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Optimal control and homogenization in a mixture of fluids separated by a rapidly oscillating

interface

Hakima Zoubairi

Abstract

We study the limiting behaviour of the solution to optimal control problems in a mathematical mixture of two homogeneous viscous fluids.

These fluids are separated by a rapidly oscillating periodic interface with constant amplitude. We show that the limit of the optimal control is the optimal control for the limiting problem

1 Introduction

The aim of this paper is to study the optimal control problem in a mixture of fluids. More precisely, we consider a mixture of two viscous, homogeneous and incompressible fluids occupying sub-domains of a bounded domain Ω ⊂ Rn+1 (n = 1 or 2). These fluids are separated by a given interface whose form is determined using a rapidly oscillating function of period ε > 0 and constant amplitudeh1>0.

We assume that the velocity and pressure of both fluids satisfy the Stokes equations. On the interface, we assume that the velocity is continuous and that the normal forces that the fluids exert each other are equal in magnitude and opposite in direction (hence, surface tension effects are neglected).

We associate an optimal control problem to these equations and our aim is to study the limiting behaviour of the solutions when the oscillating period ε tends to 0. To do so, we use some homogenization tools (see Bensoussan-Lions- Papanicolaou [4] and Sanchez-Palencia [15]) and Murat’s compactness result [11].

This work is based on the mathematical framework of Baffico & Conca [3]

for the Stokes problem, of Brizzi [5] for the transmission problem, and of Baffico

& Conca [1, 2] for the transmission problem in elasticity.

The plan of this paper is as follows. In Section 2, we present the domain with the rapidly oscillating interface, the Stokes problem posed in this domain

Mathematics Subject Classifications: 35B27, 35B37, 49J20, 76D07.

Key words: Optimal control, homogenization, Stokes equations.

2002 Southwest Texas State University.c

Submitted January 23, 2002. Published March 5, 2002.

1

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and the definition of the associated optimal control problem. In Section 3, we introduce the adjoint problem and present related convergence results. In Section 4, we prove the results announced in the previous section. In Section 5, we sketch the proof of the convergence results concerning the optimal control.

In Section 6, we present the case Ω⊂R2.

2 Setting of the problem

For n= 1 or 2, letY =]0,1[n and Ω =]0, Le i[n with Li >0,i = 1, . . . , n. Let h: ¯Y →R, h≥0 be a smooth function such that

i) h|∂Y =h1 whereh1= max{h(y) :y∈Y¯} andh1>0.

ii) Existsy0∈Y such that h(y0) = 0 and∇yh(y0) = 0.

Forzo∈R+, define Ω⊂Rn+1 by Ω =Ωe×]−z0, z0[ and Γ the boundary Ω by Γ =Ωe× {−z0} ∪∂Ωe×]−z0, z0[∪Ωe× {z0}.

To define the reference cell, we introduce the sub-domains:

11={(y, z)∈Y ×R:h(y)< z < z0} Ω12={(y, z)∈Y ×R:−z0< z < h(y)}, which are separated by the interface

Γ1={(y, z)∈Y ×R:h(y) =z}.

So that, we have the decomposition of the reference cell Λ (as in figure 2) Λ = Ω11∪Γ1∪Ω12= (Y×]−z0, z0[)

When we intersect Λ with the hyperplane {Z = z}(0 < z < h1) we obtain Y × {z} and the following decomposition forY (as in figure 2)

Y =Y?(z)∪γ(z)∪O(z), where

Y?(z) ={y∈Y :h(y)> z}, O(z) ={y∈Y :h(y)< z}, γ(z) ={y∈Y :h(y) =z}.

Letε >0 be a small positive parameter. We extendhbyY-periodicity toRn, we restrict this function toΩ (this function is still denoted bye h). Let

hε(x) =h(x

ε) x∈Ω.e Now we introduce

ε1={(x, z)∈Ωe×R:hε(x)< z < z0}

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Figure 1: The reference cell Λ Ωε2={(x, z)∈Ωe×R:−z0< z < hε(x)} and the rapidly oscillating interface is therefore defined by

Γε={(x, z)∈Ωe×R:hε(x) =z}.

So that, we obtain the following decomposition of Ω (see figure 2):

Ω = Ωε1∪Γε∪Ωε2.

Finally, as in figure 2, we set Ω1=Ωe×]h1, z0[, Ωm=Ωe×]0, h1[, Ω2=Ωe×]−z0,0[.

We notice that Ω = Ω1∪Ωm∪Ω2.

The Stokes Problem

Let the viscosity of the problem defined by µε1χε12χε2,

where µ1, µ2 >0, µ16=µ2, andχεi correspond to the characteristic functions of Ωεi (i= 1,2).

We denote by~v = (v, vn+1), a vector of Rn+1. Throughout this paper, C denotes various real positive constants independent ofε. We also denote by| · | the n-dimensional Lebesgue measure and by (ek)1kn the canonical basis of Rn (yk is thek-th component ofy∈Rn in this basis).

