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Tutorial: Multi-Species Lattice Boltzmann Modelsand Applications
Pietro Asinari, PhD
Dipartimento di Energetica, Politecnico di Torino, Torino, Italy,e-mail: [email protected],
home page: http://staff.polito.it/pietro.asinari
19th Discrete Simulation of Fluid DynamicsCNR and University of Rome, "Tor Vergata", Rome, Italy, on July 5-9, 2010
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 1 / 51
[email protected]://staff.polito.it/pietro.asinari
Abstract of this tutorial
The aim of this tutorial is to discuss a numerical scheme based on theLattice Boltzmann Method (LBM) for gas mixture modeling, which fullyrecovers Maxwell-Stefan diffusion model in the continuum limit, withoutthe restriction of the mixture-averaged approximation [1]. Themixture-averaged approximation is used to express the molarconcentration gradient by means of a Fickian expression and a properdiffusion coefficient, both designed in such a way to be consistent withthe Maxwell-Stefan formulation only in some limiting cases.Unfortunately (a) this approximation yields to Fickian diffusioncoefficients depending on both the fluid transport properties and thelocal flow and (b) it is not valid in general. The key idea for avoidingthese problems is to start from a Bhatnagar-Gross-Krook-type kineticmodel for gas mixtures, recently proposed by Andries, Aoki andPerthame [2] (the so-called AAP model).
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 2 / 51
Outline of this tutorial
1 Kinetic theory of rarefied gas mixturesFull Boltzmann equationsExchange relation for momentum
2 Lattice Boltzmann solvers and applicationsSingle-relaxation-time Lattice Boltzmann schemeAndries-Aoki-Perthame (AAP) modelLBM formulation by variable transformation
3 Validation and simple applications
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 3 / 51
Kinetic theory of rarefied gas mixtures
Outline Compass
1 Kinetic theory of rarefied gas mixturesFull Boltzmann equationsExchange relation for momentum
2 Lattice Boltzmann solvers and applicationsSingle-relaxation-time Lattice Boltzmann schemeAndries-Aoki-Perthame (AAP) modelLBM formulation by variable transformation
3 Validation and simple applications
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 4 / 51
Kinetic theory of rarefied gas mixtures Full Boltzmann equations
Full Boltzmann equations for gas mixtures
The simultaneous Boltzmann equations for a mixture withoutexternal force can be written as [3, 4, 5, 6]:
∂tfσ + ξ ·∇fσ + aσ ·∇ξfσ = Qσ =∑ς
Qσς , (1)
where Qσς = Qςσ, ς 6= σ, is the cross collision term for twodifferent species σ (with mass mσ) and ς (with mass mς ).Obviously, for an N -component system, there will be N suchequations. In general, the collision term is
Qσς=̇
∫R3ξ
∫g·n
Kinetic theory of rarefied gas mixtures Full Boltzmann equations
Collision kernel and post–collision velocities
The weight Kσς(g,n) = Kςσ(g,n) is a volumetric particle flux orcollision kernel. In the following, we will discuss only the case ofMaxwell molecules [7], namely Kσς(|g · n|/|g|), where the collisionkernel is independent of the relative velocity.In the previous equation, the post–collision test and field particlevelocities ξ′ and ξ′∗ are given by
ξ′ = ξ +2µσςmσ
(g · n) n, (3)
ξ′∗ = ξ∗ −2µσςmς
(g · n) n, (4)
which means that there are many possible outcomes (ξ′, ξ′∗) froma given pair of incoming (test and field) particle velocities (ξ, ξ∗),depending on the impact direction n.The reduced mass µσς is defined as
µσς = mσmς/(mσ +mς). (5)
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 6 / 51
Kinetic theory of rarefied gas mixtures Full Boltzmann equations
