Multiphase Flow Laboratory

2023

E. Ezzatneshan, R. Sadraei, Influence of vibration on droplet dynamics in a three-dimensional porous medium, Physics of Fluids 35(7), (2023).

In this study, the effects of vibration on droplet dynamics inside a three-dimensional (3D) porous medium are investigated with a focus on frequency, amplitude, and surface wettability. A lattice Boltzmann method based on the Allen–Cahn equation (A-C LBM) is used. The results show that the volume of the drained drop and drainage duration of the droplet are significantly affected by the contact angle. The hydrophilic nature of the pores causes the droplet to spread inside the medium and resist the vibration force, resulting in a lower discharged liquid volume and delayed drainage. In contrast, a hydrophobic surface repels the droplet and leads to quicker drainage. It is also observed that the speed of droplet drained from the porous medium is higher for hydrophobic conditions, causing the separated drop to rebound and jump back toward the medium after impacting the surrounding wall boundaries. A thorough investigation is conducted on the combined implication of the surface adhesion, amplitude, and frequency of vibration on the first separation time of the droplet from the porous medium and full drainage duration. The results show that with increasing the hydrophobicity, the required vibration amplitude for complete drainage has decreased. In this way, the interplay between the adhesive force and the vibration force impedes the liquid phase separation from the hydrophilic porous medium at a low vibration amplitude. However, the results demonstrate that even in these conditions, an increase in the vibration frequency can enhance the separation and improve the drainage of the liquid phase from the pores.

E. Ezzatneshan, K. Hejranfar, An unstructured preconditioned central difference finite volume multiphase Euler solver for computing inviscid cavitating flows over arbitrary two- and three-dimensional geometries, Computers & Mathematics with Applications, (2023).

In the present work, a numerical method is adopted and applied for simulating the inviscid cavitating flows around two- and three-dimensional geometries on unstructured meshes. The algorithm uses the preconditioned multiphase Euler equations discretized by a cell-centered central difference finite volume scheme with suitable dissipation terms. The interface capturing method with three transport equation-based cavitation models, namely the Merkle et al., Singhal et al. and Kunz et al. models are employed for the mass transfer between the liquid and vapor phases to be calculated. The simulations of the steady inviscid cavitating flows are performed around different two- and three-dimensional geometries, namely the NACA0012, NACA66(MOD) and two-element NACA4412-4415 hydrofoils, the hemispherical head shape body and the twisted NACA0009 hydrofoil, and the results are obtained over these geometries with the three cavitation models used for different flow conditions. The effects of different numerical parameters on the accuracy of the solution are also examined by a sensitivity study. The present results are compared with those of performed by other researchers which exhibit good agreement. It is indicated that the solution method adopted based on the preconditioned finite volume multiphase Euler flow solver on unstructured meshes is capable of accurately predicting the surface pressure distribution and the cavity shape over the arbitrary two- and three-dimensional geometries.

2022

E. Ezzatneshan, A. Salehi, H. Vaseghnia, Study of micro-heater shape and wettability effects on inception of boiling phenomenon using a multiphase lattice Boltzmann method, International Journal of Thermal Sciences, (2022).

In the present paper, the inception of boiling phenomenon and the resulting bubble dynamics are investigated using a multiphase lattice Boltzmann method (LBM). Two equations of state (EoS), namely the Peng-Robinson (P-R) and the Redlich-Kwong-Soave (R-K-S), are employed and their capability for the prediction of the bubble nucleation, growth, and departure is evaluated against the available data in the literature. The effect of the micro-heater surface wettability and shape, namely the grooved and cavity, are studied on the separation time and velocity of the detached bubble at different flow conditions. Based on the present results, the boiling phenomenon nucleates from sharp-angled edges of the groove and cavity that are the region with higher temperature gradients. For the hydrophilic heated surface, the growth of the bubble continues on the sharp edges with the minimum dry spot on the surface. However, the hydrophobic heated surface tends to attract the forming bubble in the inception process, so that the boiling bubble is flattened on the heated surface after formation and tends to stick to the micro-heater. It is found that the bubble separation time from the micro-heater surface is longer, and the departure velocity is lower for the hydrophobic heated surface in comparison with the hydrophilic wetting condition. Also, the present study shows that the bubble separation from the grooved micro-heater occurs faster and with a higher velocity in comparison with the cavity micro-heater at a certain contact angle that demonstrates the significant effect of the surface shape on the boiling phenomenon. Hence, the grooved micro-heater is incorporated with two triangular pillars. The results demonstrate that using pillars provides more time for the bubble to grow before departure from the surface due to the blockage of the liquid flow coming from the lateral sides to the bubble formation site.

E. Ezzatneshan, A. A. Khosroabadi, On Accuracy of Lattice Boltzmann Method Coupled with Cahn-Hilliard and Allen-Cahn Equations for Simulation of Multiphase Flows at High-Density Ratios, J. Applied Fluid Mechanics, (2022).

In this work, the accuracy of the multiphase lattice Boltzmann method (LBM) based on the phase-field models, namely the Cahn-Hilliard (C-H) and Allen-Cahn (A-C) equations, are evaluated for simulation of two-phase flow systems with high-density ratios. The mathematical formulation and the schemes used for discretization of the derivatives in the C-H LBM and A-C LBM are presented in a similar notation that makes it easy to implement and compare these two phase-field models. The capability and performance of the C-H LBM and A-C LBM are investigated, specifically at the interface region between the phases, for simulation of flow problems in the two-dimensional (2D) and three-dimensional (3D) frameworks. Herein, the equilibrium state of a droplet and the practical two-phase flow problem of the rising bubble are considered to evaluate the mass conservation capability of the phase-filed models employed at different flow conditions and the obtained results are compared with available numerical and experimental data. The effect of employing different equations proposed in the literature for calculating the relaxation time on the accuracy of the implemented phase-field LBMs in the interfacial region is also studied. The present study shows that the LBM based on the A-C equation (A-C LBM) is advantageous over that based on the C-H equation in dealing with the conservation of the total mass of a two-phase flow system. Also, the results obtained by the A-C LBM is more accurate than those obtained using the C-H LBM in comparison with other numerical results and experimental observations. The present study suggests the A-C LBM as a sufficiently accurate and computationally efficient phase-field model for the simulation of practical two-phase flows to resolve their structures and properties even at high-density ratios.