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Figure 2: Decomposition ofY whenn= 1 (left) and whenn= 2 (rigut)

Figure 3: Rapidly oscillating interface (left) and its homogenized version (right)

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We define the optimal control as follows. LetUad⊂L2(Ω)n+1 be a closed non empty convex subset. LetN >0 be a given constant. For~θ∈ Uadthe state equation is given by the Stokes problem

−div(2µεe(~uε)) =f~ε− ∇pε+θ~ in Ω div~uε= 0 in Ω

~

uε= 0 on∂Ω.

(2.1)

where (~uε, pε) are respectively the velocity and the pressure of the fluid,~θis the control and f~ε is a density of external forces defined by f~ε =f~1χε1+f~2χε2

withf~i∈L2(Ω)n+1(i= 1,2). The rate-of-strain tensore(~uε) is e(~uε) = 1

2 ∇~uε+t∇~uε . The cost function is

Jε(~θ) = 1 2 Z

e(~uε) :e(~uε)dx+N 2

Z

|~θ|2dx. (2.2) The optimal control~θε?is the function inUadwhich minimizesJε(~θ) for~θ∈ Uad, in other words

Jε(~θε?) = min

~θ∈Uad

Jε(~θ). (2.3)

This problem is standard and admits an unique optimal solution~θ?ε∈ Uad (see Lions [10]). Our aim is to study the limiting behaviour of ~θ?ε as ε → 0. In particular, it can be shown that (for a subsequence)

?ε* ~θ? weakly inL2(Ω)n+1.

Our objective is to characterizeθ~?as the optimal control of a similar problem with limiting tensorsAandBand to identify these tensors. The homogenization of the problem (2.1) is thanks to Baffico & Conca [3]. About the optimal control, Saint Jean Paulin & Zoubairi [12] studied the problem of a mixture of two fluids periodically distributed one in the other. Also this type of problem has been studied by Kesavan & Vanninathan [9] in the periodic case for a problem which the state equation is a second order elliptic problem with rapidly oscillating coefficients and by Kesavan & Saint Jean Paulin [7] and [8] in the general case (with H-convergence).

In this paper, we adapt these methods to the Stokes problem following the technique used by Baffico & Conca [2] and [3], by Kesavan & Saint Jean Paulin [7] and [8], and by Saint Jean Paulin & Zoubairi [12].

Definition We defineL20(Ω) by L20(Ω) =

g∈L2(Ω) : Z

g(y)dy= 0 .

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The problem (2.1)–(2.3) can be reduced to a system of equations by introducing the adjoint state (~vε, p0ε)∈H1(Ω)n+1×L20(Ω). Thus we get

−div(2µεe(~uε)) =f~ε− ∇pε+~θ in Ω div(2µεe(~vε)−e(~uε)) =−∇p0ε in Ω

div~uε= div~vε= 0 in Ω

~

uε=~vε= 0 on∂Ω,

(2.4)

the optimal control~θε? is characterized by the variationnal inequality

ε?∈ Uad and Z

(~vε+N ~θε?).(~θ−~θ?ε)dx≥0∀~θ∈ Uad.

3 Convergence results

The homogenized adjoint problem

Letµ=µ(y, z) (wherey∈Y andz∈]0, h1[), be the variable viscosity given by µ(y, z) =µ1χO(z)(y) +µ2χY?(z)(y).

Let us introduce some functions as solution of the Stokes problem defined onY. These functions, introduced by Baffico & Conca [3], are associated to the state equation.

Let 1≤k, l≤n, and let (χkl, rkl1) be the solution of

−div

y (2µey(−χkl+Pkl)) =−∇yrkl1 in Y divy χkl= 0 in Y

χkl, r1kl Y-periodic,

(3.1)

wherePkl= 12 ykel+ylek

. We defineMklthen×nmatrix byMkl=ey(Pkl).

We notice that [Mkl]ij = 12ikδjlilδjk) for all 1 ≤ i, j, k, l ≤ n. We also define then+ 1×n+ 1 matrix

[Ekl]ij =1

2(δikδjlilδjk) ∀1≤i, j, k, l≤n+ 1.

For eachz∈]0, h1[, problem (3.1) admits an unique solution in H]1(Y)n/R

× L20(Y) (see Sanchez-Palencia [15] or Baffico & Conca [6]).

Now, we consider the periodic problem

−div

y (µ∇y(−ϕk+ 2yk)) = 0 in Y ϕk Y-periodic.

(3.2) For eachz∈]0, h1[ fixed, problem (3.2) has an unique solution inH1(Y) up to an additive constant.

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LetA(z) be the fourth-order tensor whose coefficients are defined by

aijkl=





1[Ekl]ij h1< z < z0 agijkl 0< z < h1

2[Ekl]ij −z0< z < h1

1≤i, j, k, l≤n+ 1 (3.3)

with

agijkl =

























 R

Y

ey(−χkl+Pkl)

ijdy 1≤i, j, k, l≤n

1 2

R

Y µ∂(ϕ∂yk+2yk)

i dy 1≤i, k≤n j, l=n+ 1

1 2

R

Y µ∂(ϕ∂yl+2yl)

i dy 1≤i, l≤n j, k=n+ 1

1 2

R

Y µ∂(ϕ∂yk+2yk)

j dy 1≤j, k≤n i, l=n+ 1

1 2

R

Y µ∂(ϕ∂yl+2yl)

j dy 1≤j, l≤n i, k=n+ 1

1 2

R

Y 2µdy i, j, k, l=n+ 1

0 otherwise.

