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Heat Transfer Research 48(9):811–826 (2017) 1064-2285/17/$35.00 © 2017 by Begell House, Inc. www.begellhouse.com 811 1. INTRODUCTION The lattice Boltzmann method (LBM) (Succi, 2001) has been developed to be a powerful numerical method for fluid flow simulation in the last three decades. Traditional computational fluid dynamic (CFD) methods, such as finite volume method (FVM) and finite difference method (FDM), solve difference mass and momen- tum equations to obtain the velocity, density, and pressure directly. Different from them, the LBM solves the mesoscopic density distribution parameters first and then uses them to calculate the macroscopic parameters that can also satisfy the mass and momentum equations. The LBM has been applied to many fluid flow and heat transfer problems, including incompressible fluid flow (Zou et al., 1995; Li et al., 2014d), natural convection (Li et al., 2016a,b; Rahmati et al., 2009), porous media fluid flow (Chen and Doolen, 1998; Guo and Zhao, 2002), and phase change problems (Chatterjee and Chakraborty, 2005; Li et al., 2014c, 2015b). Comparing with the traditional methods, the LBM shows its advantages in easy code settings, applicability in parallel computing (Wang and Aoki, 2011), and suitability to complex fluid flow. Several hybrid methods are developed to take advantages of both the CFD and LBM (Li et al., 2014a,b). The LBM can also be coupled with the Monte Carlo method to solve fluid flow and heat transfer problems (Li et al., 2015a). Some basic problems still remain in this algorithm. For example, there are many boundary condition setting methods in the LBM without reaching agreement. Latt and Chopard (2008) compared five straight common boundary conditions and suggested that all the methods can yield macroscopic results of the same MASS BALANCE IN LATTICE BOLTZMANN METHOD WITH DIRICHLET VELOCITY BOUNDARY CONDITION Zheng Li, 1,2 Mo Yang, 1 Ya-Ling He, 3 & Yuwen Zhang 2,* 1 College of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China 2 Department of Mechanical and Aerospace Engineering, University of Missouri, Columbia, MO 65211, USA 3 School of Energy and Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, China *Address all correspondence to: Yuwen Zhang, E-mail: [email protected] Original Manuscript Submied: 7/28/15; Final DraReceived: 4/14/16 Many dierent methods can be used to treat open boundary conditions in the laice Boltzmann method. The Zou–He meth- od, nite dierence velocity gradient method, and regularized method are reviewed and compared for the Dirichlet velocity condition for Poiseuille ow with dierent Reynolds numbers. Using the same convergence criterion, all the numerical pro- cedures are carried on until steady states are reached. The obtained velocities and pressures are checked and compared with analytical solutions and mass balances for dierent methods. The results indicate that all the numerical data agreed well with the analytical solutions and the Zou–He method results satisfy the mass balance beer than the others. KEY WORDS: Zou–He method, nite dierence velocity gradient method, regularized method, laice Boltz- mann method, Dirichlet velocity condition

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Page 1: MASS BALANCE IN LATTICE BOLTZMANN METHOD WITH …faculty.missouri.edu/zhangyu/Pubs/260_HTR-14588.pdfMass Balance in Lattice Boltzmann Method 815 Volume 48, Issue 9, 2017 bility than

Heat Transfer Research 48(9):811–826 (2017)

1064-2285/17/$35.00 © 2017 by Begell House, Inc. www.begellhouse.com 811

1. INTRODUCTION

The lattice Boltzmann method (LBM) (Succi, 2001) has been developed to be a powerful numerical method for fl uid fl ow simulation in the last three decades. Traditional computational fl uid dynamic (CFD) methods, such as fi nite volume method (FVM) and fi nite difference method (FDM), solve difference mass and momen-tum equations to obtain the velocity, density, and pressure directly. Different from them, the LBM solves the mesoscopic density distribution parameters fi rst and then uses them to calculate the macroscopic parameters that can also satisfy the mass and momentum equations. The LBM has been applied to many fl uid fl ow and heat transfer problems, including incompressible fl uid fl ow (Zou et al., 1995; Li et al., 2014d), natural convection (Li et al., 2016a,b; Rahmati et al., 2009), porous media fl uid fl ow (Chen and Doolen, 1998; Guo and Zhao, 2002), and phase change problems (Chatterjee and Chakraborty, 2005; Li et al., 2014c, 2015b). Comparing with the traditional methods, the LBM shows its advantages in easy code settings, applicability in parallel computing (Wang and Aoki, 2011), and suitability to complex fl uid fl ow. Several hybrid methods are developed to take advantages of both the CFD and LBM (Li et al., 2014a,b). The LBM can also be coupled with the Monte Carlo method to solve fl uid fl ow and heat transfer problems (Li et al., 2015a).

