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This numerical investigation employs advanced computational fluid dynamics techniques to examine the intricate physics of cavitating flows around a pitching NACA 66 hydrofoil. Unsteady simulations are conducted using both large eddy simulation (LES) and K-Omega SST turbulence models. The pitching motions range from 0 to 15°, encompassing four cavitation numbers (3.25, 3.5, 3.75, 4) and a Reynolds number of 750,000. The study comprehensively analyzes various flow field characteristics, including pressure distribution, velocity, vorticity, shear stress, vapor volume fraction, and turbulence kinetic energy. The kinematics of pitching significantly influence hydrodynamic loads and the surrounding flow structures in cavitating conditions. Flow visualizations illustrate the evolution of sheet/cloud cavitation, reentrant jet, and vortex dynamics throughout a pitching cycle. Notable findings demonstrate the impact of cavitation number on cavity shape and the relationship between angle of attack, reentrant jet location, and vortex size. Additionally, negative vorticity dilatation is observed to be correlated with cavity size. • The cavitation and supercavitation structure around a 2D, 3D pitching hydrofoil is studied using LES. • The impact of cavitation number on cavity shape and the relationship between angle of attack, reentrant jet location, and vortex size was studied. • Determination of the effect of pitching motion in cavitation flow on hydrodynamic coefficients and surrounding flow structure [ABSTRACT FROM AUTHOR]

In this paper we numerically evaluate the recently developed Aoki et al. slip and jump conditions in high-speed rarefied gas flows for the first time. These slip and jump conditions are developed to be employed with the Navier–Stokes–Fourier equations. They were derived based on the Boltzmann equation with the first order Chapman–Enskog solution, and the analysis of the Knudsen layer. Four aerodynamic configurations are selected for a comprehensive evaluation of these conditions such as sharp-leading-edge flat plate, vertical plate, wedge and circular cylinder in cross-flow with the Knudsen number varying from 0.004 to 0.07, and argon as the working gas. The simulation results using the Aoki et al. boundary conditions show suitable agreement with the DSMC data for slip velocity and surface gas temperature. The accuracy of these boundary conditions is superior to the conventional Maxwell, Smoluchowski and Le boundary conditions. [ABSTRACT FROM AUTHOR]

In this paper, wavelet analysis of the cavitating flow over a sphere is reported. Unsteady and dynamic behaviors of cavitation were captured using large eddy simulation (LES) turbulence approach and Sauer mass transfer models. Numerical simulation is implemented under the framework of OpenFOAM within the interPhaseChangeFoam solver. The simulation is conducted over a wide range of cavitation numbers. Two more essential variations, pressures and kinetic energies, were considered at specific points in front and behind of the sphere's body for sufficient simulation period. The oscillations global frequency modes and spectral content of the cavity cloud are computed and analyzed using Fourier and continuous wavelet transformations. The computed results show that the flow fluctuations enhance by increasing the cavitation number. The low-frequency fluctuations play a pivotal role in the cavitating flow and possess almost the same magnitude in all investigated cavitation numbers. The frequencies enhance as the simulation time increases in all cases. One of the primary frequencies that happened in all cavitation numbers in the cavity cloud separation is due to a Strouhal number within the range of 0.046 and 0.05. Therefore, this Strouhal number can be used for the purpose of cloud cavitation detection. • LES simulation of cavitating flow around sphere in OpenFOAM. • Providing a thorough understanding of the wavelet analysis around a sphere for the first time. • Reporting the oscillations global frequency modes and spectral content of the sphere cavity cloud. [ABSTRACT FROM AUTHOR]

Rarefied gas flows in nano-scale isosceles triangular cavities with a motion away from the square corner are considered over a wide range of flow rarefactions regimes, i.e., 0.01 ≤ Kn ≤ 10, using the direct simulation Monte Carlo (DSMC) method. We put emphasis to the rarefaction effects on the vortical behaviors and heat transfer patterns of rarefied monatomic and diatomic gases. We show that the primary vortices appearing in the triangular cavity are Moffat-type vortices, which disappear as velocity slip increases over the walls. However, an additional vortex is observed at Kn > 1 on the inclined wall attributable to the balance of the thermally driven flows from the cold to the hot region and flow induced by the primary vortex. We also report variations in the effective length scale of the fluid circulation with the Knudsen number. Our investigations specify that the competition between the Fourier term and shear stress gradient component of the heat flux constitutive relation results in the cold-to-hot transfer in triangular cavities. Also, heat flux patterns predicted by the asymptotic theory of the Boltzmann equation for the weakly non-linear flow and by the linearized form of the Regularized 13 moments (R13) equations are compared with the DSMC solution suitably at low Knudsen number regimes. [ABSTRACT FROM AUTHOR]