(3.4)

This tensor (introduced by Baffico & Conca [3]) is the homogenized tensor associated to the state equation. By the same way, we introduce other test functions which will be associated to the adjoint state.

Let (ψkl, r2kl) be the solution of

−div

y (2µeykl) +ey(−χkl+Pkl)) =−∇yrkl2 inY divy ψkl= 0 inY

ψkl, r2kl Y-periodic,

(3.5)

For each z ∈]0, h1[, problem (3.5) has an unique solution in H]1(Y)n/R

× L20(Y).

As we did for the problem (3.2), we introduce the scalar problem

−div

y (2µ∇yψk+∇y(−ϕk+ 2yk)) = 0 in Y ψk Y-periodic.

(3.6)

Forz∈]0, h1[ fixed, this problem admits an unique solution inH1(Y) up to an additive constant.

LetB(z) be the fourth order tensor which coefficients are given by

bijkl=





[Ekl]ij h1< z < z0

bgijkl 0< z < h1 [Ekl]ij −z0< z < h1

1≤i, j, k, l≤n+ 1 (3.7)

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with

bgijkl=

























 R

Y 2µ[eykl)]ij+ [ey(−χkl+Pkl)]ij

dy 1≤i, j, k, l≤n

1 4

R

Y∂ψ∂yk

i +∂(ϕ∂yk+2yk)

i

dy 1≤i, k≤n j, l=n+ 1

1 4

R

Y∂ψ∂yl

i +∂(ϕ∂yl+2yl)

i

dy 1≤i, l≤n j, k=n+ 1

1 4

R

Y∂ψ∂yk

j +∂(ϕ∂yk+2yk)

j

dy 1≤j, k≤n i, l=n+ 1

1 4

R

Y∂ψ∂yl

j +∂(ϕ∂yl+2yl)

j

dy 1≤j, l≤n i, k=n+ 1

1 i, j, k, l=n+ 1

0 otherwise.

(3.8) Now we give a result concerning some properties of tensor A.

Proposition 3.1 (Baffico & Conca [3]) The coefficients of A in (3.3) sat- isfy:

a)aijkl(z) =aklij(z) =aijlk(z) ∀1≤i, j, k, l≤n+ 1, ∀z∈]−z0, z0[ b) there existsα >0 such that for all ξ,n+ 1×n+ 1 symmetric matrix,

A(z)ξ:ξ≥αξ:ξ ∀z∈]−z0, z0[.

Now we give a result concerning some symmetry and ellipticity properties of the tensorB.

Proposition 3.2 The coefficients of B(voir(3.7)) are such that:

a)bijkl(z) =bklij(z) =bijlk(z)for1≤i, j, k, l≤n+ 1, for allz∈]−z0, z0[ b) There existsβ >0 such that for all ξ, the n+ 1×n+ 1 symmetric matrix,

B(z)ξ:ξ≥βξ:ξ ∀z∈]−z0, z0[.

Proof. Throughout this proof, we adopt the convention of summation over repeated indices. To prove a), we first study the coefficients of tensorB with indexes 1 ≤ i, j, k, l ≤n. The symmetry of these coefficients is evident when z∈]h1, z0[ and z∈]−z0, h1[.

Let study the case wherez∈]0, h1[. In this case (cf (3.8)) bijkl=bgijkl =

Z

Y

eykl)

ij+

ey(−χkl+Pkl)

ij

dy. (3.9) Following the ideas in [7, 12, 14], we transform the above expression to obtain a symmetric form. Let (Ykl, rkl3) be the solution of

−div

y (2µey(−Ykl+Pkl)) =−∇yr3kl inY divy Ykl= 0 inY

Ykl, rkl1 Y-periodic,

(3.10)

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IntroducingYklthe solution of the previous local problem, the coefficients (3.9) can be rewritten as

bgijkl= Z

Y

ey(−Ykl+Pkl)

ijdy+ Z

Y

eykl)

ij

eykl−Ykl)

ij

dy.

(3.11) The first term of the second integral of the right-hand side of this equation is evaluated as follows (using the fact that [eykl)

ij= [eykl)

ji) Z

Y

eykl)

ijdy= Z

Y

eykl)

βmδβiδmjdy

= Z

Y

µ

eykl)

βmβiδmjβjδmi)dy

= Z

Y

eykl)

βm

ey(Pij)

βmdy.

Using successively (3.1),(3.5) and (3.10), we have Z

Y

eykl)

βm

ey(Pij)

βmdy= Z

Y

eykl)

βm

eyij)

βmdy

= Z

Y

eykl−Pkl)

βm

eyij)

βmdy

= Z

Y

eykl−Ykl)

βm

eyij)

βmdy.