Some basic problems still remain in this algorithm. For example, there are many boundary condition setting methods in the LBM without reaching agreement. Latt and Chopard (2008) compared fi ve straight common boundary conditions and suggested that all the methods can yield macroscopic results of the same

MASS BALANCE IN LATTICE BOLTZMANN METHOD WITH DIRICHLET VELOCITY BOUNDARY CONDITION

Zheng Li,1,2 Mo Yang,1 Ya-Ling He,3 & Yuwen Zhang2,*

1College of Energy and Power Engineering, University of Shanghai for Science and Technology,  Shanghai 200093, China 2Department of Mechanical and Aerospace Engineering, University of Missouri, Columbia,  MO 65211, USA3School of Energy and Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, China

*Address all correspondence to: Yuwen Zhang, E-mail: [email protected]

Original Manuscript Submitt ed: 7/28/15; Final Draft Received: 4/14/16

Many diff erent methods can be used to treat open boundary conditions in the latt ice Boltzmann method. The Zou–He meth-od, fi nite diff erence velocity gradient method, and regularized method are reviewed and compared for the Dirichlet velocity condition for Poiseuille fl ow with diff erent Reynolds numbers. Using the same convergence criterion, all the numerical pro-cedures are carried on until steady states are reached. The obtained velocities and pressures are checked and compared with analytical solutions and mass balances for diff erent methods. The results indicate that all the numerical data agreed well with the analytical solutions and the Zou–He method results satisfy the mass balance bett er than the others.

KEY WORDS: Zou–He method, fi nite diff erence velocity gradient method, regularized method, latt ice Boltz-mann method, Dirichlet velocity condition

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812 Li et al.

accuracy for several cases. Collison and streaming are two basic processes in the LBM. Their boundary con-ditions fall into two categories: (1) recovering the unknown density distribution step after streaming step on the boundary, (2) replacing all density distributions. These methods yield different results when the boundary speed is not zero. In that case, the density distributions entering into the system differ from those leaving the system which violates the local mass balance (Yin et al., 2012).

Numerous efforts have been made to study the cases of nonzero velocity to discuss local mass balance for the LBM boundary. A curved boundary treatment was proposed (Mei et al., 1999, 2002). The no-slip curved boundary is approximated by a series of stairs. The velocities on the stairs obtained from the difference are not equal to zero. Correspondingly, it may violate the local mass balance. Bao et al. (2008) and Verschaeve and Muller (2010) applied different settings for the local mass balance on a curved boundary. The veloci-ty on the moving boundary is also nonzero in the LBM. Coupanec and Verschaeve (2011) derived a mass conserving boundary condition for tangentially moving walls in the LBM. An enhanced mass conserving closure scheme was employed for the LBM hydrodynamics under open boundary conditions (Hollis et al., 2006). There are some different opinions on the local mass balance. It was argued that this local mass bal-ance is fundamentally fl awed leading incorrect pressure (Ladd and Verberg, 2001; Nguyen and Ladd, 2002). Ginzbourg and d’Humieres (1996) demonstrated that the total mass balance can be reached even though the local mass balance is not satisfi ed in the LBM boundary conditions (Filippova and Hanel, 1998). Mass and momentum transfer across the curved boundary in the LBM are reviewed in Yin et al. (2012). They con-cluded that the adding of momentum to the density distribution reduction had no direct infl uence on fl ow and pressure fi elds, but the incorrect fl uid–particle interaction might affect the results of simulation of particulate suspensions. Mass balance is the conservation law based on macroscopic parameters (velocity and density) and the local mass balance has no direct relation with the conservation law, since it is based on the meso-scopic density distribution parameter. In this paper, the LBM with three common boundary conditions [the Zou–He method (Zou and He, 1997), fi nite difference velocity gradient method, and regularized method] are employed to solve the fl uid fl ow problem with the Dirichlet velocity boundary. All three methods cannot

NOMENCLATURE

α lattice acceleration u horizontal velocity in lattice unit

c lattice speed v vertical velocity in lattice unit

cs lattice sound speed U nondimensional horizontal velocity

ea velocity in every direction V nondimensional vertical velocity

f density distribution V velocity

I unit tensor Greek symbols

K Chapman–Enskog expansion coeffi cient

P nondimensional pressure Π stress tensor

Q lattice weight tensor τ relaxation time

r lattice location vector ω lattice weight

t lattice time Ω collision operator

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satisfy the local mass balance for the nonzero boundary. These three boundary conditions will be discussed based on the mass conservation law directly.