2021

E. Ezzatneshan, H. Vaseghnia, Dynamics of an acoustically driven cavitation bubble cluster in the vicinity of a solid surface, Physics of Fluids, (2021).

The dynamics of a cavitation bubble cluster under the influence of an acoustic field is a complex multiphase system that can be observed in acoustic cavitation. In the present study, a three-dimensional (3D) computational technique based on the multiphase lattice Boltzmann method (LBM) with multiple relaxation time (MRT) is adopted to investigate the acoustically driven cavitation bubble cluster dynamics near a solid wall at different wetting conditions. Herein, the Peng–Robinson-Stryjek-Vera (PRSV2) equation of state (EoS) with an acentric factor is incorporated in the LBM to accurately impose the physical properties of actual fluids. The validity and capability of the adopted MRT-LBM are confirmed by the excellent agreement of the present results compared to the computed data based on the Rayleigh-Plesset (R-P) equation for a heterogeneous cavitation phenomenon. The obtained results for the acoustically driven cavitation bubble cluster dynamics demonstrate that the shielding effect of top bubbles prevents the pressure pulse from reaching the lower bubbles. Therefore, the cluster core and the bubbles near the solid surface are more affected by the destruction of the upper layer bubbles than the acoustic field. Also, it is found that the wettability of the solid wall significantly affects the irradiated impulsive pressure waves around the cluster. To justify this result from the physical point of view, the magnitude of the primary and secondary Bjerknes forces is measured and accordingly, the growth and collapse of bubbles in the cluster under the influence of the acoustic field are discussed in detail.

E. Ezzatneshan, R. Goharimehr, Study of fingering dynamics of two immiscible fluids in a homogeneous porous medium with considering wettability effects using a pore-scale multicomponent lattice Boltzmann model, European Journal of Computational Mechanics, (2021).

In the present study, a pore-scale multicomponent lattice Boltzmann method (LBM) is employed for the investigation of the immiscible-phase fluid displacement in a homogeneous porous medium. The viscous fingering and the stable displacement regimes of the invading fluid in the medium are quantified which is beneficial for predicting flow patterns in pore-scale structures, where an experimental study is extremely difficult. Herein, the Shan-Chen (S-C) model is incorporated with an appropriate collision model for computing the interparticle interaction between the immiscible fluids and the interfacial dynamics. Firstly, the computational technique is validated by a comparison of the present results obtained for different benchmark flow problems with those reported in the literature. Then, the penetration of an invading fluid into the porous medium is studied at different flow conditions. The effect of the capillary number (Ca), dynamic viscosity ratio (M), and the surface wettability defined by the contact angle (Teta) are investigated on the flow regimes and characteristics. The obtained results show that for M<1, the viscous fingering regime appears by driving the invading fluid through the pore structures due to the viscous force and capillary force. However, by increasing the dynamic viscosity ratio and the capillary number, the invading fluid penetrates even in smaller pores and the stable displacement regime occurs. By the increment of the capillary number, the pressure difference between the two sides of the porous medium increases, so that the pressure drop ∆p along with the domain at Teta=40 is more than that of computed for Teta=80. The present study shows that the value of wetting fluid saturation Sw at Teta=40 is larger than its value computed with Teta=80 that is due to the more tendency of the hydrophilic medium to absorb the wetting fluid at Teta=40. Also, it is found that the magnitude of Sw computed for both the contact angles is decreased by the increment of the viscosity ratio from Log(M)=-1 to 1. The present study demonstrates that the S-C LBM is an efficient and accurate computational method to quantitatively estimate the flow characteristics and interfacial dynamics through the porous medium.

E. Ezzatneshan, R. Goharimehr, A Pseudopotential Lattice Boltzmann Method for Simulation of Two-phase Flow Transport in Porous Medium at High-Density and High–Viscosity Ratios, Geofluids, (2021).

In this work, the capability of a multiphase lattice Boltzmann method (LBM) based on the pseudopotential Shan-Chen (S-C) model is investigated for simulation of two-phase flows through porous media at high-density and high–viscosity ratios. The accuracy and robustness of the S-C LBM are examined by the implementation of the single-relaxation time (SRT) and multiple-relaxation-time (MRT) collision operators with integrating the forcing schemes of the shifted velocity method (SVM) and the exact difference method (EDM). Herein, two equations of state (EoS), namely the standard Shan-Chen (SC) EoS and Carnahan-Starling (CS) EoS, are implemented to assay the performance of the numerical technique employed for simulation of two-phase flows at high-density ratios. An appropriate modification in the forcing schemes is also used to remove the thermodynamic inconsistency in the simulation of two-phase flow problems studied at low reduced temperatures. The comparative study of these improvements of the S-C LBM is performed by considering an equilibrium state of a droplet suspended in the vapor phase. The solver is validated against the analytical coexistence curve for the liquid-vapor system and the surface tension estimation from the Laplace Law. Then, according to the results obtained, a conclusion has been made to choose an efficient numerical algorithm, including an appropriate collision operator, a realistic EoS and an accurate forcing scheme, for simulation of multiphase flow transport through a porous medium. The patterns of two-phase flow transport through the porous medium are predicted using the present numerical scheme in different flow conditions defined by the Capillary number and the dynamic viscosity ratio. The results obtained for the non-wetting phase saturation, penetration structure of the invading fluid and the displacement patterns of two-phase flow in the porous medium are comparable with those reported in the literature. The present study demonstrates that the S-C LBM with employing the MRT-EDM scheme, CS EoS and the modified forcing scheme is efficient and accurate for estimation of the two-phase flow characteristics through the porous medium.

E. Ezzatneshan, A. A. Khosroabadi, Droplet spreading dynamics on hydrophobic textured surfaces: A Lattice Boltzmann study, Computers and Fluids, (2021).