Moreover using (3.10), we can rewrite the last integral as Z

Y

eykl−Ykl)

βm

eyij)

βmdy

= Z

Y

eykl−Ykl)

βm

eyij−Yij)

βmdy +

Z

Y

eykl−Ykl)

βm

ey(Yij)

βmdy

= Z

Y

eykl−Ykl)

βm

eyij−Yij)

βmdy +

Z

Y

eykl−Ykl)

βm

ey(Pij)

βmdy

= Z

Y

eykl−Ykl)

βm

eyij−Yij)

βmdy+ Z

Y

eykl−Ykl)

ijdy Thus the second integral of the right-hand side of (3.11) can be rewritten as

Z

Y

eykl)

ij

eykl−Ykl)

ij

dy

= Z

Y

eykl−Ykl)

βm

eyij−Yij)

βmdy.

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Let us now consider the first integral in (3.11). Multiplying the first equation of (3.10) byYij and integrating by parts we have

Z

Y

ey(−Ykl+Pkl)

βm

ey(Yij)

βmdy= 0.

Using the fact that

ey(Pij)

βm=12 δβiδmjβjδmi

, we obtain Z

Y

ey(−Ykl+Pkl)

βm

ey(−Yij+Pij)

βmdy

= Z

Y

ey(−Ykl+Pkl)

βm

ey(Pij)

βmdy

= Z

Y

ey(−Ykl+Pkl)

ijdy.

Then using definition (3.11), we derive bgijkl =

Z

Y

ey(−Ykl+Pkl) :ey(−Yij+Pij)dy+

Z

Y

eykl−Ykl) :eyij−Yij)dy.

(3.12) It is immediate from the above form that the coefficients ofBsatisfybgijkl=bgklij. On the other hand, sinceey(Pkl) =ey(Plk) then by uniqueness of problem (3.1), we haveχkllk (up to an additive constant) and thenbijkl=bijlk.

We now study the coefficientsbijklwithi=k=n+1 and 1≤j, l≤n. From the definition of B (cf (3.7)), these coefficients are symmetric whenz ∈]h1, z0[ and z ∈]−z0, h1[. To prove the symmetry when z ∈]0, h1[, we proceed as we did before. These coefficients are as follows

bn+1jn+1l^ =1 4

Z

Y

∂(−ϕl+ 2yl)

∂yj

+ 2µ∂ψl

∂yj

dy. (3.13)

Letτk be the solution of

−∆y(−τk+ 2yk) = 0 inY

τk Y-p´eriodic. (3.14)

The expression (3.13) can be rewritten as follows (usingτl):

bn+1jn+1l^ =1 4

Z

Y

∂(−τl+ 2yl)

∂yj

dy+1 4

Z

Y

2µ∂ψl

∂yj −∂(ϕl−τl)

∂yj

dy. (3.15) Using exactly the same technique used above, we obtain (using (3.2), (3.6) and (3.14))

Z

Y

2µ∂ψl

∂yj

=1 2

Z

Y

∂(ϕl−τl)

∂yk

∂(ϕj−τj)

∂yk

dy+ Z

Y

∂(ϕl−τl)

∂yj

dy.

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Therefore, Z

Y

2µ∂ψl

∂yj −∂(ϕl−τl)

∂yj

dy=1 2

Z

Y

∂(ϕl−τl)

∂yk

∂(ϕj−τj)

∂yk

dy.

Concerning the first integral in (3.15), we consider τl the solution of (3.14):

multiplying the first equation byτj and integrating by parts, we have Z

Y

∂(−τl+ 2yl)

∂yk

∂τj

∂ykdy= 0, so we derive

Z

Y

∂(−τl+ 2yl)

∂yk

∂(−τj+ 2yj)

∂yk

dy= 2 Z

Y

∂(−τl+ 2yl)

∂yj

dy.

Finally, we obtain the following expression bn+1jn+1l^ = 1

8 Z

Y

∇(−τl+ 2yl).∇(−τj+ 2yj)dy+1 8

Z

Y

∇(ϕl−τl).∇(ϕj−τj)dy.

(3.16) From the form of (3.16), it is evident thatbn+1jn+1l^ =bn+1ln+1j^ . By construc- tion ofB, we also havebn+1ln+1j^ =bn+1ljn+1^ . For the other nonzero terms, the same method can be used to obtain

bin+1kn+1^ =bkn+1in+1^ =bin+1n+1k^ bin+1n+1l^ =bin+1ln+1^ =bn+1lin+1^ bn+1jkn+1^ =bn+1jn+1k^ =bkn+1n+1j^ .

To prove part b), we first notice that the coerciveness ofBwhenz∈]h1, z0[ andz∈]−z0, h1[ is evident. Whenz∈]0, h1[, from the form ofbgijkl1≤i, j, k, l≤ n(see (3.12)) and the form ofbn+1jn+1l^ 1≤j, l≤n(see (3.16), we have thatB

is elliptic.

