LAAS-CNRS, University of Toulouse 7 av. du Colonel Roche, 31077 Toulouse cedex 4, France. e-mail: [email protected], [email protected] Abstract: The problem under consideration relates to a model of porous wall devoted to aircraft motors noise reduction. For such a medium, the parameters of the propagation equation depend on the frequency, so the corresponding time-model involves non rational convolution operators. Consequently, the impedance of the wall is a non rational function of the frequency. On the basis of complex analysis and causality properties, we introduce inﬁnite dimensional state formulations of these operators. The coupling of the so-obtained model with a standard aeroacoustic one then leads to a time-local system whose analysis is simpliﬁed thanks to the existence of an energy functional in the sense of which the global dissipativity is insured. Some numerical results are given to illustrate the theoretical results. Keywords: porous medium, aeroacoustic waves, boundary control, diﬀusive representation, state representation, non rational operators, energy functional, energy balance. 1. INTRODUCTION Aircraft motors noise reduction is currently an important challenge for aerospace industry. In the case of hot zones such as exhaust nozzles, a speciﬁc porous wall was proposed in Gasser [2003] for absorption of a wide part of the energy of incident acoustic waves. The frequency model of such a material which describes the wave propagation inside the porous medium, is given on (ω, z, x) ∈ R×]0, 1[×]0, X[ by:

e(x) iω ρeﬀ (iω) u ˆ + ∂z Pˆ = 0 (1) e(x) iω χeﬀ (iω) Pˆ + ∂z u ˆ = 0, where u and P designate the Fourier transforms of the velocity and the pressure in the porous medium, e(x) denotes the thickness of the porous wall and the parameters ρeﬀ (iω) and χeﬀ (iω) are respectively the so-called eﬀective density of Pride et al. (Pride [1993]) and the eﬀective compressibility of Lafarge (Lafarge [1993]). These parameters are expressed (Gasser [2003]): ⎧ 1 ⎨ ρ (iω) = ρ (1 + a (1+b iω) 2 ) eﬀ iω (2) iω ⎩ χeﬀ (iω) = χ (1 − c 1 ), iω+a (1+b iω) 2

with: 1 8μ 8μ ,a= , a = , P0 ρ0 Λ 2 ρ0 Λ2 1 γ−1 1 , b = , 0 < c = < 1, b= 2a 2a γ where the physical parameters ρ0 , P0 , μ, γ, α∞ , Λ, Λ are respectively the density and pressure at rest, the dynamic ρ = ρ0 α ∞ , χ =

viscosity, the speciﬁc heat ratio, the tortuosity, the high frequency characteristic length of the viscous incompressible problem and the high frequency characteristic length of the thermal problem. The model (1) is completed by the following limit conditions: u|z=1 = 0 (wave reﬂection at z = 1) (3) P|z=0 = w. When coupling this system with the ﬂuid medium, two connection conditions at the interface Γ are necessary: ﬂuid P|Γ = P|z=0 ﬂuid and u|Γ · n = φ u|z=0 , where n is the outgoing unit normal on Γ and φ > 0 is the porosity coeﬃcient of the material (Gasser [2003]). Then, w and y := u|z=0 (4) can be interpreted respectively as the input and the output of (1), the correspondence w → y deﬁning the impedance operator of the porous wall. Parameters ρeﬀ and χeﬀ depend on the frequency ω, so the time model obtained by inverse Fourier transform of (1), is written: e(x) (∂t ◦ ρeﬀ (∂t )) u + ∂z P = 0 (5) e(x) (∂t ◦ χeﬀ (∂t )) P + ∂z u = 0, where ρeﬀ (∂t ) and χeﬀ (∂t ) are the (causal) convolution operators associated to the symbols ρeﬀ and χeﬀ respectively. Model (5) is not time-local because of the presence of convolution operators ρeﬀ (∂t ) and χeﬀ (∂t ). We propose in section 2 a new formulation of such operators by means of a suitable diﬀusive input-output dif-

ferential system. This formulation allows us to obtain, in section 3, a time-local augmented input-output model equivalent to (1,3,4). We show that in the sense of an explicitly known functional, this model is consistent from the energy dissipation point of view. Then in section 4, the dissipativity of the whole problem is easily deduced from a suitable global energy functional. In section 5, we compute the analytic expression of the porous wall impedance from which we build a reduced diﬀusive state formulation of the associated operator. Finally, some numerical results and simulations are presented in section 6 .