Indifferentiability Principle and H–theorem
Indifferentiability Principle: when all the masses mσ andcross-sections Kσς are identical, the total distribution f =
∑σ fσ
obeys the single species Boltzmann equation.H–theorem: in the case of a mixture, the entropy inequality(H–theorem) is ∑
σ
∫R3ξQσ log(fσ)dξ ≤ 0. (6)
These properties of the full Boltzmann equations for mixtures willguide the design of BGK–type simplified models and consequentlyof the Lattice Boltzmann solvers.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 7 / 51
Kinetic theory of rarefied gas mixtures Exchange relation for momentum
Exchange relation for momentum (1 of 3)
The momentum equation is
∂t(ρσuσ) +∇ ·Πσ = ρσaσ +∫R3ξmσξQσdξ. (7)
Taking into account the symmetries of the elementary collisionyields (see for example Eq. (48) in the Ref. [8])∫
R3ξmσξQσdξ =
∑ς
〈∫g·n
Kinetic theory of rarefied gas mixtures Exchange relation for momentum
Exchange relation for momentum (2 of 3)
Let us express the surface element dn in Eq. (10) by using g aspolar axis, φ as polar angle and θ as azimuthal angle.Hence Kσς(|g · n|/|g|) = Kσς(cosφ) = Kσς(φ) and consequently∫
R3ξmσξQσdξ =
∑ς
2µσς
〈∫g·n
Kinetic theory of rarefied gas mixtures Exchange relation for momentum
Exchange relation for momentum (3 of 3)
Consequently
1
2
∫ 2π0
∫ π0Kσς(φ) |g| cosφ
00
cosφ
sinφdφdθ = χσς g. (12)where
χσς =1
2
∫ 4π0
Kσς(φ) (cosφ)2 dn. (13)
Coming back to Eq. (10) yields∫R3ξmσξQσdξ =
∑ς
2µσςχσς 〈g〉σς = p∑ς
Bσςyσyς(uς−uσ), (14)
where
Bσς =2µσςχσς n
2
p= Bςσ, (15)
where n is the total particle number density, p is the pressure andyσ is the molar concentration, as it will be discussed next.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 10 / 51
Lattice Boltzmann solvers and applications
Outline Compass
1 Kinetic theory of rarefied gas mixturesFull Boltzmann equationsExchange relation for momentum
2 Lattice Boltzmann solvers and applicationsSingle-relaxation-time Lattice Boltzmann schemeAndries-Aoki-Perthame (AAP) modelLBM formulation by variable transformation
3 Validation and simple applications
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 11 / 51
Lattice Boltzmann solvers and applications Andries-Aoki-Perthame (AAP) model
Simplified kinetic models
The full Boltzmann equations for gas mixtures are much moreformidable to analyze than the equation for a single-speciessystem [3, 4, 5, 6].A popular approach is to derive simplified model Boltzmannequations which are more manageable to solve. Numerous modelequations are influenced by Maxwell’s approach to solve theBoltzmann equation by using the properties of the Maxwellmolecule and the linearized Boltzmann equation [12].The simplest model equations for a binary mixture is that by Grossand Krook [13, 12], which is an extension of thesingle-relaxation-time model for a pure system — the celebratedBhatnagar-Gross-Krook (BGK) model.Many other models exist: Sirovich [14, 15], Hamel [16, 17], ...
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 12 / 51
Lattice Boltzmann solvers and applications Andries-Aoki-Perthame (AAP) model
Basic consistency constraints
1 The Indifferentiability Principle, which prescribes that, if aBGK-like equation for each species is assumed, this set ofequations should reduce to a single BGK-like equation, whenmechanically identical components are considered, i.e. the singlefluid description should be recovered [18].
2 The relaxation equations for momentum and temperature shouldbe as close as possible to those derived by means of the fullBoltzmann equations [19].
3 All the species should tend to a target equilibrium distributionwhich is a Maxwellian, centered on a proper macroscopic velocity,common to all the species.
