface cell model, the validity of the model should be, therefore, limited to the low-Reynolds number conditions. The developed model is compared to data from three representative experimental studies in the literature and it is found that the model can better capture the scatters in experimental data than the original free surface cell model. The model is also compared with three representative models and demonstrates reasonable results. While the deterministic free surface cell model underestimates the velocity, the cell model with truncated fractal distribution being incorporated can predict high velocity with a wide range of particle concentration heterogeneity. Full article

(This article belongs to the Section Flow of Multi-Phase Fluids and Granular Materials)

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16 pages, 4940 KiB

Open AccessArticle

Three-Dimensional Simulations of Viscous Incompressible Fluid Flows on Grids with Unmatched Grid Interfaces

by Andrey Kozelkov, Andrey Kurkin, Aleksey Korotkov, Sergey Lashkin and Elmira Lashkina

Fluids 2022, 7(11), 346; https://doi.org/10.3390/fluids7110346 - 9 Nov 2022

Cited by 1 | Viewed by 1291

Abstract

The paper describes a numerical method that considers specific computational fluid dynamics (CFD) aspects of viscous incompressible flow simulations in the vicinity of interfaces between unmatched fragments of unstructured grids. The method is based on the general grid interface (GGI) principle, which involves [...] Read more.

The paper describes a numerical method that considers specific computational fluid dynamics (CFD) aspects of viscous incompressible flow simulations in the vicinity of interfaces between unmatched fragments of unstructured grids. The method is based on the general grid interface (GGI) principle, which involves conservative flux interpolation and does not require original grid modification at unmatched interfaces. A method is presented which combines adjacent unmatched fragments of an unstructured grid into a single domain by means of virtual interfaces considering connections between adjacent cells through virtual faces. The performance of the method is illustrated by the finite-volume discretization of the transport equation in the region of matched interfaces and its modification for the case of unmatched interfaces. The efficiency of the proposed method is demonstrated by three-dimensional CFD simulations with grid models composed of unmatched unstructured grid fragments. The simulation results are compared with equivalent simulations on matched grids. The influence of unmatched interfaces on the convergence rate and accuracy of the solution is assessed. Full article

(This article belongs to the Special Issue Applications in Computational Fluid Dynamics)

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24 pages, 17680 KiB

Open AccessArticle

Heat Transfer in a Non-Isothermal Collisionless Turbulent Particle-Laden Flow

by Hamid Reza Zandi Pour and Michele Iovieno

Fluids 2022, 7(11), 345; https://doi.org/10.3390/fluids7110345 - 7 Nov 2022

Cited by 11 | Viewed by 1868

Abstract

To better understand the role of particle inertia on the heat transfer in the presence of a thermal inhomogeneity, Eulerian–Lagrangian direct numerical simulations (DNSs) have been carried out by using the point–particle model. By considering particles transported by a homogeneous and isotropic, statistically [...] Read more.

To better understand the role of particle inertia on the heat transfer in the presence of a thermal inhomogeneity, Eulerian–Lagrangian direct numerical simulations (DNSs) have been carried out by using the point–particle model. By considering particles transported by a homogeneous and isotropic, statistically steady turbulent velocity field with a Taylor microscale Reynolds number from 37 to 124, we have investigated the role of particle inertia and thermal inertia in one- and two-way coupling collisionless regimes on the heat transfer between two regions at uniform temperature. A wide range of Stokes numbers, from 0.1 to 3 with a thermal Stokes-number-to-Stokes-number ratio equal to 0.5 to 4.43 has been simulated. It has been found that all moments always undergo a self-similar evolution in the interfacial region between the two uniform temperature zones, the thickness of which shows diffusive growth. We have determined that the maximum contribution of particles to the heat flux, relative to the convective heat transfer, is achieved at a Stokes number which increases with the ratio between thermal Stokes and Stokes number, approaching 1 for very large ratios. Furthermore, the maximum increases with the thermal Stokes-to-Stokes number ratio whereas it reduces for increasing Reynolds. In the two-way coupling regime, particle feedback tends to smooth temperature gradients by reducing the convective heat flux and to increase the particle turbulent heat flux, in particular at a high Stokes number. The impact of particle inertia reduces at very large Stokes numbers and at larger Reynolds numbers. The dependence of the Nusselt number on the relevant governing parameters is presented. The implications of these findings for turbulence modelling are also briefly discussed. Full article

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12 pages, 4700 KiB

Open AccessArticle

Deep Learning Forecasts a Strained Turbulent Flow Velocity Field in Temporal Lagrangian Framework: Comparison of LSTM and GRU

by Reza Hassanian, Ásdís Helgadóttir and Morris Riedel

Fluids 2022, 7(11), 344; https://doi.org/10.3390/fluids7110344 - 3 Nov 2022

Cited by 10 | Viewed by 2565

Abstract

The subject of this study presents an employed method in deep learning to create a model and predict the following period of turbulent flow velocity. The applied data in this study are extracted datasets from simulated turbulent flow in the laboratory with the [...] Read more.