Here we examine partial and supercavitation over a sphere at a constant Reynolds number of 1.5 × 10 6 and a broad range of cavitation numbers (0.36 <σ< 1). Large eddy simulation (LES) and Sauer mass transfer model were used to simulate the dynamic and unsteady cavitation around the sphere. Also, the compressive volume of fluid (VOF) method is used to track the cavity interface. The two-phase flow solver of the OpenFOAM package, intephaseChangeFoam is employed. Large-eddy simulation of cavitating flow over the sphere is compared with the non-cavitating flow at the same Reynolds number. This work provides a thorough understanding of the fluid dynamic characteristics of the sphere cavitation such as vorticity field, turbulent kinetic energy, pressure, velocity, streamlines and boundary layer. Also, detailed analyses of the instantaneous cavity leading edge and separation point location, vortex shedding, streamwise velocity fluctuation and evolution of the cavity are reported. Characteristics of the wake of the cavitating flows are compared with the single-phase results. We report that cavitation suppresses instability in the near wake region and delays the three-dimensional breakdown of the vortices. The volume fraction contours of the cavity cloud obtained from the numerical simulations are compared with the experimental data at the same working condition with a suitable quantitative accuracy. [ABSTRACT FROM AUTHOR]

In this study we use the direct simulation Monte Carlo (DSMC) to elaborate on the heat transfer patterns in the pressure-driven rarefied flow through micro/nanochannels. Finite length planar micro/nanochannels are considered using with symmetrical wall heat flux boundary conditions, and the gas flow is considered to be in slip and transition regimes. When considering zero-conductive or cooled walls, the DSMC solution predicts a possibility of the anti-Fourier heat transfer, i.e., the transfer of heat from cold-to-hot regions of the flow field. It turns out that the competition between the contributions of temperature gradient and pressure gradient (shear stress) in the heat flux results in three different heat transfer regimes. The regimes consist of complete hot-to-cold heat transfer regime, the entire anti-Fourier regime, and localized anti-Fourier regime. While the heat flux due to the shear stress is directed from the outlet towards the inlet, the Fourier term is strongly influenced by viscous slip heating, which then acts as a heat source, and contributes to patterns of heat flux on the fluid layer adjacent to the walls. Furthermore, the heat flux regimes for complete or localized cold-to-hot transfer are classified according to the magnitude of the normalized heat flux and the Knudsen number. Additionally, effects of heat flux condition on the mass flow rate are discussed. [ABSTRACT FROM AUTHOR]

The aim of the current work is to evaluate the accuracy and efficiency of a newly suggested collision scheme in the Direct Simulation Monte Carlo (DSMC) method; Simplified Bernoulli Trials implemented over transient adaptive subcells, SBT-TAS, for two different unsteady flow test cases. First, the performance of the SBT-TAS scheme is evaluated via simulating the shock-tube test case through a broad range of Knudsen numbers. The shock-tube investigations indicated that the SBT-TAS scheme could predict solutions as accurate as the nearest neighbor (NN) scheme. We show that the implemented SBT-TAS scheme requires at least 3–4 particles per each subcell to predict intermolecular collision frequency accurately. Then, a two-dimensional cylinder at Mach number of 0.6 is considered to investigate the accuracy and efficiency of the SBT-TAS in the prediction of the unsteady vortexes shedding. On the same grid and using the same number of particles per cell (PPC), the SBT and No Time Counter (NTC) collisions schemes fail to predict the correct vortex shedding behind the cylinder accurately and the structure of vortices are smeared out while the SBT-TAS and NN collisions schemes predict the strength of the vortices and their oscillation frequency with the same level of accuracy. Our results indicate that SBT-TAS preserves its accuracy in prediction of the aforementioned phenomenon while it uses relatively the same sample sizes and number of particles compared with its NN counterpart. [ABSTRACT FROM AUTHOR]

In this paper, cavitation and supercavitation around 3D hemispherical head-form body and a conical cavitator were simulated. Dynamic and unsteady behaviors of cavitation were solved using large eddy simulation (LES) and k-ω SST turbulence models, as well as Kunz and Sauer mass transfer models. In addition, the compressive volume of fluid (VOF) method is used to track the cavity interface. Simulation is performed under the framework of the OpenFOAM package. The main contribution of this work is to present a correlation between the cavity length and diameter for hemispherical head-form bodies for the first time. Moreover, we provide a detailed comparison between different turbulence and mass transfer models over a broad range of cavitation numbers, especially in small cavitation numbers, including σ =0.07, 0.05, 0.02 for two cases, which is not reported previously. Our numerical results are compared with the available experimental data and a broad set of analytic relations for the cavity characteristics such as cavity length and diameter with suitable accuracy. Discussions on boundary layer separation and re-entrant jet behavior, which play a significant role in the bubble shedding in the cavity closure region, are presented. [ABSTRACT FROM AUTHOR]