4 The non-negativity of the distribution functions for all the speciesshould be satisfied.
5 A generalized H theorem for mixtures should hold.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 13 / 51
Lattice Boltzmann solvers and applications Andries-Aoki-Perthame (AAP) model
Simplified AAP model
Let us consider a simplified version of the AAP model, proposedby Andries, Aoki, and Perthame [2], which is based on only oneglobal (i.e., taking into account all the species ς) operator for eachspecies σ, namely
∂t̂fσ + ξ ·∇̂fσ = λσ[fσ(∗) − fσ
], (16)
where
fσ(∗) =ρσ
(2πϕσ/3)exp
[−3 (ξ − u
∗σ)
2
2ϕσ
], (17)
and
u∗σ = uσ +∑ς
m2
mσmς
BσςBmm
xς(uς − uσ). (18)
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 14 / 51
Lattice Boltzmann solvers and applications Andries-Aoki-Perthame (AAP) model
Properties of simplified AAP model
The target velocity can be easily recasted as
u∗σ = u+∑ς
(m2
mσmς
BσςBmm
− 1)xς(uς − uσ). (19)
If mσ = m for any σ, then (Property 1)
u∗σ = u+∑ς
(m2
mm
BmmBmm
− 1)xς(uς − uσ) = u. (20)
Clearly (Property 2)
∑σ
xσu∗σ = u+
∑σ
∑ς
(m2
mσmς
BσςBmm
− 1)xσxς(uς − uσ) = u.
(21)
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 15 / 51
Lattice Boltzmann solvers and applications Andries-Aoki-Perthame (AAP) model
Diffusive scaling
In the following asymptotic analysis [20], we introduce thedimensionless variables, defined by
xi = (lc/L) x̂i, t = (UTc/L) t̂. (22)
Defining the small parameter � as � = lc/L, which corresponds tothe Knudsen number, we have xi = � x̂i.Furthermore, assuming U/c = �, which is the key of derivation ofthe incompressible limit [20], we have t = �2 t̂. Then, AAP model isrewritten as
�2∂fσ∂t
+ � ξi∂fσ∂xi
= λσ[fσ(∗) − fσ
]. (23)
In this new scaling, we can assume ∂αfσ = ∂fσ/∂α = O(fσ) and∂αM = ∂M/∂α = O(M), where α = t, xi and M = ρσ, qσi whereqσi = ρσuσi.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 16 / 51
Lattice Boltzmann solvers and applications Andries-Aoki-Perthame (AAP) model
Regular Knudsen expansion
Clearly the solution of the BGK equation depends on �. Thesolution for small � is investigated in the form of the asymptoticregular expansion
fσ = f(0)σ + �f
(1)σ + �
2f (2)σ + · · · . (24)
ρ and qσi are also expanded:
ρσ = ρ(0)σ + �ρ
(1)σ + �
2ρ(2)σ + · · · , (25)
qσi = �q(1)σi + �
2q(2)σi + · · · , (26)
since the Mach number is O(�), the perturbations of qσi starts fromthe order of �. Consequently
fσ(∗) = f(0)σ(∗) + �f
(1)σ(∗) + �
2f(2)σ(∗) + · · · , (27)
Regular expansion means ∂αf(k)σ = O(1) and ∂αM (k) = O(1).
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 17 / 51
Lattice Boltzmann solvers and applications Andries-Aoki-Perthame (AAP) model
Asymptotic analysis of AAP model
Collecting the terms of the same order yields
f (k)σ = f(k)σ(∗) − g
(k)σ , (28)
g(0)σ = 0, (29)
g(1)σ = τσ∂Sf(0)σ(∗), (30)
g(2)σ = τσ[∂tf(0)σ(∗) + ∂Sf
(1)σ(∗) − τσ∂
2Sf
(0)σ(∗)], (31)
· · · ,
where ∂S = ξi∂/∂xi and τσ = 1/λσ.The previous coefficients of the regular expansion allows one toderive the macroscopic equations recovered by the AAP model.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 18 / 51
Lattice Boltzmann solvers and applications Andries-Aoki-Perthame (AAP) model
Tuning the single species relaxation frequency
Taking the first order moments of g(1)σ yields
λσρ(0)σ [u
∗(1)σ − u(1)σ ] = ∇p(0)σ , (32)
where p(k)σ = ϕσρ(k)σ /3.