The subject of this study presents an employed method in deep learning to create a model and predict the following period of turbulent flow velocity. The applied data in this study are extracted datasets from simulated turbulent flow in the laboratory with the Taylor microscale Reynolds numbers in the range of 90 <

R_{λ}

< 110. The flow has been seeded with tracer particles. The turbulent intensity of the flow is created and controlled by eight impellers placed in a turbulence facility. The flow deformation has been conducted via two circular flat plates moving toward each other in the center of the tank. The Lagrangian particle-tracking method has been applied to measure the flow features. The data have been processed to extract the flow properties. Since the dataset is sequential, it is used to train long short-term memory and gated recurrent unit model. The parallel computing machine DEEP-DAM module from Juelich supercomputer center has been applied to accelerate the model. The predicted output was assessed and validated by the rest of the data from the experiment for the following period. The results from this approach display accurate prediction outcomes that could be developed further for more extensive data documentation and used to assist in similar applications. The mean average error and R² score range from 0.001–0.002 and 0.9839–0.9873, respectively, for both models with two distinct training data ratios. Using GPUs increases the LSTM performance speed more than applications with no GPUs. Full article

(This article belongs to the Special Issue Machine Learning in Fluid Flow and Heat Transfer)

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11 pages, 11104 KiB

Open AccessArticle

Computational Fluid Dynamics Model for Analysis of the Turbulent Limits of Hydrogen Combustion

by Ivan Yakovenko, Alexey Kiverin and Ksenia Melnikova

Fluids 2022, 7(11), 343; https://doi.org/10.3390/fluids7110343 - 1 Nov 2022

Cited by 4 | Viewed by 2194

Abstract

This paper presents a novel numerical approach for assessing the turbulent limits of hydrogen combustion. In the framework of this approach, the premixed combustion is studied numerically in the externally generated turbulent field with defined parameters. Two-dimensional calculations are carried out for hydrogen–air [...] Read more.

This paper presents a novel numerical approach for assessing the turbulent limits of hydrogen combustion. In the framework of this approach, the premixed combustion is studied numerically in the externally generated turbulent field with defined parameters. Two-dimensional calculations are carried out for hydrogen–air mixtures of different compositions, and all the possible modes of near-limit combustion are reproduced. Among these modes are: combustion in the form of spatially separated individual kernels and combustion in the form of kernels with subsequent quenching. The critical conditions between the mentioned two modes correspond to the turbulent limits of hydrogen combustion, which are necessary for the evaluation of the hazardous risks related to hydrogen explosions. Full article

(This article belongs to the Special Issue Next-Generation Methods for Turbulent Flows)

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11 pages, 1451 KiB

Open AccessArticle

Transient Electrophoresis of a Cylindrical Colloidal Particle

by Hiroyuki Ohshima

Fluids 2022, 7(11), 342; https://doi.org/10.3390/fluids7110342 - 29 Oct 2022

Cited by 8 | Viewed by 1702

Abstract

We develop the theory of transient electrophoresis of a weakly charged, infinitely long cylindrical colloidal particle under an application of a transverse or tangential step electric field. Transient electrophoretic mobility approaches steady electrophoretic mobility with time. We derive closed-form expressions for the transient [...] Read more.

We develop the theory of transient electrophoresis of a weakly charged, infinitely long cylindrical colloidal particle under an application of a transverse or tangential step electric field. Transient electrophoretic mobility approaches steady electrophoretic mobility with time. We derive closed-form expressions for the transient electrophoretic mobility of a cylinder without involving numerical inverse Laplace transformations and the corresponding time-dependent transient Henry functions. The transient electrophoretic mobility of an arbitrarily oriented cylinder is also derived. It is shown that in contrast to the case of steady electrophoresis, the transient Henry function of an arbitrarily oriented cylinder at a finite time is significantly smaller than that of a sphere with the same radius and mass density as the cylinder so that a cylinder requires a much longer time to reach its steady mobility than the corresponding sphere. Full article

(This article belongs to the Section Flow of Multi-Phase Fluids and Granular Materials)

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12 pages, 773 KiB

Open AccessFeature PaperArticle

Numerical Simulation of Mixing Fluid with Ferrofluid in a Magnetic Field Using the Meshless SPH Method

by Mohsen Abdolahzadeh, Ali Tayebi, Mehrdad Ahmadinejad and Božidar Šarler

Fluids 2022, 7(11), 341; https://doi.org/10.3390/fluids7110341 - 29 Oct 2022

Cited by 1 | Viewed by 1981

Abstract

In this study, a numerical investigation of the effect of different magnetic fields on ferrofluid-fluid mixing processes in a two-dimensional microchannel is performed An improved version of smoothed particle hydrodynamics, SPH, by shifting particle algorithm and dummy particle boundary condition, is implemented to [...] Read more.