Now we introduce the homogenized problem. Let (~u, p) and (~v, p0) be in H1(Ω)n+1×L20(Ω)2

and be the solution of

−div Ae(~u)

=f~− ∇p+~θ in Ω div Ae(~v)− Be(~u)

=−∇p0 in Ω div~u= div~v= 0 in Ω

~

u=~v= 0 on∂Ω,

(3.17)

where f~is the weak limit off~ε inL2(Ω)n+1 and we precise it later, (4.10).

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Remark 3.3 Since Propositions 3.1 and 3.2 hold, problem (3.17) admits an unique solution.

Now we state the main result of this paper, and we will prove it in the next section.

Theorem 3.3 Under some regularity hypotheses concerning the solutions of (3.1), (3.2), (3.5), and (3.6) (detailed in Section 4) the solutions (~uε, pε) and (~vε, p0ε) of (2.4) are such that~uε * ~u weakly inH1(Ω)n+1,~vε * ~v weakly in H1(Ω)n+1,pε* pweakly inL20(Ω),p0ε* p0 weakly inL20(Ω), where(~u, p)and (~v, p0)are the unique solutions of (3.17).

4 Proof of the convergence result

A priori estimates Let

ξε= 2µεe(~uε), (4.1)

qε= 2µεe(~vε)−e(~uε). (4.2) Proposition 4.1 The sequences (~uε, pε),(~vε, p0ε),ξε andqε are such that (up to subsequences)

~

uε* ~uweakly inH1(Ω)n+1 ~vε* ~v weakly in H1(Ω)n+1 pε* pweakly inL20(Ω) p0ε* p0 weakly inL20(Ω)

ξε* ξ weakly inL2(Ω)n+1×n+1 qε* qweakly in L2(Ω)n+1×n+1. Proof. Using~uεas a test function in the first equation of (2.4), we can easily see that there exists a constantC >0 independent of εsuch that

k~uεkH1(Ω)n+1 ≤C; (4.3) therefore, we have for a subsequence (still denoted byε)

~

uε* ~uweakly inH1(Ω)n+1. (4.4) Similarly, multiplying the second equation of (2.4) by~vε, integrating by parts and using (4.3), we obtain

k~vεkH1(Ω)n+1 ≤C, so we have (for a subsequence)

~

vε* ~vweakly inH1(Ω)n+1. (4.5) Now sincekdiv (2µεe(~uε))kH−1(Ω)n+1is bounded, we havek∇pεkH−1(Ω)n+1 ≤C, this implies (see Temam [16])|pε|L20(Ω)≤C, we derive (for a subsequence),

pε* pweakly inL20(Ω).

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Also by similar arguments, we get

p0ε* p0 weakly inL20(Ω).

The boundedness of kξεkL2(Ω)(n+1)2 provides from (4.3) and we derive by ex- traction of subsequences

ξε* ξweakly inL2(Ω)(n+1)2.

Similarly, the boundedness ofkqεkL2(Ω)(n+1)2 provides from (4.3) and (4.1.6), so we can extract a subsequence such that

qε* q weakly inL2(Ω)(n+1)2. (4.6) Since (~uε, pε) and (~vε, p0ε) are solutions of (2.4) and since Proposition 4.1 holds, we obtain that (~u, p),(~v, p0), ξ andqsatisfy in the distribution sense

−div ξ

=f~− ∇p+~θ in Ω div q

=−∇p0 in Ω div~u= div~v= 0 in Ω

~

u=~v= 0 on∂Ω,

(4.7)

where f~ is the weak limit of f~ε in L2(Ω)n+1. This limit can be identified explicitly (cf [3] or [5]). Indeed, the characteristic functions χε1 (i= 1,2) are such that

χε

i * ρandχε

2 *(1−ρ) weakly ? in L(Ω), (4.8) where

ρ(x, z) =





1 in Ω1

|O(z)|

|Y| in Ωm

0 in Ω2.

(4.9) Then we have

f~ε* ~f =f~1ρ+f~2(1−ρ) weakly inL2(Ω)n+1. (4.10) Proposition 4.2 (Baffico & Conca [3]) Under the hypotheses (4.12), (4.15) and (4.28) (introduced in the next subsections), ξ=Ae(~u), where A is defined by (3.4).

To prove Theorem 3.3, we have to show thatq, ~uand~v are related by

q=Ae(~v)− Be(~u). (4.11)

Using the same method as Baffico & Conca[3], we show that the identifi- cation of q is carried out in Ω1,Ωmand Ω2 independently. In Ω1 and Ω2, this identification will pose no particular problem. In Ωm, following the ideas of Baffico & Conca ([3]), there is three steps: we first identify the components [q]ij

of q for 1≤ i, j ≤ n, and then [q]n+1j for 1 ≤ j ≤ n and finally we identify [q]n+1n+1. To do so, we use some suitable test functions and the energy method ( cf Bensoussan, Lions & Papanicolaou [4] or Sanchez-Palencia [15]).

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Identification of [q]

ij

1 ≤ i, j ≤ n in Ω

m

In what follows, we construct at first the test functions which allow us the identification of [q]ij, then we introduce the regularity conditions that these functions must satisfy and finally we establish the identification.