A three-dimensional (3D) lattice Boltzmann method based on the Allen-Cahn equation (A-C LBM) is employed to study the spreading dynamics of impacting droplets on the hydrophobic flat and textured surfaces. At first, the simulation of the equilibrium state of a droplet and also the impacting droplet spreading on a flat surface are considered at various wetting conditions to examine the accuracy and efficiency of the present numerical technique. The obtained results for these cases show excellent agreement with available theoretical, numerical, and experimental data in the literature that confirms the validity of the A-C LBM employed for simulation of such complex interfacial dynamics. Upon validation, the implemented A-C LBM is applied for investigation of the droplet impingement on the hydrophobic textured surface and the results are compared with those obtained by considering the flat surface. The present study shows that using microscale textures on a hydrophobic surface dramatically reduces the contact area between the impacting droplet and the solid wall due to the momentum redirection phenomenon. Indeed, the hydrophobic surface used with inplane circular ridges causes the spreading lamella to eject out-of-plane with a liquid bowl shape. This mechanism converts a part of the horizontal momentum of the spreading to the vertical momentum that prevents droplets to have intense interaction. For an impacting droplet at a high Weber number, it is concluded that the geometrical parameters of the ridge dictate the morphology of the spreading on the hydrophobic surface. However, the droplet dynamics at a low Weber number depend on both the texture size and the surface wettability, so that the physical mechanisms of the droplet spreading and lamella ejection dramatically change by variation of these parameters. The obtained results show that when the adhesion force of the substrate is dominant (lower contact angles), the wetting property of the surface plays an effective role than the texture geometry in the formation of the liquid bowl at low Weber numbers. Also, the present study demonstrates the capability of the A-C LBM for the prediction of the studied multiphase flow structures and characteristics with extremely complex interface phenomena.

E. Ezzatneshan, A. Salehi, H. Vaseghnia, Study on forcing schemes in thermal lattice Boltzmann method for simulation of natural convection flow problems, Heat Transfer, (2021), 50(8), pp. 7604-7631.

The present study addresses the effect of various schemes for applying an external force term on the accuracy and performance of the thermal lattice Boltzmann method (LBM) for simulation of free convection problems. Herein, the forcing schemes of Luo, shifted velocity method (SVM), Guo, and exact difference method (EDM) are applied with considering three velocity discrete models of D2Q4, D2Q5, and D2Q9. The accuracy and performance of these schemes are evaluated with the simulation of three natural convection problems, namely the free convection in a closed cavity, in a square enclosure with a hot obstacle inside, and the Rayleigh-Benard problem. The obtained results based on the present thermal LBM with different forcing schemes and velocity discrete models are compared with the existing experimental and numerical data in the literature. This comparison study indicates that imposing all employed forcing schemes leads to similar performance for the simulation of free convection problems studied at the middle range of Rayleigh numbers. It is found that the Luo forcing scheme is simple for implementation in comparison with the other three forcing schemes and provides the results with acceptable accuracy at moderate Rayleigh numbers. At higher Rayleigh numbers, however, the Guo scheme is not only numerically stable but a more precise forcing scheme in comparison with the other three methods. It is illustrated that employing the discrete velocity model of D2Q4 has more appropriate numerical stability along with less computational cost in comparison with two other discrete velocity models for simulation of natural convection heat transfer.

E. Ezzatneshan, Noise prediction around two-dimensional airfoils using an efficient theoretical algorithm, Journal of Vibration and Sound 18, (2021) (In Persian).

In the present study, an efficient numerical algorithm based on theoretical methods is developed for the prediction of the noise of aerodynamic flows around two-dimensional geometries. The theoretical formulation for calculating different types of noise is proofed and a solver with a negligible computational cost is developed accordingly. By using the present noise solver, the aeroacoustic characteristics around two test cases are predicted and the obtained results are presented in comparison with the available numerical and experimental data. The present results for noise around NACA0012 and S822 are in excellent agreement with those reported in the literature which shows the accuracy and performance of the present solver based on the theoretical formulation. Employing this quick solver is very crucial to save costs in the preliminary design process of low noise geometries in aerospace engineering. The extension of the present two-dimensional noise solver for the prediction of noise properties around three-dimensional geometries is undergoing.

2020

E. Ezzatneshan, H. Vaseghnia, Simulation of Collapsing Cavitation Bubble in Various Liquids by Lattice Boltzmann Model Coupled with Redlich-Kwong-Soave Equation of State, Physical Review E 102, (2020), pp. 053309.

A computational technique based on the pseudo-potential multiphase lattice Boltzmann method (LBM) is employed to investigate the collapse dynamics of cavitation bubbles of various liquids in the vicinity of the solid surface with different wettability conditions. The Redlich-Kwong-Soave (R-K-S) equation of state (EoS) that includes an acentric factor is incorporated to consider the physical properties of water (H_2 O), liquid nitrogen (LN_2), and liquid hydrogen (LH_2) in the present simulations. The accuracy and performance of the present multiphase LBM are examined by simulation of the homogenous and heterogeneous cavitation phenomena. The good agreement of the results obtained based on the present solution algorithm in comparison with the available data confirms the validity and capability of the multiphase LBM employed. Then, the cavitation bubble collapse near the solid wall is studied by considering the H_2 O, LN_2 and LH_2 fluids, and the wettability effect of the surface on the collapse dynamics is investigated. The obtained results demonstrate that the collapse phenomenon for the H_2 O is more aggressive than that of the LH_2 and LN_2. The cavitation bubble of the water has a shorter collapse time with an intense liquid jet, while the collapse process in the LN_2 takes a longer time due to the larger radius of its bubble at the rebound. Also, this study demonstrates that the increment of the hydrophobicity of the wall causes less energy absorption by the solid surface from the liquid phase around the bubble that leads to form a liquid jet with higher kinetic energy. Therefore, the bubble collapse process occurs more quickly for hydrophobic surfaces, regardless of the fluids considered. The present study shows that the pseudo-potential LBM with incorporating an appropriate EoS and a robust forcing scheme is an efficient numerical technique for simulation of the dynamics of the cavitation bubble collapse in different fluids.