2. PROBLEM STATEMENT

Poiseuille fl ow is employed to test the boundary methods in the LBM for the Dirichlet velocity condition. Figure 1 shows 2D incompressible fl uid fl ow between two parallel fl at plates. The channel's height and length are h and l, respectively. Fully developed fl ow can be reached at the channel outlet since l is greater than 10h.

This problem is governed by the following equations:

( ) ( )0

u vt x y

∂ ρ ∂ ρ∂ρ+ + =

∂ ∂ ∂ , (1)

( ) ( ) ( ) 2 2

2 2u uu vu p u u

t x y x x y

⎛ ⎞∂ ρ ∂ ρ ∂ ρ ∂ ∂ ∂+ + = − + μ +⎜ ⎟⎜ ⎟∂ ∂ ∂ ∂ ∂ ∂⎝ ⎠

, (2)

( ) ( ) ( ) 2 2

2 2v uv vv p v v

t x y y x y

⎛ ⎞∂ ρ ∂ ρ ∂ ρ ∂ ∂ ∂+ + = − + μ +⎜ ⎟⎜ ⎟∂ ∂ ∂ ∂ ∂ ∂⎝ ⎠

, (3)

which are subject to the following boundary conditions:

( )0: 0x u u y v= = = , (4)

: / 0 0x l u x v= ∂ ∂ = = , (5)

/ 2 : 0 0y h u v= = = , (6)

/ 2 : 0 0y h u v= − = = . (7)

In addition, the Reynolds number is defi ned using the maximum velocity umax in the channel:

maxRe u h

. (8)

FIG. 1: Physical model

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3. LATTICE BOLTZMANN METHOD

The statistical behavior of fl uid fl ow can be expressed by the following Boltzmann equation (Chen and Dool-en, 1998):

( )collision+ + =∂ ∂ ∂⋅ ⋅ Ω

∂ ∂ ∂f f f ft

εε

ar

, (9)

where f and Ω are the density distribution and collision operator, respectively. The lattice Boltzmann method is executed on a regular grid. For a 2D problem, the density distributions at each computing node have nine directions to move to the nearby nodes, which is shown in Fig. 2. This is referred to as the D2Q9 model in the LBM (Chen and Doolen, 1998).

The velocities at each node are

(0, 0) 1

( cos , sin ) 2, 3, 4, 52 2

(2 1) (2 1)2 ( cos , sin ) 6, 7, 8, 94 4

⎧⎪ =⎪ π π⎪= − − =⎨⎪

+ π + π⎪− − =⎪⎩

a

aa ae c a

a ac a

, (10)

where c is the lattice speed. Then Eq. (9) can be differenced in these nine directions as follows:

( ) ( ) ( )collision, , 1, 2, ... , 9+ Δ + Δ − = Ω =a a a af x e t t t f x t f a , (11)

where tΔ is the discrete time step. One LBM iteration includes two steps: collision and streaming. The collision step is local to each node:

( ) ( ) ( )*collision, , 1, 2, ... , 9= + Ω =a a af x t f x t f a , (12)

where *f is the post-collision density distribution. Many methods exist in the LBM to simplify the collision term. The algorithms can be single-relaxation time model (SRT), double-relaxation time model, or multiple relaxation time model. The double-relaxation and multiple-relaxation time models have better numerical sta-

FIG. 2: Nine directions in D2Q9 model

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bility than SRT. On the other hand, SRT has valid advantages in a simple model setting. SRT is employed since numerical stability is not challenging for all the test cases in this article. This model simplifi es the col-lision operator by the following equation:

( ) ( )eq

collision1= – 1, 2, ... , 9Ω − =τa a af f f a , (13)

where τ is the single relaxation time and eqaf is the direction equilibrium distribution

eq2 41 11 : 1, 2, ... , 9

2

⎛ ⎞= ρω + ⋅ + =⎜ ⎟⎜ ⎟

⎝ ⎠a a

s sf a

c ca ae V Q VV , (14)

where cs is the speed of sound which is equal to 3c . The scalar lattice weights aω and tensors aQ are defi ned as

4 191 2, 3, 4, 591 6, 7, 8, 936

a

a

a

a

⎧ =⎪⎪⎪ω = =⎨⎪⎪

=⎪⎩

, (15)