Letwkl=−χkl+Pkl and σkl = 2µey(wkl) where (χkl, rkl1) be the solution of (3.1). We assume that (χkl, r1kl), as function of (y, z)∈ Y×]0, h1[, satisfies the regularity hypothesis

a) χkl∈L2loc(0, h1, H]1(Y)n)∩(L2loc(]0, h1[×Rn))n b) ∂

∂z (χkl)i

∈L2loc(0, h1, L2](Y)n)∩L2loc(]0, h1[×R)

1≤i, j≤n (4.12) We define the following functions by extension byY-periodicity toRn+1and by restriction to Ωm:

wε,kl(x, z) =εwkl(x ε, z) rε,kl1 (x, z) =rkl1(x

ε, z) σε,kl(x, z) =σkl(x

ε, z)

(4.13)

It is easy to check that

divxε,kl) =−∇xr1ε,kl in Ωm divx (wε,kl) = div

x (Pkl) =δkl in Ωm. (4.14) We also need the Murat’s compactness result [11].

Lemma 4.3 If the sequence(gn)n belongs to a bounded subset ofW1,p(Ω)for some p >2, and(gn)n ≥0 in the following sense i.e., for allφ ∈ D(Ω) such that φ ≥ 0 then for all n > 0hgn, φi ≥ 0. Then (gn)n belongs to a compact subset ofH1(Ω).

If we suppose thatrε,kl1 satisfy

a) rε,kl1 ∈Lploc(Ωm) for somep >2, locally bounded b) ∂

∂z r1ε,kl

≥0 in distribution sense,

(4.15) Then using Lemma 4.3 and hypothesis (4.12), we have the following result.

Proposition 4.4 (Baffico & Conca [3]) If (4.12) and (4.15) hold. Then for allΩ0 ⊂⊂Ωm, we have the following convergence

a)wε,kl* Pkl weakly in H1(Ω0)n b) ∂z (wε,kl)i

→0strongly in L2(Ω0)n,1≤i≤n c) r1ε,kl→0weakly in L2(Ω0)n

d) ∂z rε,kl1

→0 strongly inH1(Ω0)n,1≤i≤n e) σε,kl* σkl=mY(2µey(wkl)weakly inL2(Ω0)n×n.

(4.16)

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In the same way, we assume that the solution (ψkl, rkl2) of (3.5) satisfies the following convergence hypothesis:

a) ψkl∈L2loc(0, h1, H]1(Y)n)∩(L2loc(]0, h1[×Rn))n b) ∂

∂z (ψkl)i

∈L2loc(0, h1, L2](Y)n)∩L2loc(]0, h1[×R)

1≤i, j≤n (4.17) We define

ψε,kl(x, z) =εψkl(x

ε, z), and rε,kl2 (x, z) =rkl2(x

ε, z) (4.18) so we obtain

−div

xεexε,kl) +ex(wε,kl)

=−∇xrε,kl2 in Ωm divx ψε,kl= 0 in Ωm

(4.19)

If we suppose thatrε,kl2 satisfy

a) r2ε,kl∈Lploc(Ωm) for somep >2, locally bounded b) ∂

∂z rε,kl2

≥0 in distribution sense,

(4.20)

then using Lemma 4.3, we derive

∂z rε,kl2

→0 strongly inH1(Ω0)n.1≤i≤n We have the following result concerning these functions

Proposition 4.5 If (4.12) and (4.15) hold. Then for allΩ0⊂⊂Ωm, we have a) ψε,kl*0 weakly inH1(Ω0)n

b) ∂

∂z (ψε,kl)i

→0 strongly inL2(Ω0)n, 1≤i≤n c) rε,kl2 *0 weakly inL2(Ω0)n.

(4.21)

To prove this proposition, we use well-known results concerning the convergence of periodic functions.

We shall prove now the principal result of this section

Proposition 4.6 If (4.12), (4.15), (4.17), and (4.20) hold, then[qε]kl *[q]kl

weakly in L2(Ωm) (up to a subsequence) for all1≤k, l≤nwhere [q]kl= 1

|Y|

n

X

i,j=1

Z

Y

2µ[ey(−χij+Pij)]kldy e(~v)

ij

− 1

|Y|

n

X

i,j=1

Z

Y

2µ[ey(−ψij)]kl+ [ey(−χij+Pij)]kl

dy e(~u)

ij

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Proof. Letφ∈ D(Ωm) andw~ε,kl= (wε,kl,0). Multiplying the second equation of (2.4) byφ ~wε,kl, integrating by parts and using (4.2), we obtain

− Z

m

∇p0ε.(φ ~wε,kl)dxdz

=− Z

m

(qε∇φ). ~wε,kldxdz+ Z

m

e(~uε) :∇w~ε,klφdxdz

− Z

m

εe(~vε) :∇w~ε,klφdxdz.

Developing the second and third integral of the right-hand side of the above equation,

− Z

m

∇p0ε.φ ~wε,kldxdz

=− Z

m

(qε∇φ). ~wε,kldxdz+ Z

m

ex(uε) :ex(wε,kl)φdxdz

− Z

m

εex(vε) :ex(wε,kl)φdxdz− Z

m

n

X

j=1

[qε]n+1j

∂z (wε,kl)j φdxdz.