If λσ is selected as λσ = pBmm/ρ, then the previous expressionbecomes
1/p(0)∇p(0)σ =∑ς
Bσς yσyς [u(1)ς − u(1)σ ], (33)
which clearly proves that the leading terms of the macroscopicequations recovered by means of the AAP model are consistentwith Maxwell-Stefan model.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 19 / 51
Lattice Boltzmann solvers and applications LBM formulation by variable transformation
D2Q9 lattice
Let us define the AAP model for a set of discrete velocities,
�2∂fσ∂t
+ �Vi∂fσ∂xi
= λσ[fσ(∗) − fσ
], (34)
where Vi is a list of i-th components of the velocities in theconsidered lattice and f = fσ(∗), fσ is a list of discrete distributionfunctions (change in the notation !!) corresponding to thevelocities in the considered lattice.Let us consider the two dimensional 9 velocity model, which iscalled D2Q9, namely
V1 =[
0 1 0 −1 0 1 −1 −1 1]T, (35)
V2 =[
0 0 1 0 −1 1 1 −1 −1]T. (36)
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 20 / 51
Lattice Boltzmann solvers and applications LBM formulation by variable transformation
Rule of computation for the list
The components of the molecular velocity V1 and V2 are the listswith 9 elements. Before proceeding to the definition of the localequilibrium function fσ(∗), we define the rule of computation for thelist.Let h and g be the lists defined by h = [h0, h1, h2, · · · , h8]T andg = [g0, g1, g2, · · · , g8]T . Then, hg is the list defined by[h0g0, h1g1, h2g2, · · · , h8g8]T . The sum of all the elements of the listh is denoted by 〈h〉, i.e. 〈h〉 =
∑8i=0 hi.
Then, the (dimensionless) density ρσ and momentum qσi = ρσuσiare defined by
ρσ = 〈fσ〉, qσi = 〈Vifσ〉. (37)
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 21 / 51
Lattice Boltzmann solvers and applications LBM formulation by variable transformation
Continuous equilibrium moments
Let us introduce the following function
fe(ρ, ϕ, u1, u2) =ρ
(2πϕ/3)exp
[−3 (ξ − u)
2
2ϕ
]. (38)
Let us define 〈〈·〉〉 =∫ +∞−∞ · dξ1dξ2 and the generic equilibrium
moment mpq = 〈〈fe ξp1ξq2〉〉.
All the equilibrium moments appearing in the Euler system ofequations are the following: m00, m10, m01, m20, m02, m11. Due tothe lattice deficiencies, only the equilibrium moments m21, m12must be considered in addition, for recovering the desiredNavier-Stokes system of equations. The moment m22 is arbitrarilyselected in order to complete the set of discrete moments.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 22 / 51
Lattice Boltzmann solvers and applications LBM formulation by variable transformation
Simplified continuous equilibrium moments
Collecting the previous results yields
m̄c(ρ, ϕ, u1, u2) = ρ [1, u1, u2,
u21 + ϕ/3, u22 + ϕ/3, u1u2,
u1 u22 + u1ϕ/3, u
21 u2 + u2ϕ/3,
ϕ (u21u22 + u
21ϕ/3 + u
22ϕ/3 + ϕ/9)]
T .
The previous analytical results involve high order terms (like u1 u22)which are not strictly required, in order to recover the macroscopicequations we are interested in.