In this study, we performed numerical investigations of the cavitating and supercavitating flow behind a three-dimensional disk with a particular emphasis on detailed comparisons of various turbulence and mass transfer models. Simulations were performed using the OpenFOAM package and flows at three different cavitation numbers, ( σ = 0.2, 0.1, and 0.05) were considered. Large eddy simulation (LES) and k – ω shear stress transport turbulence approaches were coupled with various mass transfer model types (e.g., Kunz, Schnerr–Sauer, and Zwart models). The Zwart mass transfer model was added to the standard OpenFOAM package. A compressive volume of fluid method was used to track the interface between the liquid and vapor phases. Our numerical results in terms of the cavity length, diameter, and drag coefficient compared fairly well with experimental data and a broad set of analytical relations. Moreover, this study provides a better understanding of the cavitation dynamics behind disk cavitators. Our results indicate that the most accurate solutions will be obtained by applying an LES turbulence approach combined with the Kunz mass transfer model. [ABSTRACT FROM AUTHOR]

The accuracy of numerical simulations of rarefied gas flows, in particular the Navier-Stokes-Fourier (N–S–F) equations, depends on the employed surface boundary conditions. In the literature, the combination of the second-order slip/jump conditions has primarily been used for either the Burnett or the BGK Burnett equations for hypersonic gas flows. In this work, we suggest the second-order temperature jump condition in a new form. The second-order slip/jump conditions are implemented in the framework of OpenFOAM to employ with the N–S–F equations for low-speed nanoscale and hypersonic rarefied gas flows. We investigate both the first and second-order slip/jump boundary conditions for low speed rarefied gas flow in the pressure-driven backward facing step nanochannel as well as hypersonic gas flows over the flat plate and past a circular cylinder in cross-flow. Simulation results show that the combination of the second-order slip/jump (in new form) conditions predicts better surface properties than those of the first-order slip/jump conditions for all cases studied by comparing the Burnett and DSMC data. Especially, the N–S–F simulation results of the second-order slip/jump (in new form) conditions of the cylinder case can capture the Burnett data at Kn = 0.1, while those of the first-order conditions do not. [ABSTRACT FROM AUTHOR]

Flow regimes through micro/nano nozzles experience the continuum behavior as well as the rarefied condition. The occurrence of broad rarefaction regimes due to unique geometry/flow conditions makes the particle-based simulation of micro/nano nozzles computationally expensive. Therefore, the primary aim of the present study is to implement the Simplified Bernoulli trials (SBT) collision algorithm in place of the conventional “No-Time-Counter” scheme in the direct simulation Monte Carlo (DSMC) method, thus reducing the required particle number. We show that the SBT scheme employed on the proper number of grid cells could reduce the computational costs and memory of micro/nano nozzle flow simulations. We also investigated the physical behavior of gas flows in micro/nano nozzles. We report the bimodal behavior of the velocity, temperature and Mach number at the throat at lower Knudsen numbers and the occurrence of Mach < 1 in the throat. We also considered back pressure effects on the flow field and the impact of gas species on the micro/nano nozzle performance. We identified an efficiency reduction due to rarefaction and improvements of the micro/nano nozzle performance due to increasing of the throat dimension. Performance reduction at the nanoscale is also discussed. [ABSTRACT FROM AUTHOR]

In the current work, the effect of shear work due to the velocity slip on the non-equilibrium heat transfer in a pressure driven micro/nanochannel is evaluated under the constant wall heat flux boundary condition. As our simulation tool, the DSMC method is employed. Implementation of the wall heat flux in the DSMC method is performed using the “modified Iterative” technique. We investigate the effects of rarefaction, property variations and compressibility. The numerical results show that shear stress on the walls significantly affects all aspects of the flow behavior and heat transfer through micro/nanochannels such as heat flux rates. We also analyze the counter-gradient heat flow (cold to hot heat transfer) phenomenon appearing at the cooling conditions. It is observed that viscous dissipation affects the heat flux applied to the walls and may overcome the wall heat flux, i.e., in the case of low cooling wall heat flux condition, shear work may completely heat the flow field. Nusselt number singularity is also discussed. [ABSTRACT FROM AUTHOR]

