Study on lattice Boltzmann method/ large eddy simulation and its application at high Reynolds number flow

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1 Research Article Study on lattice Boltzmann method/ large eddy simulation and its application at high Reynolds number flow Advances in Mechanical Engineering 1 8 Ó The Author(s) 2015 DOI: / aime.sagepub.com Haiqing Si and Yan Shi Abstract Lattice Boltzmann method combined with large eddy simulation is developed in the article to simulate fluid flow at high Reynolds numbers. A subgrid model is used as a large eddy simulation model in the numerical simulation for high Reynolds flow. The idea of subgrid model is based on an assumption to include the physical effects that the unresolved motion has on the resolved fluid motion. It takes a simple form of eddy viscosity models for the Reynolds stress. Lift and drag evaluation in the lattice Boltzmann equation takes momentum-exchange method for curved body surface. First of all, the present numerical method is validated at low Reynolds numbers. Second, the developed lattice Boltzmann method/large eddy simulation method is performed to solve flow problems at high Reynolds numbers. Some detailed quantitative comparisons are implemented to show the effectiveness of the present method. It is demonstrated that lattice Boltzmann method combined with large eddy simulation model can efficiently simulate high Reynolds numbers flows. Keywords Lattice Boltzmann method, subgrid model, wall boundary condition, computational fluid dynamics Date received: 5 October 2014; accepted: 22 January 2015 Academic Editor: Oronzio Manca Introduction Recently, the lattice Boltzmann method (LBM) has emerged as a well-known alternative of computational technique in fluid dynamics for modeling fluid flow in a way that is consistent with the Navier Stokes equation, 1,2 due to its intrinsic advantages over conventional Navier Stokes schemes. The LBM is an innovative numerical method based on kinetic theory to simulate various hydrodynamic systems; it is a reasonable candidate for simulation of turbulence, flow-induced noise, and sound propagation. The development of the lattice Boltzmann equation (LBE) was independent of the continuous Boltzmann equation. It was introduced to solve some of the difficulties of the lattice gas automata (LGA). A parameter matching procedure based on the Chapman Enskog analysis of the LGA needs to construct a set of relaxation equations so that the correct hydrodynamic equations are derived. Compared to the second-order Navier Stokes equations, the LBM is derived from a set of first-order partial differential equations the LBM formulation does not include nonlinear convection term while Navier College of Civil Aviation and Flight, Nanjing University of Aeronautics and Astronautics, Nanjing, China Corresponding author: Haiqing Si, College of Civil Aviation and Flight, Nanjing University of Aeronautics and Astronautics, Nanjing , China. sihaiqing@126.com Creative Commons CC-BY: This article is distributed under the terms of the Creative Commons Attribution 3.0 License ( which permits any use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access pages ( openaccess.htm).