(4.22) Letψ~ε,kl= (ψε,kl,0). Multiplying the first equation of (2.4) byφ ~ψε,kl, integrat- ing by parts and using definition (4.1), we obtain (after algebraic developments)

Z

m

(f~ε− ∇pε+~θ).φ ~ψε,kldxdz

= Z

m

ε∇φ). ~ψε,kl+ Z

m

εex(uε) :exε,kl)φdxdz +

Z

m n

X

j=1

ε]n+1j

∂z (ψε,kl)j

φdxdz.

Integrating by parts now the second integral of the right-hand side, we have Z

m

(f~ε− ∇pε+~θ).φ ~ψε,kldxdz

= Z

m

ε∇φ). ~ψε,kl− Z

m

divxεexε,kl)

.(uεφ)dxdz

− Z

m

(2µεexε,kl)∇xφ).uεdxdz+ Z

m

n

X

j=1

ε]n+1j

∂z (ψε,kl)j φdxdz.

Using the equation thatψε,kl satisfy (see (4.19)), Z

m

(f~ε− ∇pε+~θ).φ ~ψε,kldxdz

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= Z

m

ε∇φ). ~ψε,kl− Z

m

uεdiv

x ex(wε,kl)

φdxdz+ Z

m

xr2ε,kl.(φuε)dxdz

− Z

m

εexε,kl)∇xφ

.uεdxdz+ Z

m

n

X

j=1

ε]n+1j

∂z (ψε,kl)j φdxdz.

Integrating again by parts the second integral, Z

m

(f~ε+~θ− ∇pε).φ ~ψε,kldxdz

= Z

m

ε∇φ). ~ψε,kl− Z

m

ex(uε) :ex(wε,kl)φdxdz +

Z

m

xr2ε,kl.(φuε)dxdz− Z

m

εexε,kl)∇xφ .uεdxdz

− Z

m

ex(wε,kl)∇xφ

.uεdxdz+ Z

m n

X

j=1

ε]n+1j

∂z (ψε,kl)j

φdxdz.

Adding (4.22) and the above equation, we obtain Z

m

(f~ε+~θ).φ ~ψε,kldxdz− Z

m

∇pε.φ ~ψε,kldxdz− Z

m

∇p0ε.φ ~wε,kldxdz

=− Z

m

(qε∇φ). ~wε,kldxdz− Z

m

εex(vε) :ex(wε,kl)φdxdz

− Z

m n

X

j=1

[qε]n+1j

∂z (wε,kl)j

φdxdz+ Z

m

ε∇φ). ~ψε,kl +

Z

m

xrε,kl2 .(φuε)dxdz− Z

m

(bε,klxφ).uεdxdz +

Z

m

n

X

j=1

ε]n+1j

∂z (ψε,kl)j

φdxdz.

(4.23)

where

bε,kl= 2µεexε,kl) +ex(wε,kl). (4.24) We obtain easily that (using Problem (3.5))

divx (bε,kl) =−∇xrε,kl2 in Ωm.

We now pass to the limit in (4.23) as εtends to 0. In order to do so, we need some preliminaries results.

By Definition (4.24) and classical arguments concerning the convergence of periodic functions, we conclude that for all Ω0 ⊂⊂Ωm,

bε,kl* bkl= mY(2µeykl) +ey(wkl)) weakly inL2(Ω0)n×n and div

x (bkl) = 0 in Ωm. (4.25)

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By the convergence (4.16) a), we have

~

wε,kl→P~kl= (Pkl,0) strongly inL2(Ω0)n+1. Also by (4.21)a), we get

ψ~ε,kl→0 strongly inL2(Ω0)n+1.

Now passing to the limit in (4.23) taking into account the precedent convergence results, we obtain

− Z

m

∇p0.φ ~Pkldxdz

=− Z

m

(q∇φ). ~Pkldxdz− Z

m

σkl :ex(v)φdxdz− Z

m

(bklxφ).udxdz Integrating by parts the right-hand side of the above expression, using the second equation of (4.1.14) and the expression (4.25), we obtain

0 = Z

m

q:e(P~kl)φdxdz− Z

m

σkl:ex(v)φdxdz+ Z

m

ex(u) :bklφdxdz Since

e(P~kl)

ij = Mkl

ij, then we obtain in the distribution sense [q]kl=

n

X

i,j=1

σkl

ij

ex(v)

ij

n

X

i,j=1

bkl

ij

ex(u)

ij.