Three-dimensional computational fluid dynamics analyses have been employed to study the compressible and turbulent flow of the shock train in a convergent–divergent nozzle. The primary goal is to determine the behavior, location, and number of shocks. In this context, full multi-grid initialization, Reynolds stress turbulence model (RSM), and the grid adaption techniques in the Fluent software are utilized under the 3D investigation. The results showed that RSM solution matches with the experimental data suitably. The effects of applying heat generation sources and changing inlet flow total temperature have been investigated. Our simulations showed that changes in the heat generation rate and total temperature of the intake flow influence on the starting point of shock, shock strength, minimum pressure, as well as the maximum flow Mach number. [ABSTRACT FROM AUTHOR]

In this study, rarefied supersonic and subsonic gas flow around a NACA 0012 airfoil is simulated using both continuum and particle approaches. Navier-Stokes equations subject to the first order slip/jump boundary conditions are solved under the framework of OpenFOAM package. The DSMC solver of the package, i.e., dsmcFoam, has been improved to include a newly presented "simplified Bernoulli trial (SBT)" scheme for inter-molecular collision modeling. The use of SBT collision model permits to obtain accurate results using a much lower number of simulator particles. We considered flow at different angles of attacks and Knudsen numbers at both the subsonic and supersonic regimes. The computed density and surface pressure distributions are compared with the experimental and numerical data and suitable accuracy was observed. We investigate variations of the lift and drag coefficients with the Knudsen number and angle of attack. At low Kn number in supersonic regime, our results for lift coefficient agree well with the linearized theory; however, the deviation starts as soon as the angle of attack goes beyond 15 ° or shock wave forms above the airfoil. Along with this, we have observed that drag coefficient increases with the Kn number increasing. We also investigated the effect of Kn number on the leading edge shock position and structure, drag polar ( C L / C D ) , and slip velocity over the airfoil. [ABSTRACT FROM AUTHOR]

In the present study, we investigate the characteristics of thermal cavities in the rarefied flow regime using the Direct Simulation Monte Carlo (DSMC). We use a recently developed iterative technique to impose a desired wall heat flux boundary condition in the DSMC algorithm. Fluid mechanics and heat transfer behavior are studied over the walls and in the domain of the thermal cavity over a wide range of Knudsen number in the slip and transition regimes. The vortice behavior is described at different Knudsen numbers in detail. We numerically justify unconventional flow movement from the cold region to the hot region. Finally, we consider the effects of molecular structural parameters such as molecular mass and degree of freedom on the thermal behavior of the thermal cavity flows. [ABSTRACT FROM AUTHOR]

The present work uses the direct simulation Monte Carlo (DSMC) method to investigate thermally-driven rarefied flow induced by the partially heated diamond in a channel. Two types of flows are generated in the medium: the radiometric flow induced from the middle tip of the diamond with various temperatures on its side and the rightward thermal edge flow induced by the temperature difference between the diamond and the medium. The accommodation coefficients of the top and bottom borders were considered, and it is observed that the thermal edge flow is weakened if we set the top and bottom channels' walls as specular. In this work, we studied the influence of the geometrical parameters and operating conditions on the rarefied gas flow under specular conditions of the top and bottom walls. This includes the sizes of the diamond, the working pressure represented by the Knudsen number (Kn), and the temperature difference. The results show that varying the accommodation coefficients of channel walls affect the velocity field and results in the generation of vortices behind the diamond in the channel. We show that the drag force of the diamond is proportional to the temperature difference and increases slightly with the increase of Kn. [ABSTRACT FROM AUTHOR]

Abstract: In the current work, for the first time, we extend the application of a second order slip/jump equations introduced by Karniadakis et al. for the simulation of high speed, high Knudsen ( ) number flows over a nano-scale flat plate and a micro-scale cylinder. The NS equations subject to a second order slip/jump boundary conditions are solved using the Petrov–Galerkin Finite Element discretization. We compare our numerical solution for flow and thermal field with the solution of the DSMC and Generalized Hydrodynamic (GH) techniques, as well as a recently developed slip/jump boundary condition, i.e., Paterson equation. Current results demonstrate the suitable accuracy of the employed boundary conditions for different set of test cases. Our numerical solutions are obtained with much less numerical costs compared to alternative boundary conditions. [Copyright &y& Elsevier]