2 2 Advances in Mechanical Engineering Stokes solvers have to treat such nonlinear one. Moreover, it is necessary for the incompressible Navier Stokes equations to solve the Poisson s equation, and then the pressure can be obtained. However, the pressure in the LBM is determined directly from the state equation; the computing cost can be decreased. Furthermore, it has been shown that complex boundary geometries are treated easily in LBM. As a result, LBM has widely emerged as a computational tool for practical engineering applications in the simulations of fluid flows 3,4 and aerospace industries. 5 It is well known that the LBM is often used as a direct numerical simulation tool without any assumptions for the relationship between the turbulence stress tensor and the mean strain tensor. Thus, the smallest captured scale in the LBM is the lattice unit, and the largest scale depends on the characteristic length scale in the simulation. These scales are often determined by the available computer memory. Consequently, the LBM is able to resolve relatively low Reynolds number flows. Numerical studies 6 have shown that LBM can result in the numerical instability for simulating high Reynolds number flows if unresolved small-scale effects on large-scale dynamics are not considered. A better option is to combine the LBM and large eddy simulation (LES) model in order to solve the problem at high Reynolds numbers. A subgrid model is often used as an LES model in the numerical simulation for traditional Navier Stokes equation. The idea of subgrid model 7 is based on an assumption to include the physical effects that the unresolved motion has on the resolved fluid motion. The model often takes a simple form of eddy viscosity models for the Reynolds stress. Smagorinsky model 7 is one of all subgrid models, including the standard Smagorinsky model and dynamic subgrid model. The standard Smagorinsky model uses a positive eddy viscosity to represent smallscale energy damping. Nevertheless, the shortcoming of the standard one is that the eddy viscosity could be large and positive at some scales smaller than test grids, while it can be large and negative at others sometimes. 7 Compared with the standard one, dynamic subgrid models 8 could include the dependence of the subgrid model coefficients on local quantities to overcome these side effects. Bhatnagar, Gross, and Krook 9 (BGK) collision model. In contrast to the traditional schemes of solving the discretized Navier Stokes equations, LBM approach focuses on the microscopic scales via the discrete Boltzmann equation and tracks particle distributions on a lattice. The standard LBE involving BGK model can be written into the following form f a (x + e ax dt, y + e ay dt, t + dt) f a (x, y, t) dt = f a eq(x, y, t) f a(x, y, t), a = 0, 1,..., M t ð1þ where f a is the density distribution function along the a-direction at position (x, y), fa eq is the local equilibrium distribution function, dt is the time-step; e a (e ax, e ay )is the discrete velocity of particles at time t, M is the number of the direction for the particle velocity, and t is the relaxation time, and its definition is related to the kinematics viscosity and temperature by the following formulation t = n T + Dt 2 ð2þ The macroscopic quantities, density and momentum density, can be obtained directly from the distribution function f a, and their definitions are in the following form r(x, y, t)= XM a = 0 f a (x, y, t), ru(x, y, t)= XM a = 0 f a (x, y, t)e a ð3þ The pressure p can be obtained by the state equation of an ideal gas. Under some suppositions, the full compressible viscous Navier Stokes equations can be derived from LBEs using a multi-scale analysis. In the simulation, the particle velocity model D2Q9 10 is used in the present code. The discrete velocity set e a, equilibrium distribution function f eq, and a weighting factors w a are given by 8 < (0, 0) a = 0 e a = ( p cos ffiffiffi u a, sin u a )c, u a =(a 1)p=2 a = 1, 2, 3, 4 : 2 ( cos ua, sin u a )c, u a =(a 5)p=2 + p=4 a = 5, 6, 7, 8 f eq = w a a r 1 + 3(e a u) c 2 + 9(e a u) 2 2c 4 3u2 2c 2 8 < 4=9, a = 0 w a = 1=9, a = 1, 2, 3, 4 : 1=36, a = 5, 6, 7, 8 ð4þ ð5þ ð6þ LBM The governing equation of LBM for describing fluid flows is Boltzmann equation modeled by adopting the For the treatment of boundary condition, Zou and He s 11 non-equilibrium bounce-back scheme is applied on the far-field boundary in the computational domain due to its second-order accuracy. The boundary

3 Si and Shi 3 condition for the particle distribution functions on the curved wall is handled with second-order accuracy based on Mei et al. s 12 work. Lift and drag evaluation in the LBE takes momentum-exchange method for curved boundary. Mei has shown that it is reliable, accurate, and easy to implement for both twodimensional and three-dimensional flows. Subgrid model for LES In the LES, the most common subgrid model is the Smagorinsky one, where the anisotropic part of the Reynolds stress term is modeled as 2CD 2 S S ij ð7þ in which S qffiffiffiffiffiffiffiffiffiffiffi = 2S ij S ij is the magnitude of the largescale strain rate tensor S ij = 1 u i + u j ð8þ 2 x j x i Figure 1. Computational domain. C is the Smagorinsky constant, and D is the filter width. Next, LBM is modified to simulate the filtered physical variables in the LBE. First, the filtered particle distribution f a is denoted as follows fa (x)= ð f a (x)g(x, x 0 )dx 0 ð9þ Then the LBE can be written into a kinetic equation for the filtered particle distribution function fa (x + e ax dt, y + e ay dt, t + dt) f a (x, y, t) dt = fa eq(x, y, t) f a (x, y, t) t * ð10þ The above equation for f a has the similar form as LBE, with the only difference in the relaxation time t *. Here, the turbulence relaxation time t t is introduced into the present relaxation time to consider the effects of small-scale fluid motion. Therefore, the efficient relaxation time can be written into the following t = t + t t Then the total viscosity can be denoted as follows n = n + n t ð11þ ð12þ where n =(2t 1)=6 is the laminar viscosity, and n t = t t =3 is the turbulence viscosity. According to the present Smagorinsky subgrid model, n t should be CD 2 S. Figure 2. Computational mesh. Numerical results Circular cylinder in low Reynolds numbers flow To validate the program developed in the article, low Reynolds numbers flow around a circular cylinder is first simulated here. As depicted in Figure 1, the fluid with the velocity U = 0:1 flows around the cylinder from the left to the right. Re =(UD)=n is the Reynolds number, and D is diameter of the cylinder. Uniform grids in Figure 2 are used in the simulation; cylinder is immersed in the present grid. Two different cases for Re = 20 and Re = 40 are shown in the following figures. Figure 3 reveals stream line in the flow field for the different cases; it is clearly seen that their stream lines between upper half-plane and lower half-plane are