mc(ρ, ϕ, u1, u2) = ρ [1, u1, u2,
u21 + ϕ/3, u22 + ϕ/3, u1u2,
u1/3, u2/3,
(u21 + u22)/3 + ϕ/9]
T
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 23 / 51
Lattice Boltzmann solvers and applications LBM formulation by variable transformation
Design of discrete local equilibrium
On the selected lattice, the discrete integrals mσ(∗), correspondingto the previous continuous ones, can be computed by means ofsimple linear combinations of the discrete equilibrium distributionfunction fσ(∗) (still unknown), namely mσ(∗) = Mfσ(∗) where M isa matrix defined as
M = [1, V1, V2, V21 , V
22 , V1V2, V1V
22 , V
21 V2, V
21 V
22 ]T . (39)
We design the discrete local equilibrium such asmσ(∗) = mc(ρσ, ϕσ, u
∗σ1, u
∗σ2), or equivalently
fσ(∗) = M−1mc(ρσ, ϕσ, u
∗σ1, u
∗σ2). In particular the latter provides
the operative formula for defining the local equilibrium andconsequently the scheme.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 24 / 51
Lattice Boltzmann solvers and applications LBM formulation by variable transformation
Discrete operative formula
Eq. (34) is formulated for discrete velocities, but it is stillcontinuous in both space and time.Since the streaming velocities are constant, the Method ofCharacteristics is the most convenient way to discretize space andtime and to recover the simplest formulation of the LBM scheme.Applying the second-order Crank–Nicolson yields
f+σ = fσ + (1− θ)λσ[fσ(∗) − fσ
]+ θ λ+σ
[f+σ(∗) − f
+σ
], (40)
where θ = 1/2.The previous formula would force one to consider quitecomplicated integration procedures [21]. A simple variabletransformation has been already proposed in order to simplify thistask [22].
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 25 / 51
Lattice Boltzmann solvers and applications LBM formulation by variable transformation
Variable transformation
(Step 1) Let us apply the transformation fσ → gσ defined by
gσ = fσ − θ λσ[fσ(∗) − fσ
]. (41)
(Step 2) Let us compute the collision and streaming step leadingto gσ → g+σ by means of the modified updating equation
g+σ = gσ + λ′σ
[fσ(∗) − gσ
], (42)
where λ′σ = λσ/(1 + θλσ).(Step 3) Finally let us come back to the original discretedistribution function g+σ → f+σ by means of
f+σ =g+σ + θ λ
+σ f
+σ(∗)
1 + θ λ+σ. (43)
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 26 / 51
Lattice Boltzmann solvers and applications LBM formulation by variable transformation
Problem for mixtures
In case of mixtures, the problem arises from the (Step 3), whichrequires both λ+σ and f
+σ(∗), depending on the updated
hydrodynamic moments at the new time step.Since the single component density is conserved, Eq. (41) yields
ρ+σ = 〈g+σ 〉, (44)
consequently it is possible to compute p+σ , ρ+, p+ and λ+σ .However this is not the case for the single component momentum,because this is not a conserved quantity and hence the first ordermoments for g+σ and f+σ differ [23], namely
〈Vi g+σ 〉 = ρ+σ u+σi − θ λ+σ ρ
+σ (u
∗+σi − u
+σi) =
= ρ+σ u+σi − θ p
+∑ς
Bσς y+σ y
+ς (u
+ςi − u
+σi). (45)
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 27 / 51
Lattice Boltzmann solvers and applications LBM formulation by variable transformation
Solution: solving locally a linear system of equations
In the general case, Eq. (45) can be recasted as
〈Vi g+σ 〉 = q+σi − θ λ+σ
∑ς
χσς (x+σ q
+ςi − x
+ς q
+σi), (46)
where q+σi = ρ+σ u
+σi and
χσς =m2
mσmς
BσςBmm
. (47)
Finally, grouping together common terms yields
〈Vi g+σ 〉 =
[1 + θ λ+σ
∑ς
(χσς x+ς )
]q+σi − θ λ
+σ x
+σ
∑ς
(χσς q+ςi). (48)
Clearly the previous expression defines a linear system ofalgebraic equations for the unknowns q+σi.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 28 / 51
Validation and simple applications
Outline Compass
1 Kinetic theory of rarefied gas mixturesFull Boltzmann equationsExchange relation for momentum
2 Lattice Boltzmann solvers and applicationsSingle-relaxation-time Lattice Boltzmann schemeAndries-Aoki-Perthame (AAP) modelLBM formulation by variable transformation
3 Validation and simple applications
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 29 / 51
Validation and simple applications
Single-relaxation-time MixLBM code
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 30 / 51
Validation and simple applications
Basic algorithm
The proposed numerical code is formulated not in the standardway (and it is quite inefficient from the computational point ofview).Even though it is not an efficient implementation, the proposedformulation is much more similar to any other explicit finitedifference (FD) scheme.This offers some advantages:
1 it makes easier to implement hybrid schemes, i.e. to mix up kineticand conventional schemes on the same discretization;
2 it makes easier to compare the LBM scheme with other FDschemes, mainly in terms of updating rule;
3 it makes easier to implement simple boundary conditions, based onthe concept of local equilibrium.