Qa = eaea – cs2I a = 1, 2, ... , 9 , (16)

where I is the unit tensor. The local collision step can be fulfi lled by the settings given above. The streaming step follows the collision step and it takes the post-collision distributions to the nearby nodes:

( ) ( )*, , 1, 2, , 9a a af x e t t t f x t a+ Δ + Δ = = ⋅ ⋅ ⋅ . (17)

The macroscopic variables can be obtained by moments of the density distributions. The quantities ρ, ρV, and Π correspond to the density distribution momentums of 0, 1, and 2:

9

1a

af

=ρ = ∑ , (18)

9

1=ρ = ∑ a

afaeV , (19)

9

1a

af

=∏ = ∑ aQ , (20)

2sp c= ρ . (21)

Applying the following Chapman–Enskog expansion equations:

1

Kr r∂ ∂

=∂ ∂

, (22)

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816 Li et al.

1 2

K Kt t t∂ ∂ ∂

= +∂ ∂ ∂

,

(23)

0 1 2 2a a a af f Kf K f= + + (24)

to Eq. (11), the macroscopic governing equations can be obtained from the LBM:

( ) 0∂ρ+ ∇ ⋅ ρ =

∂tV , (25)

( ) ( ) ( ) ( )22

1 12

⎡ ⎤⎛ ⎞∂ ρ ⎛ ⎞⎢ ⎥+ ∇ ⋅ ρ = −∇ + ∇ ⋅ ρΔ τ − ∇ + ∇ − ∇ ⋅ ρ⎜ ⎟⎜ ⎟ ⎜ ⎟∂ ⎝ ⎠⎢ ⎥⎝ ⎠⎣ ⎦

Ts

sp t c

t c

VVV V V VVV . (26)

To reach the Navier–Stokes equations, the relaxation time τ is related to ν by

212 st c⎛ ⎞Δ τ − = ν⎜ ⎟

⎝ ⎠. (27)

The product 2 2aK f in Eq. (24) shows no effect in this process. Equation (26) differs from the momentum

equation due to presence of the term ( )2

⎡ ⎤⎛ ⎞ν⎢ ⎥∇ ⋅ ρ − ∇ ⋅ ρ⎜ ⎟⎜ ⎟⎢ ⎥⎝ ⎠⎣ ⎦sc

VVV . It can be neglected when the Mach num-

ber is low (less than 0.3), which is the case in consideration. In the multiscale analysis process, the following equations are also obtained:

eq0

12

1, 2, , 9:

a a

aa

s

f fa

Kfc

⎧ =⎪

= ⋅ ⋅ ⋅ρτω⎨ = − ∇⎪⎩

aQ V. (28)

Neglecting the high-order term effect, the density distributions are approximated as

eq2 : 1, 2, , 9a

a as

f f ac

ρτω= − ∇ = ⋅ ⋅ ⋅aQ V . (29)

This equation plays important roles in many boundary conditions.

4. IMPLEMENTATION OF BOUNDARY CONDITIONS

The density distributions in some directions are unknown on the boundary before the collision step. Figure 3 shows the left boundary in a 2D domain.

The density distributions f2, f6, and f9, shown by dark vectors, are unknown after the streaming step. The LBM boundary conditions are designed to fi nd these unknown density distributions. Some boundary con-ditions substitute the unknown density distributions and keep the known ones. In contrast, some methods replace all the density distributions on boundaries before the collision step. The boundary conditions also differ from each other by the relation with the other near the nodes. Some methods obtain the boundary distributions based on the local computation node information only, while others need the nearby nodes in-formation to recover the boundary distributions. This article compares three common methods [the Zou–He

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method (Zou and He, 1997), the fi nite difference velocity gradient method (Skordos, 1993), and the regular-ized method (Latt, 2007)] for the Dirichlet velocity boundary conditions. Table 1 summarizes the categories for these three methods. Boundary velocities are known in the Dirichlet condition. Three boundary conditions are shown in Fig. 3 for the Dirichlet velocity condition based on the left boundary.

4.1 Zou–He Boundary (BC1)

This method recovers the missing density distributions based on the local information only (Zou and He, 1997). In this 2D problem, Eqs. (18) and (19) become

1 2 3 4 5 6 7 8 9

2 6 9 4 7 8

3 6 7 5 8 9

f f f f f f f f ff f f f f f uf f f f f f v

+ + + + + + + + = ρ⎧⎪ + + − − − = ρ⎨⎪ + + − − − = ρ⎩

, (30)

where u and v are known from the Dirichlet velocity condition. Four independent unknowns (ρ , f2, f6, and f9) in three equations indicate that one degree of freedom is left. Meanwhile, the density distribution can be obtained directly from Eq. (30):

FIG. 3: Boundary and fl uid nodes

TABLE 1: The categories of boundary conditions

Method Replace All fa Local

Zou–He (BC1) No Yes

Finite difference (BC2) Yes No

Regularized (BC3) Yes Yes

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818 Li et al.