E. Ezzatneshan, R. Goharimehr, Study of spontaneous mobility and imbibition of a liquid droplet in contact with fibrous porous media with considering wettability effects, Physics of Fluids 32, (2020), pp. 113303.

In this paper, droplet mobility and penetration into a fibrous porous medium are studied with considering different physical and geometrical properties for the fibers. An in-depth insight of the droplet imbibition into the fibrous medium is beneficial for improving membrane products in different applications. Herein, a multiphase lattice Boltzmann method (LBM) is employed as an efficient numerical algorithm for predicting the multiphase flow characteristics and the interfacial dynamics affected due to the interaction between the droplet and fibrous substrates considered. This computational technique is validated by comparison of the present results obtained for different benchmark two-phase flow problems with those reported in the literature, which shows good agreement and confirms its accuracy and efficiency. The droplet spreading and penetration into the fibrous porous geometries are then studied by considering various porous topologies, intrinsic contact angles, and fiber sizes. This study shows that the intrinsic contact angle has a great influence on the capillary pressure and consequently on the droplet imbibition into the porous medium. The droplet easily penetrates the porous substrate by decreasing the intrinsic contact angle of the fibers and vice versa. It is also concluded that by coating the fibrous porous medium with a narrow layer of thin fibers, the surface resistance to liquid penetration significantly increases. The present results illustrate that the droplet size impacts the directional wicking ability of the fibrous porous structure used in this study. This property should be considered to produce appropriate two-layer membranes for different applications.

E. Ezzatneshan, Study of Unsteady Separated Fluid Flows using a Multi-block Lattice Boltzmann Method, Aircraft Engineering and Aerospace Technology, (2020).

In the present work, the near wake statistics of separated fluid flows are investigated by employing the lattice Boltzmann method (LBM) in a two-dimensional (2-D) framework. A multi-block technique is applied to accurately resolve the flow characteristics by the grid refinement near the wall and preserve the stability of the numerical solution based on the LBM implemented at relatively high Reynolds numbers. Numerical simulations are performed for studying the vorticity dynamics of a dipole colliding with the wall in a bounded flow and the wake structure and separated flow properties past a circular cylinder at the values of Reynolds numbers. According to the present numerical results, the rolling-up of the boundary layer occurs due to the shear-layer instabilities near the surface which causes a boundary layer detachment from the wall and consequently leads to the formation of small-scale vortices. These shear-layer vortices shed at higher frequencies than the large-scale Strouhal vortices which result in small-scale high-frequency fluctuations in the velocity field in the very near wake of the cylinder. By comparing the present results with those provided by experimental investigations and other numerical solutions, it is also concluded that the multi-block lattice Boltzmann method employed is a robust and sufficiently accurate computational technique for simulation of separated fluid flows even at relatively high Reynolds numbers. The efficiency of this method for predicting the statistical features of the flow problems studied in the present work is comparable with the flow solvers based on the Navier-Stokes equations.

E. Ezzatneshan, A. Khosroabadi, A. Fattahi, Studying of Droplet Impingement on Hydrophilic and Hydrophobic Curved Surfaces by Lattice Boltzmann Method based on Allen-Cahn Equation, Journal of Mechanical Engineering, Amirkabir University of Technology, (2020), (In Persian).

In this paper, an efficient lattice Boltzmann method (LBM) is applied for two-dimensional simulation of two-phase flow problems at high density and viscosity ratios. The present LBM employs the Allen-Cahn equation to model the interfacial dynamics between two phases. To predict the characteristics of droplet dynamics in a wide range of impact velocities, an appropriate collision operator is implemented to ensure the stability of the numerical solutions. The performance of the numerical algorithm developed is examined by studying droplet dynamics at different flow conditions. Herein, the equilibrium state of a droplet on the flat and curved walls is verified with considering the wetting properties, namely the hydrophilic and hydrophobic characteristics, for solid surfaces. The multiphase flow pattern and interfacial dynamics of an impinging droplet on a cylinder surface and a semicircular cavity are also investigated and the results obtained are compared with the available analytical, numerical and experimental data. The present study demonstrates that the curved walls with considering the wettability effects significantly affect the droplet dynamics, depending on the properties of the liquid phase and the flow conditions. This work also shows that the LBM with the Allen-Cahn equation and using the multi-relaxation time collision operator is more stable for simulation of liquid-gas systems at density ratio 1000 and viscosity ratio 100 which makes this method more suitable for predicting practical flow characteristics.

E. Ezzatneshan, H. Vaseghnia, Study of cavitation inception using multiphase lattice Boltzmann method with incorporating equations of state, Journal of Mechanical Engineering, Amirkabir University of Technology, (2020), (In Persian).

The study of cavitation inception is crucial in several hydraulic machines, e.g. pumps, nozzles and sprays. In the present study, a multiphase lattice Boltzmann method (LBM) is implemented for simulation of the cavitation bubbles dynamics and characteristics of cavitating flows. The effect of employing various equations of state (EoS) is investigated on the computing of interaction forces and the phase separation between the liquid and its vapor in the cavitating flows. Herein, the cubic EoSs of Shan-Chen (SC) and Carnahan-Starling (CS) and the non-cubic EoS of Peng-Robinson (PR) are applied. The exact difference method is imposed to improve the numerical stability for simulation of two-phase flow systems. The accuracy and efficiency of the present method are examined by comparison of the results obtained for the homogeneous and heterogeneous cavitation with those reported in the literature. Then, the implemented multiphase LBM is used for studying the inception and growth of the cavitation bubbles in the throat of a venturi. The effect of hydrophobicity and hydrophobicity of the nozzle wall on the cavitation dynamics is investigated and a detailed discussion is made for the results from the numerical and physical point of view. Evaluation of the present results shows that the multiphase LBM with incorporating an appropriate equation of state has an excellent capability for prediction of the bubble dynamics and cavitating flow characteristics in applied geometries.