Anyway the basic sequence of collision and streaming step ispreserved.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 31 / 51
Validation and simple applications
Schematic view
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 32 / 51
Validation and simple applications
Main loop
fd(1:nx,1:ny,0:8,1:species)
do t = 1,nt,1do i = 1,nx,1
do j = 1,ny,1call UpdateLatticeData(...,f(:,:));do s = 1,species,1
fd_new(i,j,:,s) = f(:,s);call HydrodynamicMoments(...);
enddoenddo
enddofd(:,:,:,:) = fd_new(:,:,:,:);
enddo
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 33 / 51
Validation and simple applications
UpdateLatticeData loop
do k=0,8,1iI = i + Incr(k,1); jI = j + Incr(k,2);do s=1,species,1
! BCs rs,uxs,uys in I(i+,j+)enddodo s=1,species,1call EquilibriumDistribution(...,feq(:))lambda(s)=...;do ik=0,8,1fc(ik)=f(ik,s)+lambda(s)*(feq(ik)-f(ik,s));enddof_new(BB(k),s) = fc(BB(k));
enddoenddo
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 34 / 51
Validation and simple applications
UpdateLatticeData loop with variable transformation
do k=0,8,1do s=1,species,1call EquilibriumDistribution(...,feq(:))lambda(s)=...;TRANSFORMATION f(:,s) -> g(:,s)do ik=0,8,1gc(ik)=g(ik,s)+lambda’(s)*(feq(ik)-g(ik,s));enddog_new(BB(k),s) = gc(BB(k));
enddoenddoBACK-TRANSFORMATION g_new(:,s) -> f_new(:,s)COMPUTE CONSERVED MOMENTSSOLVE LINEAR SYSTEM FOR NON-CONSERVED MOMENTS
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 35 / 51
Validation and simple applications
Ternary mixture
In case of ternary mixture Eq. (14) reduces to
n∇y1 = B12y1k2 +B13y1k3 − (B12y2 +B13y3)k1, (49)
n∇y2 = B21y2k1 +B23y2k3 − (B21y1 +B23y3)k2, (50)
n∇y3 = B31y3k1 +B32y3k2 − (B31y1 +B32y2)k3. (51)
The molecular weights are mσ = [1, 2, 3], the internal energiesare [eσ = 1/3, 1/6, 1/9] and consequently ϕσ = [1, 1/2, 1/3].The theoretical Fick diffusion coefficient is Dσ = α/mσ, whereα ∈ [0.002, 0.8] and the theoretical Maxwell–Stefan diffusionresistance is given by [28]
Bσς = β
(1
mσ+
1
mς
)−1/2, β ∈ [5, 166]. (52)
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 36 / 51
Validation and simple applications
Solvent test case
A component of a mixture is called solvent if its concentration ispredominant in comparison with the other components of themixture [10].Let us suppose that, in our ternary mixture, the component 3 is asolvent. In particular, the initial conditions for the solvent test caseare given by
p1(0, x) = ∆p
[1 + tanh
(x− L/2δx
)]+ ps, (53)
p2(0, x) = ∆p
[1− tanh
(x− L/2δx
)]+ ps, (54)
p3(0, x) = 1− 2 (∆p+ ps), (55)
where clearly p(0, x) =∑
σ pσ = 1 and ∆p = ps = 0.01.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 37 / 51
Validation and simple applications
Solvent test case: simplified transport coefficients
Hence y3 ∼= 1 and consequently y1 ∼= 0 and y2 ∼= 0. Under theseassumptions, Eqs. (49, 50) reduce to
∇y1 = −B13y1(u1 − v) = B13y1(v − u1), (56)
∇y2 = −B23y2(u2 − v) = B23y2(v − u2), (57)
Consequently the measured diffusion resistances are given by
B∗13 =1
D∗1=
∂y1/∂x
y1(v − u1), (58)
B∗23 =1
D∗2=
∂y2/∂x
y2(v − u2), (59)
where, in this test, the Maxwell–Stefan model reduces to the Fickmodel.