In this study, a numerical investigation of the effect of different magnetic fields on ferrofluid-fluid mixing processes in a two-dimensional microchannel is performed An improved version of smoothed particle hydrodynamics, SPH, by shifting particle algorithm and dummy particle boundary condition, is implemented to solve numerical continuity, ferrohydrodynamics-based momentum and mass transfer equations. SPH is formulated through the irregular arrangement of the nodes where the fields are approximated using the fifth-order Wendland kernel function. After validating the computational approach, the influence of the number (from one to three) of parallel electrical wires positioned perpendicular to the microchannel on the mixing efficiency is studied for the first time. It has originally been found that the mixing efficiency highly non-linearly depends on the Reynolds number and the number of electrical wires. For Re ≤ 20 the mixing efficiency is almost the same for two and three electrical wires and about two times higher than one electrical wire. For Re ≥ 80, the mixing efficiency of three wires is much higher than one and two electrical wires. Optimum performance of the micromixer is achieved with three electrical wires, since the mixer performs well on a broader range of Re than the other two studied cases. The outcomes of this study, obtained by a meshless method, are important for the industrial design of micromixers. Full article

(This article belongs to the Special Issue The Recent Advances in Magnetorheological Fluids)

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21 pages, 26266 KiB

Open AccessArticle

Experimental and CFD Investigation of Directional Stability of a Box-Wing Aircraft Concept

by Gueraiche Djahid, Karpovich Elena, Pikulev Maxim, Kuznetsov Alexander, Sergey Popov and Manoranjan Sinha

Fluids 2022, 7(11), 340; https://doi.org/10.3390/fluids7110340 - 27 Oct 2022

Cited by 1 | Viewed by 2110

Abstract

This study aimed to explore the directional stability issues of a previously studied light box-wing aircraft model with a pusher propeller engine in the fuselage aft section. Earlier configurations have included the use of fuselage together with a lifting system consisting of two [...] Read more.

This study aimed to explore the directional stability issues of a previously studied light box-wing aircraft model with a pusher propeller engine in the fuselage aft section. Earlier configurations have included the use of fuselage together with a lifting system consisting of two wings joined together at their wingtips with vertical stabilizers. However, these side vertical surfaces failed to provide the aircraft with sufficient directional stability, thus prompting the quest in this study for novel solutions that would exclude the need for a fuselage extension and a typical fin. Solutions included the use of a ducted propeller and few configurations of small “fishtail” vertical fins, which formed part of the aft fuselage itself and coupled with vortex generators on the fuselage surface to improve their interference and heal flow separation at the fuselage aft cone. The results of wind tunnel testing were supported with CFD simulations to explain the flow behavior of each of the studied solutions. Tuft visualization and computed flow patterns allowed identification of the sources of the observed low efficiency in terms of directional stability of the fishtail against a simple idle duct without a propeller. A final configuration with a duct and a modified version of the fuselage fins was achieved that provides enough yaw stability margins for a safe flight. Full article

(This article belongs to the Special Issue High Speed Flows)

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18 pages, 12231 KiB

Open AccessArticle

Computational Fluid Dynamics Approach for Oscillating and Interacting Convective Flows

by Attila Gergely and Zoltán Néda

Fluids 2022, 7(11), 339; https://doi.org/10.3390/fluids7110339 - 24 Oct 2022

Cited by 2 | Viewed by 2162

Abstract

The oscillation and collective behavior of convective flows is studied by a computational fluid dynamics approach. More specifically, the rising dynamics of heated fluid columns is simulated in gravitational field using a simplified 2D geometry. The numerical method uses the FEniCS package for [...] Read more.

The oscillation and collective behavior of convective flows is studied by a computational fluid dynamics approach. More specifically, the rising dynamics of heated fluid columns is simulated in gravitational field using a simplified 2D geometry. The numerical method uses the FEniCS package for solving the coupled Navier–Stokes and heat-diffusion equations. For the flow of a single heated fluid column, the effect of the inflow yield and the nozzle diameter is studied. In agreement with the experiments, for a constant nozzle diameter the oscillation frequency increases approximately linearly as a function of the the flow rate, while for a constant flow rate the frequency decreases as a power law with the increased nozzle diameter. For the collective behavior of two nearby flows, we find a counter-phase synchronization and a decreasing trend of the common oscillation frequency with the distance between the jets. These results are in agreement with the experiments, and our computational study also suggests that the phenomenon is present on largely different length-scales. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers, 2022)

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Fluids, Volume 7, Issue 11 (November 2022) – 21 articles

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