( )1 3 5 4 7 82

1f f f f f f

u+ + + × + +

ρ =+

. (31)

At the four corner nodes, there is not enough information for obtaining the density. Extrapolating these corner nodes' densities (Latt and Chopard, 2008) from the neighboring computing nodes is employed for all three boundary conditions in this article.

In the Zou–He boundary condition, the nonequilibrium parts are assumed to be the same in directions 2 and 4:

eq eq2 42 4f f f f= + − (32)

and 222 :

scτω

ρ∇Q V is equal to 442 :

scτω

ρ∇Q V due to the symmetry of Q ; therefore this assumption is

reasonable regarding Eq. (27). Then the other two missing density distributions are

( )( )

6 4 5 8 2 3

9 3 4 7 2 5

2 2

2 2

f f f f f f u v

f f f f f f u v

⎧ = + + × − − + ρ + ρ⎪⎨

= + + × − − + ρ − ρ⎪⎩. (33)

4.2 Finite Difference Velocity Gradient Method (BC2)

This method is based on the approximation in Eq. (29) with a slight difference (Skordos, 1993) where:

( )eq

2 : 1, 2, , 9aa a

sf f a

cτω

= − ∇ ρ = ⋅ ⋅ ⋅aQ V , (34)

is the double dot product. Differently from Eq. (29), ρ is inside the operator ∇.This setting includes the compressible effect on the strain rate, which is negligible for the incompressible

fl uid fl ow in this article. The boundary density is obtained from Eq. (30) and then ( )∇ ρV is obtained by the following equation:

( ) ( )( ) ( )

,

,

u x v x

u y v y

⎡∂ ρ ∂ ∂ ρ ∂ ⎤∇ = ⎢ ⎥

∂ ρ ∂ ∂ ρ ∂⎢ ⎥⎣ ⎦V . (35)

To maintain the second-order accuracy, three point midpoint and endpoint derivative laws are employed for the operators in Eq. (35). For the function g taken as an example, three computing nodes in one direction are shown in Fig. 4. Its fi rst derivative functions g' at different locations can be obtained from the following equation:

( ) ( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( ) ( )

1' 3 4 1 22

1' 1 22

1' 2 4 1 3 22

g i g i g i g ix

g i g i g ix

g i g i g i g ix

⎧ = − × + × + − +⎡ ⎤⎣ ⎦⎪ Δ⎪⎪ + = − + +⎡ ⎤⎨ ⎣ ⎦Δ⎪⎪

+ = − × + + × +⎡ ⎤⎪ ⎣ ⎦Δ⎩

, (36)

( )u x∂ ρ ∂ , ( )v x∂ ρ ∂ , ( )u y∂ ρ ∂ , and ( )v y∂ ρ ∂ in Eq. (35) can be obtained from Eq. (36) for all the bound-aries including the corner nodes. Then all the boundary density distributions can be obtained from Eq. (34).

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4.3 Regularized Method (BC3)

Similar to the fi nite velocity gradient method, the regularized method (Latt, 2007) also replaces all the bound-ary density distributions with the aid of Eq. (29). Instead of calculating ∇V from the nearby nodes informa-tion, the regularized method fulfi lls this step using the local information. The strain rate tensor S is defi ned as

( )12

T+⎡ ⎤∇ ∇⎢ ⎥⎣ ⎦V V . Due to the symmetry of Qa and S, Eq. (29) becomes

eq (1)4 : 1, 2, ... , 9

= + =aa a a

sf f Q a

c∏ , (37)

where the tensor (1)∏∏ is defi ned as

(1) 22 sc= − τρS∏∏ . (38)

After the solution of the boundary density ρ from Eq. (30), eqaf can be obtained from Eq. (14). The

quantity (1)∏∏ is approximated by the following equation:

9

eq(1) ( eq)

1

nna a

af

== = ∑ Q∏ ∏∏ ∏ . (39)

Not all eqnaf are known after the streaming process. For example, eq

2nf , eq

6nf , and eq

9nf are unknowns

for the left boundary. They are approximated to equal those in the opposite directions:

eq eq eq eq eq eq

72 4 6 8 9, ,n n n n n nf f f f f f= = = . (40)

Then the regularized method if fulfi lled by the above-given equations.