Theorem 3. If the possible singularities of H on γ are simple poles or branching points such that |H ◦ γ| is locally integrable, then: γ ˜ ˜ .) = Ψw (t, .) ◦ γ˜ : 1. with ν˜ = 2iπ H ◦ γ˜ and ψ(t, ˜ ξ) dξ; (H(∂t )w) (t) = ν˜ (ξ) ψ(t, (12) R

2. with γ˜n → γ in sense of measures:

1,∞ Wloc

and ν =

γ ˜ 2iπ

lim H ◦ γ˜ n in the

(H(∂t )w) (t) = ν, ψ(t, .) ,

where ψ(t, ξ) is solution of the evolution problem on (t, ξ) ∈ R∗+ ×R:

2. DIFFUSIVE REALIZATION OF CAUSAL CONVOLUTION OPERATORS

∂t ψ(t, ξ) = γ(ξ) ψ(t, ξ) + w(t), ψ(0, ξ) = 0. In this section, we present a particular case of a methodology introduced and developed in Montseny [2005] in a general framework. We consider a causal operator deﬁned, on any continuous function w : R+ → R, by t w → h(t − s) w(s) ds. (6) 0

We denote H the Laplace transform of h and H(∂t ) the convolution operator deﬁned by (6). t

t

Let w (s) = 1]−∞,t] (s) w(s) and wt (s) = w (t − s). From causality of H(∂t ), we deduce: H(∂t )(w − wt ) (t) = 0 for all t; then, we have for any continuous function w:

(H(∂t )w)(t) = L−1 (H Lw) (t) = L−1 H Lwt (t). (7) We deﬁne: Ψw (t, p) := ept Lwt (p) = (Lwt ) (−p);

(8)

by computing ∂t Lwt , Laplace inversion and use of (7), it can be shown: Lemma 1. 1. The function Ψw is solution of the diﬀerential equation: ∂t Ψ(t, p) = p Ψ(t, p) + w, t > 0, Ψ(0, p) = 0. (9) 2. For any b 0, b+i∞ 1 (H(∂t )w) (t) = H(p) Ψw (t, p) dp. (10) 2iπ b−i∞ We denote Ω the holomorphic domain of H. Let γ a closed 1 simple arc in C− ; we denote Ω+ γ the exterior + domain deﬁned by γ, and Ω− γ the complementary of Ωγ . By use of standard techniques (Cauchy theorem, Jordan lemma (Lavrentiev [1977])), it can be shown: Lemma 2. For γ ⊂ Ω such that H is holomorphic in Ω+ γ, , then: if H(p) → 0 when p → ∞ in Ω+ γ 1 H(p) Ψw (t, p) dp, (11) (H(∂t )w) (t) = 2iπ γ˜ − where γ˜ is any closed simple arc in Ω+ γ such that γ ⊂ Ωγ ˜.