Abstract: In this research, convective heat transfer of the argon gas flow through a micro/nano channel with uniform heat flux wall boundary condition is investigated using the direct simulation Monte Carlo (DSMC) method. Both of the hot wall (qw >0) and the cold wall (qw <0) cases are considered. Implementation of wall heat flux in the DSMC method is performed using the recently developed “iterative” technique. Our investigation considers heat transfer behavior in both of slip and transition flow regimes. We investigate the influence of rarefaction, i.e., Knudsen number, and viscous dissipation, i.e., Brinkman number, on the Nusselt number behavior. We use the generalized hard sphere (GHS) collision model to consider accurate variation of the heat conductivity with the temperature. The DSMC solutions for the Nusselt number are compared with different analytical expressions reported in the literature with suitable accuracy through the slip regime. We observe that the dependency of the Nusselt number on the Knudsen number decreases in nanochannels as Knudsen number increases into the transition regime, i.e., Nusselt number approaches nearly a constant value as Knudsen number goes beyond 1. Additionally, it is shown that the Nusselt number is a weak function of the Brinkman number. [Copyright &y& Elsevier]

Abstract: In this paper simulation of cavitating flow over the Clark-Y hydrofoil is reported using the large eddy simulation (LES) turbulence model and volume of fluid (VOF) technique. We applied an incompressible LES modelling approach based on an implicit method for the subgrid terms. To apply the cavitation model, the flow has been considered as a single fluid, two-phase mixture. A transport equation model for the local volume fraction of vapour is solved and a finite rate mass transfer model is used for the vapourization and condensation processes. A compressive volume of fluid (VOF) method is applied to track the interface of liquid and vapour phases. This simulation is performed using a finite volume, two phase solver available in the framework of the OpenFOAM (Open Field Operation and Manipulation) software package. Simulation is performed for the cloud and super-cavitation regimes, i.e., σ =0.8, 0.4, 0.28. We compared the results of two different mass transfer models, namely Kunz and Sauer models. The results of our simulation are compared for cavitation dynamics, starting point of cavitation, cavity’s diameter and force coefficients with the experimental data, where available. For both of steady state and transient conditions, suitable accuracy has been observed for cavitation dynamics and force coefficients. [Copyright &y& Elsevier]

Abstract: Micro/nano geometries with specified wall heat flux are widely encountered in electronic cooling and micro-/nano-fluidic sensors. In the present study we introduce a new iterative technique to impose a desired (positive/negative) wall heat flux boundary condition in the DSMC method that can be useful for simulation of Micro/Nano electro-mechanical systems (MEMS/NEMS) with given heat energy exchange. In the proposed algorithm we use the non-dimensional difference between computed and desired wall heat flux rates to improve iteratively an initial estimate of the wall temperature. A relaxation factor is applied to control the correction of wall temperature values. Effects of different numerical parameters such as number of simulator particles per cell and relaxation factor on the accuracy, performance and robustness of the iterative technique are investigated. We examine our iterative technique by analyzing heating and cooling processes in rarefied pressure-driven micro/nanoscale channel flows. Some unique behaviors are observed. For example, it is observed that contrary to the heating process, the cooling of micro/nano channel walls results in small variations in the density field. The upstream thermal creep effects in the cooling process decrease the velocity slip although the Knudsen number increases along the channel. Additionally, the cooling process changes the curvature of the pressure distribution making it below the linear incompressible one. For the cases considered here, our results indicate that flow cooling increases the mass flow rate through the channel, and vice versa. We also investigate the effects of wall heat transfer on the hydrodynamics and thermal behaviors of the 2-D micro/nano cavity flow. [Copyright &y& Elsevier]

Abstract: We investigate the efficiency of a parallel direct simulation Monte Carlo (PDSMC) algorithm in solving the rarefied subsonic flow through a nanochannel. We use MPI library to transfer data between the processors. It is observed that PDSMC solver shows ideal speed up if sufficient workload is provided for each of processors. Additionally, this study shows that the computational time and speed up of the extended PDSMC solver do not depend (or slightly depend) on the number of cells. In contrary, increasing the total number of particles would result in a better efficiency of the PDSMC. [Copyright &y& Elsevier]

Abstract: In the present work, rarefied gas flow between two parallel moving plates maintained at the same uniform temperature is simulated using the direct simulation Monte Carlo (DSMC) method. Heat transfer and shear stress behavior in the micro/nano-Couette flow is studied and the effects of the important molecular structural parameters such as molecular diameter, mass, degrees of freedom and viscosity–temperature index on the macroscopic behavior of gases are investigated. Velocity, temperature, heat flux and shear stress in the domain are studied in details. Finally, a discussion on the role of the molecular structural parameters in the decrease or increase of amounts of hydrodynamics and thermal properties of the gas is presented. [Copyright &y& Elsevier]