E. Ezzatneshan, R. Goharimehr, Effect of Multiphase Lattice Boltzmann Collision Models with Single- and Multi-Relaxation Times for Simulation of Liquid-Vapor Two-Phase Flows Using Two Different Forcing Schemes, Journal of Mechanical Engineering, Sharif University of Technology, (2020), 36 (1), (In Persian).

In this paper, the multiphase lattice Boltzmann collision models are evaluated by a comparative study for the simulation of liquid-vapor two-phase flow problems. Herein, the single-relaxation-time (SRT) scheme based on the Bhatnagar-Gross-Krook (BGK) approximation and the multiple-relaxation-time (MRT) method with two different forcing schemes are considered. The pseudo-potential Shan-Chen (SC) model is used to resolve the inter-particle interactions between the liquid and gas phases. In the standard form of the SC model, the interaction force is imposed in the momentum field which unphysically causes the density ratio to change with the variation of relaxation time. In this study, a modified form of this model is implemented to decouple these two physical parameters. Herein, the interaction force is imposed using the exact difference method (EDM). The efficiency and accuracy of the present numerical scheme based on the lattice Boltzmann method (LBM) with the SRT and MRT schemes are examined for simulation of two-phase flows in different conditions. The equilibrium state of a droplet in the periodic flow domain and on the flat surface with hydrophobic and hydrophilic wetting condition are computed to investigate the robustness and performance of the collision operators applied. The results obtained for these problems are compared with the analytical solutions which shows a good agreement. The collision of a droplet on the liquid film at various flow conditions is investigated and the predicted results are presented at a range of the Weber and Reynolds numbers. The present study demonstrates that the SRT model suffers from the spurious velocity in the interfacial region which causes numerical instabilities at moderate Reynolds and Weber numbers. It is found that the MRT model is stable for all the cases considered in the present work even at high Reynolds and Weber numbers. In terms of the computational efficiency, the SRT scheme is slightly attractive, although the computational cost of this model is not considerably lower than MRT scheme. The present study suggests the lattice Boltzmann method with the MRT collision operator incorporated with the EDM technique is robust, sufficiently accurate and computationally efficient to resolve the practical liquid-vapor two-phase flow structures and properties.

E. Ezzatneshan, L. Kazemi, A.A. Khosroabadi, Numerical Simulation of a Droplet Breakup Phenomenon in Cross Flow of Cold and Hot Gas and its Dynamic Investigation in Three-Dimensional Field, Journal of Mechanical Engineering, Tabriz University, (2020) (In Persian).

In the present paper, the simulation of droplet dynamics and its breakup process in the cross-flow of cold and hot gases are performed. Herein, the Navier-Stokes equations are used with employing two additional equations for tracking the liquid-gas interface and for computing the mass transfer due to the evaporation and condensation. Accuracy of the applied numerical algorithm is evaluated by simulation of the Stefan heat transfer problem, two-phase laminar Couette flow, and comparison of the present numerical results with those obtained based on the analytical solution. The dynamics of a droplet break-up in the cold gas are studied and the obtained results are discussed. Then, the study is carried out for the simulation of droplet dynamics in the cross-flow of hot gas and the effect of the evaporation on the break-up process is investigated. The present study shows that the aerodynamics forces and Rayleigh-Taylor and Kelvin-Helmholtz instabilities have dominant effects on the droplet dynamics in the cold gas. However, for a droplet in the cross-flow of hot gas, evaporation has a significant effect on the deformation and breakup phenomenon.

2019

E. Ezzatneshan, H. Vaseghnia, Evaluation of Equations of State in Multiphase Lattice Boltzmann Method with Considering Surface Wettability Effects, Physica A: Statistical Mechanics and its Applications 541, (2019), pp. 123258.

A two-dimensional lattice Boltzmann method (LBM) is applied to investigate the use of various equations of state (EoS) for the simulation of liquid-vapor two-phase flow systems with considering the wetting properties, namely the hydrophilic and hydrophobic characteristics, for solid surfaces. The pseudo-potential single-component multiphase Shan–Chen model is used to resolve inter-particle interactions and phase change between the liquid and its vapor. Several EoSs, including the Redlich–Kwong (R-K), Carnahan-Starling (C-S), and Peng-Robinson (P-R) in comparison with the Shan–Chen (S-C) model are considered to study their effects on the numerical simulation results in terms of density ratios, spurious velocities and the contact angle of the two-phase flow with the solid wall. Accuracy and performance of the multiphase LBM by incorporating various EoSs are examined by solving two-phase flow systems at different conditions. Herein, three test cases considered are an equilibrium state of a droplet suspended in the vapor phase, a liquid droplet located on the solid surface, and a liquid droplet motion through a grooved channel with different wetting conditions. The results obtained demonstrate that implementation of the wall boundary condition with the wettability effects significantly impacts the numerical stability of the LBM with the EoSs employed for simulation of liquid-vapor flow problems, particularly at high-density ratio. Simulation of the equilibrium state of a droplet on a surface with considering wettability effects shows that the S-C model, R-K, P-R and C-S EoSs are stable for the maximum density ratio up to 78.9, 4904.4 and 147, respectively. It is defined that the parasitic currents do not increase significantly due to imposing the wetting condition on the solid wall, however, the numerical solutions with considering wettability effects are more sensitive at high-density ratios. The present study demonstrates that the P-R EoS is more stable for simulation of high-density ratio liquid-vapor systems with reasonable spurious currents in the interfacial region for the flow problems with the periodic computational domain. However, with considering the wetting wall boundary condition, the C-S EoS produces less spurious velocity in the interface region, which leads to more precise and stable numerical simulations in comparison with the other EoSs applied for the equilibrium state of a liquid droplet on the solid surface. The results obtained also demonstrate the capability of the multiphase LBM for predicting practical flow characteristics with different EoSs implemented.

E. Ezzatneshan, Simulation of Dipole Vorticity Dynamics Colliding Viscous Boundary Layer at High Reynolds Numbers, J. Applied Fluid Mechanics 12(4) (2019), pp. 1073-1081.