5. RESULTS AND DISCUSSION

To test the three methods for the Dirichlet velocity boundary conditions, Poiseuille fl ows in Section 2 are solved with the aid of the LBM. The number Re in the test cases is equal to 5, 10, 25, and 50 to limit the compressible effects. After grid size independent test, 30 500× grids are employed for all the test cases.

An analytical solution exists in this problem once full development is reached (Latt and Chopard, 2008):

( )

2max

21 yu y uh

⎡ ⎤⎛ ⎞⎢ ⎥= − ⎜ ⎟⎝ ⎠⎢ ⎥⎣ ⎦

, (41)

where umax is the maximum velocity and then u is quadratic for the location y. The quantity px is the average pressure along y. It is linear relative to x according to the analytical solution. Assuming pΔ to be the pres-sure difference between px and pout, Δp is also linear relative to x.

FIG. 4: Difference model

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Each has same τ in Eq. (27) by adapting u0. The LBM simulation is carried on untill the steady-state con-dition is reached:

101max 10t t

t

u uu

−−−≤ . (42)

Numerical solutions are then compared with analytical results for validation. Figure 5 shows the compari-son of velocities between the LBM and analytical results for Re = 5. These two quadratic results agree with each other well. Figure 6 shows the comparison of pressure differences. They are all linear relative to x and agree with each other well. Therefore, numerical results with different boundary conditions agree with the analytical results well in this case.

Figures 7 and 8 show the velocity and pressure comparisons for Re = 10. The velocities are still quadratic relative to y and agree with the analytical results well. The maximum velocity umax increases from 0.005 to 0.01 as Re increases from 5 to 10. All three boundary conditions also lead to the same Δp results which are

FIG. 5: Velocity distribution, Re = 5

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FIG. 6: Pressure distribution, Re = 5

FIG. 7: Velocity distribution, Re = 10

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linear relative to x. The slope is also twice as that for the case of Re =5. Therefore, the LBM with different boundary conditions can lead to the results agreeing with analytical results well.

The test cases are also carried out for Reynolds numbers equal to 25 and 50. Figures 9–12 compare the velocity and pressure for these two cases. Their analytical results are also satisfi ed. Therefore, the LBM with different boundary conditions can lead to the results that agree with analytical results for all the test cases.

It is necessary to point out that these three boundary conditions all violate the local mass balance, which does not relate to mass balance directly. The parameter MB is employed to evaluate different boundary con-ditions' accuracies:

( ) ( )max

u vMB =

t x y∂ ρ ∂ ρ∂ρ

+ +∂ ∂ ∂

. (43)

It can be reached based on the macroscale parameters with the derivatives obtained from Eq. (36). This common convergence criterion in the CFD is not used widely in the LBM. This parameter grows with the velocity. To eliminate this effect, we use another error parameter:

maxerror MB u= . (44)

The error tendency with Re for different boundary conditions is shown in Fig. 13.Numerical methods fi t mass balance better with this error decreasing. It changes from 10–5 to 10–4 for the

BC1. On the other hand, BC2 and BC3 have the errors around 10–3. The BC1 error is less than the other boundary conditions' results by 10 times, at least. The BC1 results are much better than the other cases from the mass balance point of view.

6. CONCLUSIONS

Three boundary conditions (the Zou–He method, fi nite difference velocity gradient method, and the regu-larized method) are tested and compared for the Dirichlet velocity condition. Poiseuille fl ows with different

FIG. 8: Pressure distribution, Re = 10

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FIG. 9: Velocity distribution, Re = 25

FIG. 10: Pressure distribution, Re = 25

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FIG. 11: Velocity distribution, Re = 50

FIG. 12: Pressure distribution, Re = 50

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Reynolds numbers are solved for validation. The LBM velocity and pressure results with different boundary conditions agree with the analytical solutions well for all the test cases. Meanwhile, the LBM with the Zou–He boundary condition leads to the results fi tting mass balance best; therefore, the local mass balance has no relation with the macroscopic mass balance in the LBM boundary. The Zou–He boundary condition is supe-rior in the LBM for this kind of problem relating to the macroscopic mass balance.

ACKNOWLEDGMENTS

Support for this work by the U.S. National Science Foundation under grant number CBET-1404482 and by the Chinese National Natural Science Foundations under Grants 51129602 and 51276118 is gratefully ac-knowledged.

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