4 4 Advances in Mechanical Engineering Figure 3. Stream line for Re = 20 and 40. Figure 4. Vortex contours for cylinder. symmetry. Vorticity contours for the different Re can be found in Figure 4; symmetry vortex pairs attached on the cylinder are also obviously seen in the figure. Drag evaluation in the LBE takes momentum-exchange method for curved boundary. Figure 5 shows the time history of drag for these two cases. Comparisons between the present result and the referred one are given in Table 1; it is further demonstrated from the table that the program developed in the article could be able to simulate low Reynolds flow field, which provides a possibility to model high Reynolds number flow problems using the presented method. Here, the data in Table 1 are extracted at non-dimensional time 20. Next, high Reynolds flows will be tested according to the coupled LBM and LES. Table 1. Comparisons of lift and drag. Re =20 Re =40 Lift Nieuwstadt and Keller Dennis and Chang The present model Drag Nieuwstadt and Keller Dennis and Chang The present model Lid-driven cavity flow at high Reynolds numbers Lid-driven cavity flow is considered here to validate the simulation for high Reynolds flow using the present

5 Si and Shi 5 Figure 5. Time history of Cd for cylinder. Figure 6. Computational domain and grids. developed LBM/LES model. It is a classic problem for studying the complex flow in a simple geometry. As shown in Figure 6, flow is driven by the uniform velocity U on the top boundary of square cavity; vortex will be formed inside the cavity depending on the different Reynolds number Re. A uniform grid is used in the present simulation in Figure 6. Figure 7 reveals the stream line and vortex contours at Reynolds number 7500 and U = 0.1. From this figure, the complex vortex structures are found, main vortex lies in the center of the cavity, small vortex also exists in the corner of the cavity around the main vortex, and a second vortex is also clearly seen in the lower right of the cavity. Detailed quantitative comparisons of the vortex position can be found in Table 2 in order to show the ability of the present LBM/LES for Table 2. Comparisons of vorticity positions inside the cavity. Coordinates Ghia s result The present result Main vortex X Y Lower left vortex X Y Lower right vortex X Y simulating high Reynolds flow. Compared with Ghia et al. s 15 results, the calculated results in the article can keep consistent with them for the different vortex positions. It is expected that LBM/LES is used to simulate

6 6 Advances in Mechanical Engineering Figure 7. Stream lines and vortex contours for cavity flow. high Reynolds number flow around the complex bodies. NACA0012 airfoil at high Reynolds numbers NACA0012 airfoil at high Reynolds flow is simulated at Re = and Re = 6: using the present method, where Re = UC=n, C is the chord of NACA0012 airfoil. U is set to be 0.2 in the numerical simulations. The calculations at two different Reynolds numbers are performed to compare with the experimental result in order to show the ability to simulate high Reynolds numbers flow. The angle of attack (AOA) in the first calculation is 7, and a range of AOA from 0 to 14 is applied in the second one. The computational domain and mesh are depicted in Figure 8, where the different boundary conditions are imposed at the inlet, outlet, top boundary, bottom boundary, and wall. Fixed velocity and pressure are imposed at the inlet and outlet boundaries, respectively. Non-reflective boundary condition is applied at the top and bottom boundaries. In this study, density distribution function at all the far-field boundaries is estimated using the non-equilibrium bounce-back scheme from Zou and He,11 No-slip boundary is implemented on the body wall, where Guo et al. s16 non-equilibrium extrapolation scheme is used to calculate the density distribution function. Lift and drag evaluation in the LBM also takes momentum-exchange method for curved airfoil boundary. Table 3 shows the comparison of lift and drag coefficients between the present and the referred results for Re = and AOA = 7. From the table, it is clearly seen that the calculated result from LBM/LES Figure 8. Computational domain, boundary, and grid.