Abstract: In this study, we use direct simulation Monte Carlo method to simulate subsonic flow in nanochannels and micro/nanoscale backward-facing (BF) step considering a wide range of Knudsen number regimes. The nanochannel flow simulation indicates that the nanoscale flow through the nanochannel resembles unique features such as encountering negative pressure deviation behavior and observing flat velocity profiles at higher Knudsen number regimes. On the other hand, the micro/nano BF step flow simulations demonstrate that the length of separation region considerably decreases as the flow becomes more rarefied and approaches the transition regime. Meanwhile, the variations in the flow properties are much slower in the mid-transition and free-molecular regimes compared with the slip and early transition regime cases. [Copyright &y& Elsevier]

In this paper, we suggest the application of micro-scale radiometric pumps for helium‑xenon binary mixture separation. For this purpose, we consider three geometries of radiometric pumps consisting of channels with their bottom or top surfaces periodically patterned with different structures including a simple vane or double isosceles triangular ratchets. In these geometries, the channel surfaces and structures are considered as diffuse reflectors. The temperature is kept constant on the horizontal walls of the channel; thus, radiometric flow is created by subjecting the adjacent sides of the vane/ratchet to constant but unequal temperatures. Numerical simulation is performed using the direct simulation Monte Carlo (DSMC) method. The DSMC simulations are performed at a Knudsen number for mixture based on the vane/ratchet height of around 0.2. The present study has two important aspects. The first is the amount of lateral and vertical separation that can be achieved by using newly designed Knudsen pumps is described. The second is to study the behavior of the flow field in binary gas mixtures. The features of each design in gas separation will be discussed. [ABSTRACT FROM AUTHOR]

In this article, a new hybrid approach combining Fokker Planck (FP) and Direct Simulation Monte Carlo (DSMC) is introduced. The FP approach, which is an approximation of the Boltzmann equation, has been recently introduced for modeling rarefied gas flows. Compared to standard molecular methods like DSMC, it has a lower computational cost. However, the FP model may provide erroneous results in modeling some flow features, including vortex region and shock waves. The DSMC approach is accurate enough; yet, it has a high computational cost, especially at low Knudsen/low Mach number conditions. The purpose of this article is to suggest an optimized hybrid algorithm to benefit from high-speed modeling and sufficient accuracy simultaneously. We show a modified form of the gradient length Knudsen number, i.e., Kn GL /Kn, must be considered to define the range of accuracy of the FP method. We used Kn GL /Kn as a parameter for switching between DSMC and FP. The cavity flow and the nozzle flow are considered for the investigation of the proposed hybrid algorithm. The dependence of the accuracy and computational cost on Kn GL /Kn is reported. The results demonstrate that the optimized Kn GL /Kn is between 3 and 4, which gives suitable computational cost and acceptable accuracy. • Report on the evaluation of a hybrid Fokker-Plank-DSMC approach to treat micro-cavity and micro-nozzle flows. • Using a novel criterion for switching between Fokker-Plank and DSMC solvers. • A thorough study of the numerical efficiency of the hybrid scheme at various Kn and back pressure magnitudes. [ABSTRACT FROM AUTHOR]

This study employs numerical analysis to investigate the behavior of the National Advisory Committee for Aeronautics (NACA)66 modified (mod) hydrofoil under plunging motion, considering various cavitation numbers. The Large Eddy Simulation (LES) method is utilized to model turbulence. The hydrofoil's plunging motion leads to an increase in lift force and a decrease in drag force. Our research indicates that increasing the speed of the hydrofoil's heaving motion delays the onset of cavitation and enhances the formation of cavity clouds. Furthermore, during the peak oscillatory motion of the hydrofoil in the plunging phase, the detachment length of cavitation bubbles decreases. Additional investigations reveal that cavitation on the hydrofoil's surface accelerates the transition from a laminar to a turbulent boundary layer, strengthening the turbulent boundary layer and postponing the onset of flow separation. This research involves an in-depth investigation of the terms in the vorticity transport equation in cavitation inception, finding a correlation between vapor volume fraction and vorticity dilatation near the hydrofoil surface. This correlation proves crucial to the initial stages of the cavitation inception process. • The plunging motion of NACA66 mod hydrofoil boosts lift and cuts drag, enhancing aerodynamic efficiency. • Higher speeds in hydrofoil heaving delay cavitation onset, improving cavity cloud formation. • The study reveals a key link between vapor volume fraction and vorticity dilatation, crucial for understanding early cavitation inception. [ABSTRACT FROM AUTHOR]