Now since (4.16) e), (4.25) hold and since ex(v)

ij = e(~v)

ij and ex(u)

ij = e(~u)

ij for all 1≤i, j≤n, we get [q]kl= 1

|Y|

n

X

i,j=1

Z

Y

2µ[ey(−χkl+Pkl)]ijdy e(~v)

ij

− 1

|Y|

n

X

i,j=1

Z

Y

2µ[ey(−ψkl)]ij+ [ey(−χkl+Pkl)]ij

dy e(~u)

ij. (4.26) Also since the following symmetry property holds (see Proposition 3.1)

Z

Y

2µ[ey(−χkl+Pkl)]ijdy= Z

Y

2µ[ey(−χij+Pij)]kldy, and the following’s one holds too (see Proposition 3.2)

Z

Y

ey(−ψkl) +ey(−χkl+Pkl)

ij

dy

= Z

Y

ey(−ψij) +ey(−χij+Pij)

kl

dy, (4.27)

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we conclude that [q]kl = 1

|Y|

n

X

i,j=1

Z

Y

2µ[ey(−χij+Pij)]kldy e(~v)

ij

− 1

|Y|

n

X

i,j=1

Z

Y

2µ[ey(−ψij)]kl+ [ey(−χij+Pij)]kl

dy e(~u)

ij

This completes the proof.

Identification of [q]

n+1j

, 1 ≤ j ≤ n in Ω

m

Letϕk be the solution of Problem (3.2). We assume thatϕkk(y, z) satisfy the regularity hypotheses

a) ϕk ∈L2

loc(0, h1, H]1(Y))∩L2

loc(]0, h1[×Rn) b) ∂ϕ∂zk ∈L2loc(0, h1, L2](Y))∩L2loc(]0, h1[×Rn)

(4.28) Let us define ζk =−ϕk + 2yk andηk = µ∇yζk. We also define the following functions byY-periodicity:

ζε,k(x, z) =εζk(x

ε, z), ηε,k(x, z) =ηk(x

ε, z) (4.29)

It is easy to see that −divxηε,k = 0 in Ωm. We introduce a supplementary hypothesis concerning ηε,k:

a) { ηε,k

j}ε>0⊂Lp

loc(Ωm) for some p >2, locally bounded b) ∂

∂z ηε,k

j≥0 in the distribution sense.

(4.30)

Then we have the following result.

Proposition 4.7 (Baffico & Conca [3]) Assume (4.28) and (4.30). Then for all Ω0 ⊂⊂Ωm, we have

a)ζε,k*2yk weakly in H1(Ω0) b) ∂z ζε,k

→0 strongly in L2(Ω0) c) ηε,k* ηk =mYk)weakly in L2(Ω0)n d) ∂z ηε,k

j∂z ηk

j strongly inH1(Ω0),1≤j≤n.

(4.31)

In view of (4.3.5) c) and (4.3.3), we get−divxηk = 0 in Ωm. Similarly we assume thatψk, the solution of (3.6), satisfies

a) ψk ∈L2loc(0, h1, H]1(Y))∩L2loc(]0, h1[×Rn) b) ∂ψk

∂z ∈L2loc(0, h1, L2](Y))∩L2loc(]0, h1[×Rn)

(4.32)

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Let

ψε,k(x, z) =ψk(x

ε, z) and dε,k= 2µεxψε,k+∇xζε,k, (4.33) then using (3.6), (4.29) and (4.33), we get

−div

x dε,k= 0 in Ωm. (4.34)

We assume thatdε,ksatisfies the regularity conditions:

a)

dε,k

j ε>0⊂Lploc(Ωm) for somep >2, locally bounded b) ∂

∂z dε,k

j≥0 in the distribution sense.

(4.35)

We have the following result.

Proposition 4.8 Assume hypotheses (4.3.7) and (4.35) hold. Then for all Ω0⊂⊂Ωm, we have

a) ψε,k*0 weakly inH1(Ω0) b) ∂

∂z ψε,k

→0 strongly inL2(Ω0)

c) dε,k* dk =mY(2µ∇yψk+∇yζk)weakly in L2(Ω0)n d) ∂

∂z dε,k

j→ ∂

∂z dk

j strongly inH1(Ω0), 1≤j≤n.

(4.36)

Remark 4.9 From (4.34) and (4.36) c), we have−divxdk= 0 in Ωm.

Proof of Proposition 4.8 Using classical arguments concerning convergence of periodic functions, we can prove the three first assertions. For the last one, we use the compactness Lemma 4.3 (the hypotheses of this lemma hold since

we suppose that (4.35) is satisfied).

Proposition 4.10 If (4.28), (4.30), (4.32) and (4.35) hold, then up to a sub- sequence, we have [qε]n+1k *[q]n+1k weakly inL2(Ωm)∀1≤j≤n, where

[q]n+1k = 1

|Y|

n

X

i=1

Z

Y

µ∂(−ϕi+ 2yi)

∂yk

dy e(~v)

n+1i

− 1 2|Y|

n

X

i=1

Z

Y

2µ∂ψi

∂yk

+∂(−ϕi+ 2yi)

∂yk

dy e(~u)

n+1i.

Proof. Let φ∈ D(Ωm) and ~ζε,k = (0, ζε,k). Multiplying the second equation of (2.4) byφ~ζε,k, integrating by parts and using (4.2), we obtain

− Z

m

∇p0ε.~ζε,kφdxdz=− Z

m

(qε∇φ).~ζε,kdxdz+ Z

m

e(~uε) :∇~ζε,kφdxdz

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