The vorticity dynamics of a Lamb-like dipole colliding with flat boundaries are investigated for high Reynolds number flows by implementation of the lattice Boltzmann method (LBM). The standard LBM based on the single-relaxation-time collision model suffers from numerical instabilities at high Reynolds numbers. Herein, a regularized collision model is employed for the LBM to preserve the stability and accuracy of the numerical solutions at such flow conditions. The computations are performed for the normal collision of the dipole with the no-slip boundary for several Reynolds numbers in the range of . The results obtained based on the regularized lattice Boltzmann (RLB) method for the statistical flow characteristics like the vorticity field and enstrophy quantity of the dipole-wall collision problem are investigated. The present study demonstrates that the shear-layer instabilities near the wall are responsible for rolling-up of the boundary layer before it is detached from the surface for high Reynolds numbers. This detachment mechanism leads to a viscous rebound and formation of small scale vortices. The shear-layer vortices formed dramatically influence the flow evolution after the collision and result strong enhancement of the total enstrophy of the flow field. By comparing the present results with those of provided by other numerical solutions, it is also concluded that the RLB scheme implemented is robust and sufficiently accurate numerical technique in comparison with the flow solvers based on the Navier-Stokes equations for predicting the statistical features of separated fluid flows even at high Reynolds numbers.

2018

E. Ezzatneshan, K. Hejranfar, Simulation of three‐dimensional incompressible flows in generalized curvilinear coordinates using a high‐order compact finite‐difference lattice Boltzmann method, Int. J. for Numerical Methods in Fluids (2018), 89 (7), pp. 235-255.

In the present study, a high‐order compact finite‐difference lattice Boltzmann method (CFDLBM) is applied for accurately computing three‐dimensional incompressible flows in the generalized curvilinear coordinates to handle practical and realistic geometries with curved boundaries and non‐uniform grids. The incompressible form of the three‐dimensional nineteen discrete velocity (D3Q19) lattice Boltzmann method (LBM) is transformed into the generalized curvilinear coordinates. Herein, a fourth‐order compact finite‐difference scheme and a fourth‐order Runge–Kutta scheme are used for the discretization of the spatial derivatives and the temporal term, respectively, in the resulting D3Q19 lattice Boltzmann (LB) equation to provide an accurate 3‐D incompressible flow solver. A high‐order spectral‐type low‐pass compact filtering technique is applied to have a stable solution. All boundary conditions are implemented based on the solution of the governing equations in the 3‐D generalized curvilinear coordinates. Numerical solutions of different 3‐D benchmark and practical incompressible flow problems are performed to demonstrate the accuracy and performance of the solution methodology presented. Herein, the two‐dimensional cylindrical Couette flow, the decay of a three‐dimensional double shear wave, the cubic lid‐driven cavity flow with non‐uniform grids, the flow through a square duct with 90∘ bend and the flow past a sphere at different flow conditions are considered for validating the present computations. Numerical results obtained show the accuracy and robustness of the present solution methodology based on the implementation of the high‐order compact finite‐difference LB method (CFDLBM) in the generalized curvilinear coordinates for solving 3‐D incompressible flows over practical and realistic geometries.

E. Ezzatneshan, Simulation of Cavitating Flow through the Nozzle by Using Multiphase Lattice Boltzmann Method, Journal of Mechanical Engineering, Tabriz University, (2018) (In Persian).

Cavitating flow through the nozzle is numerically simulated by using the multiphase lattice Boltzmann method. The pseudo-potential Shan-Chen model is used to resolve inter-particle interactions, modeling phase change between the liquid and vapor phases and imposing the surface tension at the interface. The numerical algorithm implemented is simple for programming and efficient for simulation of multiphase cavitating flows comparing to the traditional Navier-Stokes solvers with complicated cavitation models. Efficiency and accuracy of the multiphase lattice Boltzmann method with Shan-Chen model for simulation of cavitating flows through the nozzle are examined by computing the cavitation inception, growth and collapse and the results obtained are compared with the existing numerical results in the literature. The study shows that the present computational technique is robust and efficient to predict the cavitation phenomena in the geometries studied.

E. Ezzatneshan, Comparative Study of the Lattice Boltzmann Collision Models for Simulation of Incompressible Fluid Flows, Mathematics and Computers in Simulation 156 (2018), pp. 158-177.

In this paper, several lattice Boltzmann (LB) collision models are evaluated by a comparative study for simulation of incompressible fluid flows with periodic and also with curved wall boundaries. Herein, the single-relaxation-time (SRT) scheme based on the Bhatnagar-Gross-Krook (BGK) approximation, multiple-relaxation-time (MRT), regularized lattice Boltzmann (RLB) and the entropic lattice Boltzmann (ELB) methods are considered. The doubly periodic shear layers flow problem is computed in two different Reynolds numbers to investigate the robustness and performance of the collision operators applied. Efficiency and accuracy of these techniques are also examined by computing the incompressible fluid flows around a circular cylinder at various flow conditions. The predicted results are compared with the experiments and the numerical results performed by other researchers. The present study demonstrates that the SRT model is accurate enough and efficient for simulation of fluid flows with low Reynolds numbers. This scheme, however, suffers from numerical instabilities at moderate Reynolds numbers and requires very fine grid resolutions to remain stable. It is found that the ELB scheme does not sufficiently reduce the numerical oscillation at high Reynolds number flows. This method provides fewer stability benefits while being more computationally expensive. The results obtained show that the MRT and RLB models are stable (in contrast to SRT and ELB) for all the cases considered in the present work even at high Reynolds numbers. In terms of computational efficiency and accuracy, the MRT and RLB schemes are more attractive and provide the results comparable to those of other experimental and numerical methods. The present study suggests that these two techniques based on the implementation of the lattice Boltzmann method are robust, sufficiently accurate and computationally efficient to resolve the flow structures and properties around the practical geometries even at high Reynolds numbers.

2017

E. Ezzatneshan, Study of surface wettability effect on cavitation inception by implementation of the lattice Boltzmann method, Physics of Fluids 29, 113304 (2017).