In this paper, OpenFOAM package is used for the first time to simulate the thermoacoustic refrigerator. For simulating oscillating inlet pressure, we implemented cosine boundary condition into the OpenFOAM. The governing equations are the unsteady compressible Navier–Stokes equations and the equation of state. The computational domain consists of one plate of the stack, heat exchangers, and resonator. The main result of this paper includes the analysis of the position of the cold heat exchanger versus the displacement of the pressure node at large amplitude for successful operation of the refrigerator. In addition, the effect of the input power on the successful operation of the apparatus has been investigated. It is observed that for higher temperature difference between heat exchangers, the time of steady state solution is longer. We show that to analyze and optimize the thermoacoustic devices, both heat exchangers should be considered, coefficient of performance (COP) should be checked, and the successful operation of the refrigerator should be evaluated. [ABSTRACT FROM AUTHOR]

The NACA 0012 airfoil and the sharp 25–55-deg biconic cases are usually considered in evaluating the accuracy of new forms of velocity slip and temperature jump wall boundary conditions in the Navier–Stokes–Fourier equations. These cases have been investigated by many researchers in previous works with the first order Maxwell slip and Smoluchowski jump boundary conditions. Recently, we proposed a new form of the second order temperature jump for simulating rarefied gas flows and obtained accurate results for low-speed microflows. The current work is an extension of our previous CFD work where we applied both the second order velocity slip condition and the second order temperature jump boundary condition in new forms to simulate rarefied external gas flows for these cases. The second order boundary conditions are implemented into the rhoCentralFoam solver in the OpenFOAM software. The NACA 0012 airfoil case is investigated with angles-of-attack of 0 and 10 degrees and Mach numbers of 0.8 and 2. The sharp 25–55-deg biconic case is simulated at Mach of 15.6. The simulation results obtained for the NACA 0012 airfoil show that the combination of the second order slip/jump boundary conditions gives suitable agreements with DSMC for surface gas temperature and drag coefficients in comparison with those of the first order conditions. However, the results of the biconic case figure out that the second order conditions are not superior to the first order conditions. [ABSTRACT FROM AUTHOR]

• Cavitating flow around a 3-D Hydrofoil with Wavy Leading Edges (WLE) is investigated using the LES approach in OpenFOAM. • Comparison of cavity features of wavy leading edges (WLE) hydrofoil with straight leading edge (SLE) one. • Force coefficients, re-entrant jet and separation are assessed during the cloud cavity evolution around WLE and SLE hydrofoils. • The mechanisms of the laminar separation bubble (LSB) and low-pressure zone behind the WLE hydrofoil are illustrated. • Role of vorticity stretching and vorticity dilatation with the presence of cavity for the WLE and SLE hydrofoils are investigated. The present study seeks to conduct numerical investigations of the cavitating flow characteristics around a sinusoidal wavy leading edge (WLE) 3-D hydrofoil underlying a NACA 63 4 –021 profile with an aspect ratio of 4.3. Cavitational and non-cavitational characteristics of hydrofoils are numerically examined at a chord-based Reynolds number of 7.2 × 105. The sinusoidal leading edge geometries include two WLE amplitudes of 5% and 25% and two WLE wavelengths of 25% and 50% of the mean chord length. We examined the cavitating flow around the hydrofoils in different cavitation numbers, namely σ = 0.8 and σ = 1.2. The flow over the protuberances of the WLE hydrofoil is considered at varying chord lengths and a constant angle of attack α = 6°, where significant spanwise variations in all flow properties, in contrast to the straight leading edge (SLE) hydrofoil, were observed. Large eddy simulation (LES) and Kunz mass transfer models are employed to simulate the dynamic and unsteady behavior of the cavitating flow. Besides, the compressive volume of fluid (VOF) method is used to track the cavity interface. Simulation is performed under the two-phase flow solver —interPhaseChangeFoam— of the OpenFOAM package. Compared to the SLE hydrofoil, we provided an exhaustive report of the time-averaged and instantaneous fluid dynamic characteristics of the cavitating flow around the sinusoidal leading edge hydrofoil, i.e., pressure, velocity, and vorticity fields, as well as lift and drag coefficients, and turbulent kinetic energy are reported. Furthermore, detailed analyses of the instantaneous cavity leading edge and flow separation treatment, vortical structure of the flow, vorticity stretching and dilatation, details of the spanwise flow, the formation of a low-pressure zone behind the WLE hydrofoil, streamwise velocity fluctuation, and evolution of the cavity dynamics through a complete cycle are reported. Results show that early development of the laminar separation bubble (LSBs) on the suction side of WLE hydrofoil prevents significant flow separation. Furthermore, the WLE cases exhibit a significantly reduced level of unsteady fluctuations in aerodynamic forces at the frequency of periodic vortex shedding. [ABSTRACT FROM AUTHOR]