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Validation and simple applications
Solvent test case: Fick model
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Validation and simple applications
Solvent test case: Maxwell–Stefan model
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 40 / 51
Validation and simple applications
Dilute test case
A component of a mixture is said dilute if its concentration isnegligible in comparison with the other components of the mixture[10].Let us suppose that, in our ternary mixture, the component 1 isdilute. In particular, the initial conditions for the dilute test case aregiven by
p1(0, x) = ∆p
[1 + tanh
(x− L/2δx
)]+ ps, (60)
p2(0, x) = ∆p
[1− tanh
(x− L/2δx
)]+ ps +
+(1− r) (1− 2 ∆p), (61)p3(0, x) = r (1− 2 ∆p)− 2 ps, (62)
where p(0, x) =∑
σ pσ = 1, ∆p = ps = 0.01, r = 1/2.
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Validation and simple applications
Dilute test case: Maxwell–Stefan model
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 42 / 51
Validation and simple applications
Non-Fickian test case: Stefan tube
It is essentially a vertical tube, open at one end, where the carrierflow licks orthogonally the tube opening [10]. In the bottom of thetube is a pool of quiescent liquid. The vapor that evaporates fromthis pool diffuses to the top.
p1(0, x) = p1(0, 0)1
2
[1− tanh
(x− L/2δx
)]+ ps, (63)
p2(0, x) = p2(0, 0)1
2
[1− tanh
(x− L/2δx
)]+ ps, (64)
p3(0, x) = [1− p3(0, 0)]1
2
[1 + tanh
(x− L/2δx
)]+ p3(0, 0),
(65)
where the constant ps = 10−4 has been introduced for avoiding todivide per zero.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 43 / 51
Validation and simple applications
Stefan tube
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 44 / 51
Conclusions
In the present tutorial, a LBM scheme for gas mixture modeling,which fully recovers Maxwell–Stefan diffusion model in thecontinuum limit, without the restriction of the macroscopicmixture-averaged approximation, was discussed.As a theoretical basis for the development of the LBM scheme, arecently proposed BGK-type kinetic model for gas mixtures [2]was considered. This essentially ties the LBM development to therecent progresses of the BGK-type kinetic models and opens newperspectives.In the reported numerical tests, the proposed scheme producesgood results on a wide range of relaxation frequencies.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 45 / 51
References I
[1] F. Williams.Combustion Theory.Benjamin/Cumming, California, 1986.
[2] P. Andries, K. Aoki, and B. Perthame.A consistent BGK-type model for gas mixtures.J. Stat. Phys., 106(5/6):993–1018, 2002.
[3] S. Chapman and T. G. Cowling.The Mathematical Theory of Non-Uniform Gases.Cambridge University Press, Cambridge, UK, 3rd edition, 1970.
[4] J. H. Ferziger and H. G. Kaper.Mathematical Theory of Transport in Gases.North Holland, Amsterdam, 1972.
[5] L. C. Woods.An Introduction to the Kinetic Theory of Gases and Magnetoplasmas.Oxford University Press, Oxford, UK, 3 edition, 1993.
[6] S. Harris.An Introduction to the Theory of the Boltzmann Equation.Dover, Mineola, NY, 2004.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 46 / 51
References II
[7] C. CercignaniThe Boltzmann Equation and Its Applications.Applied Mathematical Sciences, Springer-Verlag, 1987.