Cavitating flow through the orifice is numerically solved by implementation of the lattice Boltzmann method. The pseudo-potential single-component multiphase Shan-Chen model is used to resolve inter-particle interactions and phase change between the liquid and its vapor. The effect of surface wettability on the cavity formation and shape is studied with imposing an appropriate wall boundary condition for the contact angle between the liquid-vapor interface and the solid surface. Efficiency of the numerical approach presented is examined by computing the cavitation inception, growth and collapse for internal cavitating flows over a sack-wall obstacle placed inside a channel and through a convergent-divergent nozzle section. The results obtained demonstrate that hydrophobic walls act as surface nuclei and contribute to the process of cavitation inception even at high cavitation numbers. In contrast, the solid wall with hydrophilic properties shows no contribution to the onset of cavitation in the geometries studied. High values for the flow velocity corresponding to low cavitation numbers are needed to observe the cavitation inception over the geometries studied with the hydrophilic solid wall. The study shows that the present computational technique based on the implementation of the lattice Boltzmann method with the Shan-Chen model employed is robust and efficient to predict the cavitation phenomena with considering surface wettability effects and also accurate enough for computing the cavitating flow properties at different conditions.

E. Ezzatneshan, Implementation of a curved wall- and an absorbing open-boundary condition for the D3Q19 lattice Boltzmann method for simulation of incompressible fluid flows, Scientia Iranica, 2017.

In this work, a three-dimensional lattice Boltzmann method is developed for numerical simulation of the fluid flows around the arbitrary geometries in the wide range of Reynolds numbers. For efficient simulation of high Reynolds number flow structures in the turbulent regime, a large eddy simulation (LES) approach with the Smagorinsky subgrid turbulence model is employed. An absorbing boundary condition based on the concept of sponge layer is improved and implemented to damp the vorticity fluctuations near the open boundaries and regularize the numerical solution by significantly reducing the spurious reflections from the open boundaries. An off-lattice scheme with a polynomial interpolation is used for implementation of curved boundary conditions for the arbitrary geometries. The efficiency and accuracy of the numerical approach presented are examined by computing the low to high Reynolds number flows around the practical geometries, including the flow past a sphere in a range of Reynolds numbers from 102 to 104 and flow around the NACA0012 wing section in two different flow conditions. The present results are in good agreement with the numerical and experimental data reported in the literature. The study demonstrates the present computational technique is robust and efficient for solving flow problems with practical geometries.

E. Ezzatneshan, Implementation of D3Q19 Lattice Boltzmann Method with a Curved Wall Boundary Condition for Simulation of Practical Flow Problems, Int. J. Engineering (IJE); Transaction B: Applications, 30 (2017), pp. 1381-1390.

In this paper, implementation of an extended form of a no-slip wall boundary condition is presented for the three-dimensional (3-D) lattice Boltzmann method (LBM) for solving the incompressible fluid flows with complex geometries. The boundary condition is based on the off-lattice scheme with a polynomial interpolation which is used to reconstruct the curved or irregular wall boundary on the neighboring lattice nodes. This treatment improves the computational efficiency of the solution algorithm to handle complex geometries and provides much better accuracy comparing with the staircase approximation of bounce-back method. The efficiency and accuracy of the numerical approach presented are examined by computing the fluid flows around the geometries with curved or irregular walls. Three test cases considered herein for validating the present computations are the flow calculation around the NACA0012 wing section and through the two different porous media in various flow conditions. The study shows the present computational technique based on the implementation of the three-dimensional Lattice Boltzmann method with the employed curved wall boundary condition is robust and efficient for solving laminar flows with practical geometries and also accurate enough to predict the flow properties used for engineering designs.

2015

K. Hejranfar, E. Ezzatneshan, Simulation of Two-Phase Liquid-Vapor Flows Using a High-Order Compact Finite-Difference Lattice Boltzmann Method, Physical Review E, 92 (2015), 053305.

A high-order compact finite-difference lattice Boltzmann method (CFDLBM) is extended and applied to accurately simulate two-phase liquid-vapor flows with high density ratios. Herein, the He-Shan-Doolen type lattice Boltzmann multiphase model is used and the spatial derivatives in the resulting equations are discretized by using the fourth-order compact finite-difference scheme and the temporal term is discretized with the fourth-order Runge-Kutta scheme to provide an accurate and efficient two-phase flow solver. A high-order spectral-type low-pass compact nonlinear filter is used to regularize the numerical solution and remove spurious waves generated by flow non-linearities in smooth regions and at the same time to remove the numerical oscillations in the interfacial region between the two phases. Three discontinuity-detecting sensors for properly switching between a second-order and a higher-order filter are applied and assessed. It is shown that the filtering technique used can be conveniently adopted to reduce the spurious numerical effects and improve the numerical stability of the CFDLBM implemented. A sensitivity study is also conducted to evaluate the effects of grid size and the filtering procedure implemented on the accuracy and performance of the solution. The accuracy and efficiency of the proposed solution procedure based on the compact finite-difference LBM are examined by solving different two-phase systems. Five test cases considered herein for validating the results of the two-phase flows are an equilibrium state of a planar interface in a liquid-vapor system, a droplet suspended in the gaseous phase, a liquid droplet located between two parallel wettable surfaces, the coalescence of two droplets and a phase separation in a liquid-vapor system at different conditions. Numerical results are also presented for the coexistence curve and the verification of the Laplace law. Results obtained are in good agreement with the analytical solutions and also the numerical results reported in the literature. The study shows that the present solution methodology is robust, efficient and accurate for solving two-phase liquid-vapor flow problems even at high density ratios.

K. Hejranfar, E. Ezzatneshan, K. Fattah-Hesari, A Comparative Study of Two Cavitation Modeling Strategies for Simulation of Inviscid Cavitating Flows, J. Ocean Engineering, 108 (2015), pp. 257–275.