A detailed study on the performance and accuracy of the Fokker Planck (FP) approach in treating shear driven flows over a wide range of Knudsen numbers and Mach numbers at subsonic and supersonic regimes is considered. One-dimensional Couette flow and the two-dimensional cavity problem are considered. The FP method is evaluated in the Couette flow at a subsonic Mach number of 0.16 (U w = 50 m/s) and at the supersonic Mach number of 3.1 (U w = 1000 m/s), where Knudsen numbers range from 0.005 to 1. Correspondingly, the cavity flow is investigated at a wall Mach number of 0.31 (U w = 100 m/s) and wall Mach number of 0.93 (U w = 300 m/s) at Knudsen numbers ranging from 0.05 to 20. Interestingly, the results show that by increasing the wall velocity and Knudsen numbers, the accuracy of the FP approach increases in treating the cavity flow. In addition to the standard Knudsen number, we show that gradient length Knudsen number, Kn GL , should be considered to determine the range of accuracy of the FP scheme. The latter depends on the strength of the center vortex of the cavity diminishing at higher rarefied conditions. The results demonstrate that the computational efficiency of the FP approach enhances at higher lid velocity. • First report on the evaluation of the Fokker-Plank scheme at high Knudsen shear-driven flows. • A thorough study of the numerical efficiency of the FP scheme at various Kn and Mach numbers. • Presenting a novel parameter in detecting the border of the accuracy of the FP scheme. [ABSTRACT FROM AUTHOR]

In this paper, we sought to investigate unsteady cavitating turbulent flows around a twisted three-dimensional NACA 16012 hydrofoil using the large eddy simulation (LES) and volume of fluid (VOF) methods. The one-equation eddy viscosity model (OEEVM) was used to calculate the sub-grid scale (SGS) stress tensor. The compressive velocity VOF technique and the Kunz cavitation model were also employed. Numerical simulation was performed using the interPhaseChangeFoam solver within the OpenFOAM framework. Cavitation simulation was performed at three cavitation numbers within the cloud cavitation regime; that is to say, cavitation numbers: 0.95, 1.15, and 1.35. Afterwards, the details of flow predictions including the pressure, velocity, streamlines, volume fraction of water phase, and turbulent kinetic energy were reported. Cavity behaviours, including growth, shedding, and collapse of the cavity, were considered in detail. Three-dimensional cavity structures such as primary and secondary shedding and the shedding of the U-shaped horseshoe vortex were reported as well. The present work illustrated the side-entrant jets and the radially diverging re-entrant jet corresponding to the three-dimensional effect of the twisted wing. The cavity pattern and the shedding cycle frequency agreed well with the available experimental observations. • Cloud cavitation around a twisted wing with NACA16012 hydrofoil, Large Eddy Simulation (LES). • Detailed dynamic evolution of cavitation including growth, shedding and collapse in downstream at σ = 0.95, 1.15, 1.35. • Pressure fluctuations, turbulent kinetic energy, vapour volume of fraction. • Re-entrant jet and side jets. [ABSTRACT FROM AUTHOR]

Conjugated heat transfer and hydraulic performance for nanofluid flow in a rectangular microchannel heat sink with LVGs (longitudinal vortex generators) are numerically investigated using at different ranges of Reynolds numbers. Three-dimensional simulations are performed on a microchannel heated by a constant heat flux with a hydraulic diameter of 160 μm and six pairs of LVGs using a single-phase model. Coolants are selected to be nanofluids containing low volume-fractions (0.5%–3.0%) of Al 2 O 3 or CuO nanoparticles with different particle sizes dispersed in pure water. The employed model is validated and compared by published experimental, and single-phase and two-phase numerical data for various geometries and nanoparticle sizes. The results demonstrate that heat transfer is enhanced by 2.29–30.63% and 9.44%–53.06% for water-Al 2 O 3 and water-CuO nanofluids, respectively, in expense of increasing the pressure drop with respect to pure-water by 3.49%–16.85% and 6.5%–17.70%, respectively. We have also observed that the overall efficiency is improved by 2.55%–29.05% and 9.78%–50.64% for water-Al 2 O 3 and water-CuO nanofluids, respectively. The results are also analyzed in terms of entropy generation, leading to the important conclusion that using nanofluids as the working fluid could reduce the irreversibility level in the rectangular microchannel heat sinks with LVGs. No exterma (minimums) is found for total entropy generation for the ranges of parameters studied. [ABSTRACT FROM AUTHOR]