[8] P. AsinariNonlinear Boltzmann equation for the homogeneous isotropic case: Minimal deterministicMatlab program.accepted for publication in Computer Physics Communications.available on line arXiv:1004.3491v1 [physics.comp-ph].
[9] C.E. Brennen.Fundamentals of Multiphase Flow.Cambridge University Press, United Kingdom, 2005.
[10] R. Taylor and R. Krishna.Multicomponent Mass Transfer.John Wiley & Sons, New York, 1993.
[11] R. Krishna and J.A. Wesselingh.Maxwell-stefan approach to mass transfer.Chemical Engineering Science, 52(6):861–911, 1997.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 47 / 51
References III
[12] E. P. Gross and E. A. Jackson.Kinetic models and the linearized Boltzmann equation.Phys. Fluids, 2(4):432–441, 1959.
[13] E. P. Gross and M. Krook.Models for collision processes in gases: Small-amplitude oscillations of chargedtwo-component systems.Phys. Rev., 102:593–604, 1956.
[14] L. Sirovich.Kinetic modelling of gas mixtures.Phys. Fluids, 5:908–918, 1962.
[15] L. Sirovich.Mixtures of Maxwell molecules.Phys. Fluids, 9:2323–2326, 1966.
[16] B. B. Hamel.Kinetic models for binary mixtures.Phys. Fluids, 8:418–425, 1965.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 48 / 51
References IV
[17] B. B. Hamel.Two-fluid hydrodynamic equations for a neutral, disparate-mass, binary mixtures.Phys. Fluids, 9:12–22, 1966.
[18] V. Garzò, A. dos Santos, and J. J. Brey.A kinetic model for a multicomponent gas.Phys. Fluids A, 1(2):380–383, 1989.
[19] T. F. Morse.Kinetic model equations for a gas mixture.Phys. Fluids, 7:2012–2013, 1964.
[20] Y. Sone.Kinetic Theory and Fluid Dynamics.Birkhäuser, Boston, 2nd edition, 2002.
[21] P. Asinari.Semi-implicit-linearized multiple-relaxation-time formulation of lattice Boltzmann schemesfor mixture modeling.Phys. Rev. E, 73(5):056705, 2006.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 49 / 51
References V
[22] He X., Chen S., and Doolen G. D. J.A novel thermal model for the lattice boltzmann method in incompressible limit.J. Computat. Phys., 146:282, 1998.
[23] S. Arcidiacono, I. V. Karlin, J. Mantzaras, and C. E. Frouzakis.Lattice boltzmann model for the simulation of the multicomponent mixtures.Phys. Rev. E, 76:046703–1—046703–11, 2007.
[24] P. Asinari.Multiple-relaxation-time lattice boltzmann scheme for homogeneous mixture flows withexternal force.Phys. Rev. E, 77:056706, 2008.
[25] P. Asinari.Lattice Boltzmann scheme for mixture modeling: Analysis of the continuum diffusionregimes recovering Maxwell-Stefan model and incompressible Navier-Stokes equations.Phys. Rev. E, 80:056701, 2009.
[26] H. Grad.Theory of rarefied gases.In F. Devienne, editor, Rarefied Gas Dynamics, pages 100–138. Pergamon, London, 1960.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 50 / 51
References VI
[27] P. Asinari and T. Ohwada.Connection between kinetic methods for fluid-dynamic equations and macroscopicfinite-difference schemes.Comput. Math. Appl., 58:841, 2009.
[28] W.E. Stewart R.B. Bird and E.N. Lightfoot.Transport Phenomena.John Wiley & Sons, New York, 1960.
Pietro Asinari, PhD (Politecnico di Torino) Multi-Species LB Models Rome, Italy, on July 5-9, 2010 51 / 51
Kinetic theory of rarefied gas mixturesFull Boltzmann equationsExchange relation for momentum
Lattice Boltzmann solvers and applicationsSingle-relaxation-time Lattice Boltzmann schemeAndries-Aoki-Perthame (AAP) modelLBM formulation by variable transformation
Validation and simple applicationsAppendix