In the present work, two cavitation modeling strategies, namely the barotropic cavitation model and the transport equation-based model are applied and assessed for the numerical simulation of inviscid cavitating flows over two-dimensional and axisymmetric geometries. The algorithm uses the preconditioned Euler equations employing the interface capturing method for both the cavitation models. A same numerical solution procedure is used herein for discretizing the governing equations resulting from these two cavitation modeling strategies for the assessment to be valid and reliable. A central difference finite-volume scheme employing the suitable dissipation terms to account for density jumps across the cavity interface is shown to yield an effective method for solving the Euler equations. Results for steady inviscid cavitating flows over the NACA0012 and NACA66(MOD) hydrofoils and the hemispherical and ogive head shape bodies are obtained by applying these two cavitation modeling strategies and they are compared with each other for different conditions. A sensitivity study is conducted to evaluate the effects of various numerical and physical parameters involved in each cavitation model on the solution. The advantages and drawbacks of these two strategies for modeling of cavitating flows are also discussed. The present inviscid cavitation results are also compared with the experiments and the other inviscid and viscous cavitation results performed by other researchers and some conclusions are made.

2014

K. Hejranfar, E. Ezzatneshan, A High-Order Compact Finite-Difference Lattice Boltzmann Method for Simulation of Steady and Unsteady Incompressible Flows, J. Numerical Methods in Fluids, 75 (2014), pp. 713-746.

A high-order compact finite-difference lattice Boltzmann method (CFDLBM) is proposed and applied to accurately compute steady and unsteady incompressible flows. Herein, the spatial derivatives in the lattice Boltzmann equation are discretized by using the fourth-order compact finite-difference scheme and the temporal term is discretized with the fourth-order Runge-Kutta scheme to provide an accurate and efficient incompressible flow solver. A high-order spectral-type low-pass compact filter is used to stabilize the numerical solution. An iterative initialization procedure is presented and applied to generate consistent initial conditions for the simulation of unsteady flows. A sensitivity study is also conducted to evaluate the effects of grid size, filtering and procedure of boundary conditions implementation on accuracy and convergence rate of the solution. The accuracy and efficiency of the proposed solution procedure based on the compact finite-difference LBM method are also examined by comparison with the classical LBM for different flow conditions. Two test cases considered herein for validating the results of the incompressible steady flows are a 2-D backward facing step and a 2-D cavity at different Reynolds numbers. Results of these steady solutions computed by the compact finite-difference LBM are thoroughly compared with those of a compact finite-difference Navier-Stokes flow solver. Three other test cases, namely, a 2-D Couette flow, the Taylor’s vortex problem and the doubly periodic shear layers, are simulated to investigate the accuracy of the proposed scheme in solving unsteady incompressible flows. Results obtained for these test cases are in good agreement with the analytical solutions and also the available numerical and experimental results. The study shows that the present solution methodology is robust, efficient and accurate for solving steady and unsteady incompressible flow problems even at high Reynolds numbers.

K. Hejranfar, E. Ezzatneshan, Implementation of a High-Order Compact Finite-Difference Lattice Boltzmann Method in Generalized Curvilinear Coordinates, J. Computational Physics, 267 (2014), pp. 28-49.

In this work, the implementation of a high-order compact finite-difference lattice Boltzmann method (CFDLBM) is performed in the generalized curvilinear coordinates to improve the computational efficiency of the solution algorithm to handle curved geometries with non-uniform grids. The incompressible form of the discrete Boltzmann equation with the Bhatnagar–Gross–Krook (BGK) approximation with the pressure as the independent dynamic variable is transformed into the generalized curvilinear coordinates. Herein, the spatial derivatives in the resulting lattice Boltzmann (LB) equation in the computational plane are discretized by using the fourth-order compact finite-difference scheme and the temporal term is discretized with the fourth-order Runge-Kutta scheme to provide an accurate and efficient incompressible flow solver. A high-order spectral-type low-pass compact filter is used to regularize the numerical solution and remove spurious waves generated by boundary conditions, flow non-linearities and grid non-uniformity. All boundary conditions are implemented based on the solution of governing equations in the generalized curvilinear coordinates. The accuracy and efficiency of the solution methodology presented are demonstrated by computing different benchmark steady and unsteady incompressible flow problems. A sensitivity study is also conducted to evaluate the effects of grid size and filtering on the accuracy and convergence rate of the solution. Four test cases considered herein for validating the present computations and demonstrating the accuracy and robustness of the solution algorithm are: unsteady Couette flow and steady flow in a 2-D cavity with non-uniform grid and steady and unsteady flows over a circular cylinder and the NACA0012 hydrofoil at different flow conditions. Results obtained for the above test cases are in good agreement with the existing numerical and experimental results. The study shows the present solution methodology based on the implementation of the high-order compact finite-difference Lattice Boltzmann method (CFDLBM) in the generalized curvilinear coordinates is robust, efficient and accurate for solving steady and unsteady incompressible flows over practical geometries.

E. Ezzatneshan, M. Arami, T. Parhizkar, S. A. Hosseini-Kordkheili, S. Sattari, Evaluation of Optimum Performance and Economic Analysis of Micro CHP Systems in Different Sectors in Iran, Int. J. Energy&Technology, 6 (2014), pp. 1-10.

In this paper, the optimal operation strategy and economic study of Micro Combined Heat and Power) CHP (systems are performed in residential, commercial and health center sectors. The optimization model has been developed by a nonlinear programming (NLP) method. The objective function of this model is to minimize annual energy cost of the buildings. The optimum capacity and operating schedule of the Micro CHP systems are determined from the economic point of view. The annual cost saving and the rate of return of the Micro CHP systems are evaluated in the different buildings and compared with those of existing systems. A sensitivity analysis has been performed to investigate the effects of the key parameters on the adopting the Micro CHP systems to meet the energy demands of the buildings considered. This analysis indicates the profitability of Micro CHP systems is strongly sensitive to energy prices. Results obtained show if the surplus electricity sales is not considered, applying the Micro CHP systems are not economical in the buildings with the low electrical and heating demands. With considering electricity buyback, the profitability index increases for the all buildings. As well as, the annual cost is decreased by increasing the electricity buyback price. Study shows payback periods for the health, commercial and residential buildings studied are 2.5, 4.9 and 5.4 years, respectively.