7 Si and Shi 7 Table 3. Detailed comparisons of C l and C d at Re = ,AOA=7. Method PowerFlow T. Imamura (B-L model) T. Imamura (laminar) CFL3D (S-A model) Present (LBM/LES) C l ± C d ± B-L model: Baldwin-Lomax model; LBM: lattice Boltzmann method; LES: large eddy simulation; S-A model: Spalart-Allmaras model. 1.2 Experimental data GILBM Present Experimental data GILBM Present Cd Cl AOA Figure 9. Lift coefficient versus different AOA at Re = 6: could obtain good agreements with others for C l and C d from Imamura et al. 17 Detailed quantitative comparisons are performed to compare with the experimental results at Re = 6: , where a range of AOA from 0 to 14 is applied in the simulation. The relationship between C l, C d, and AOA is shown in the figure using the presented method. As depicted in Figures 9 and 10, the present result could provide reasonable agreement with the experimental and other referred one from Imamura et al. s 17 GILBM before fluid separation happens on the upper surface of the airfoil. However, it is also obviously seen that there exists some differences with the experimental result after separation in Figures 9 and 10. The similar phenomena are also found in the other numerical methods. 17 The predicted stall angle is 11 here using the present method, which approaches the experimental one. Conclusion Compared with the other methods, the present method combining the LBM and LES model could solve the AOA Figure 10. Drag coefficient versus different AOA at Re = 6: problem at high Reynolds numbers flows. As one of all subgrid models, Smagorinsky model can provide a positive eddy viscosity to represent small-scale energy damping. For the treatment of boundary condition, non-equilibrium bounce-back scheme has been applied on the far-field boundary in the computational domain. The boundary condition for the particle distribution functions on the curved wall is handled using secondorder accuracy scheme. Lift and drag evaluation in the LBE takes momentum-exchange method for curved boundary. The developed program is first to simulate low Reynolds number circular cylinder flow field; it proves that it could provide a possibility to model high Reynolds problems using the present method. Next, lid-driven cavity flow and NACA0012 airfoil flow are calculated to show the ability of combining LBM with LES for simulating high Reynolds number flow. Some comparisons demonstrate that the present calculated results could obtain good agreement with the experimental ones. Declaration of conflicting interests The authors declare that there is no conflict of interests.

8 8 Advances in Mechanical Engineering Funding This work was supported by Special Foundation of China Postdoctoral Science (grant no ), National Natural Science Foundation of China (grant no ), and the Fundamental Research Funds for the Central Universities (grant no. NS ). References 1. Shan X, Yuan XF and Chen H. Kinetic theory representation of hydrodynamics: a way beyond the Navier- Stokes equation. J Fluid Mech 2006; 550: Chen S and Doolen G. Lattice Boltzmann method for fluid flows. Annu Rev Fluid Mech 1998; 30: Adam J-L, Ricot D, Dubief F, et al. Aeroacoustic simulation of automobile ventilation outlets. J Acoust Soc Am 2008; 123(5): Balasubramanian G, Crouse B and Freed D. Numerical simulation of real world effects on sunroof buffeting of an idealized generic vehicle. In: 15th AIAA/CEAS aeroacoustics conference (30th AIAA aeroacoustics conference), Miami, FL, May 2009, AIAA Paper Reston, VA: AIAA. 5. Keating A, Dethioux P, Satti R, et al. Computational aeroacoustics validation and analysis of a nose landing gear. 15th AIAA/CEAS aeroacoustics conference (30th AIAA aeroacoustics conference), Miami, FL, May 2009, AIAA Paper Reston, VA: AIAA. 6. Sterling JD and Chen S. Stability analysis of lattice Boltzmann methods. J Comput Phys 1996; 123: Hou S, Sterling J, Chen S, et al. A lattice Boltzmann subgrid model for high Reynolds number flows. In: Kapral R (ed.) Pattern formation and lattice gas automata, vol. 6. Providence, RI: American Mathematical Society, 1996, pp Germano M, Piomeelli U, Moin P, et al. A dynamic subgrid-scale eddy viscosity model. Phys Fluid A Fluid Dynam 1991; 3(7): Bhatnagar P, Gross EP and Krook MK. A model for collision processes in gases, 1. Small amplitude processes in charged and neutral one-component systems. Phys Rev 1954; 94(3): He X and Luo LS. Theory of the lattice Boltzmann method: from the Boltzmann equation to the lattice Boltzmann equation. Phys Rev E 1997; 56: Zou Q and He X. On pressure and velocity boundary conditions for the lattice Boltzmann BGK model. Phys Fluid 1997; 9(6): Mei R, Yu D and Shyy W. Force evaluation in the lattice Boltzmann method involving curved geometry. Phys Rev E 2002; 65: Nieuwstadt F and Keller HB. Viscous flow past circular cylinder. Comput Fluid 1973; 42: Dennis SCR and Chang GZ. Numerical solutions for steady flow past a circular cylinder at Reynolds number up to 100. J Fluid Mech 1970; 42: Ghia U, Ghia KN and Shin CT. High-Re solutions for incompressible flow using the Navier-Stokes equations and a multigrid method. J Comput Phys 1982; 48(3): Guo ZL, Zheng CG and Shi BC. Non-equilibrium extrapolation method for velocity and boundary conditions in the lattice Boltzmann method. Chin Phys 2002; 11(4): Imamura T, Suzuki K, Nakamura T, et al. Flow simulation around an airfoil using lattice Boltzmann method on generalized coordinates. In: 42nd AIAA aerospace sciences meeting and exhibit, Reno, NV, 5 8 January 2004, AIAA Reston, VA: AIAA.