We now suppose that γ, γ˜ are deﬁned by functions 1,∞ of Wloc (R; C), also denoted γ, γ˜ . Under hypothesis of lemma 2, we have (Montseny [2005]): 1

Possibly at infinity

(13)

(14)

Remark 4. In the sequel, we will indiﬀerently denote ν, ψ or ν ψ dξ the duality scalar product between a continuous function ψ and a measure ν. Finally, in the particular case γ(ξ) = −|ξ|, we deduce from symmetry of the problem that there exists a measure μ such that +∞ +∞ ν ψ dξ = μ ψ dξ. −∞

0

The state equation (14) is inﬁnitedimensional. To get numerical approximations, we consider a discretization (ξ l )l=1:L of the variable ξ and approximations μL of the γ-symbol μ using atomic measures and of the form: μL =

L

μlL δ ξ l .

l=1

Let us denote ML the space of atomic measures on the mesh {ξ l }l=1:L . If ∪L ML is dense in the space of measures, we have: μL , ψ −→ μ, ψ ∀ψ. L→+∞

So we get the ﬁnitedimensional approximate state formulation of H(∂t ) : ⎧ ∂t ψ(t, ξ l ) = γ(ξ l ) ψ(t, ξ l ) + u(t), l = 1 : L ⎪ ⎪ ⎪ ⎨ ψ(0, ξ l ) = 0, l = 1 : L (15) L ⎪ ⎪ l ⎪ μL ψ(t, ξ l ). ⎩ (H(∂t )u) (t) l=1

More details can be found in Montseny [2005].

3. STATE FORMULATION OF THE POROUS MEDIUM MODEL Consider the convolution operators H1 (∂t ) and H2 (∂t ) with respective symbols H1 (p) = 1/(p ρeﬀ (p)) and H2 (p) = 1/(p χeﬀ (p)). These functions are decreasing at inﬁnity and analytic in CR− . So, by considering the contour γ deﬁned by γ(ξ) = − |ξ| , we can show that operators H1 (∂t ) and H2 (∂t ) satisfy the hypothesis of theorem 3. After computations as described in section 2, we obtain the following measures μ1 and μ2 respectively associated to H1 (∂t ) and H2 (∂t ):

√ a bξ −1 1ξ>2a + k1 δ ξ1 (ξ), π ρ ξ 2 + a2ξ − a2 √ a c b ξ−1 1ξ>2a μ2 (ξ) = π χ ξ 2 (1 − c)2 + a2 ξ − a2

(16) for any given value of x, whereas the last term expresses the instantaneous exchanged power. Moreover, thanks to the property ψ(t, 0) = 0, we deduce from (17) the positiveness of the quadratic form w → QT (w) := T wy dt for any T > 0 and any x: the passive feature of 0 the absorbent wall is restored by model (16).

μ1 (ξ) =

+

1 δ 0 (ξ) + k2 δ ξ2 (ξ), χ

4. COUPLING WITH AN AEROACOUSTIC WAVE MODEL

where: ξ1 =

√ a( 17−1) 4

> 0, ξ 2 =

a (

√

k1 =

17−1 √ ρ 17

> 0, and k2 =

√

1+16(1−c)2 −1) 4(1−c)2

c

√

> 0, > 0. 2

1+16(1−c)2 −1

χ(1−c)

√

1+16(1−c)

Let us express the system (1) under the form: u = −H1 (∂t ) ∂z P/e P = −H2 (∂t ) ∂z u/e. From results of section 2, we get the following diﬀusive formulations of operators ∂z P → u and ∂z u → P : 1 1 ∂t ψ 2 = −ξ ψ 2 − ∂z u ∂t ψ 1 = −ξ ψ 1 − ∂z P and e e u := μ1 , ψ 1 , P := μ2 , ψ 2 . Then we get the following augmented model, deﬁned on (t, z, x, ξ) ∈ R∗+ ×]0, 1[×Γ × R+ and input-output equivalent to (1,3,4): ⎧ 1 ⎪ ⎪ ⎪ ∂t ψ 1 = −ξ ψ 1 − μ2 , ∂z ψ 2 ⎪ e ⎪ ⎪ ⎪ 1 ⎪ ⎪ ⎪ ⎨ ∂t ψ 2 = −ξ ψ 2 − e μ1 , ∂z ψ 1 μ1 , ψ 1 (t, 1, x, .) = 0 ⎪ ⎪ ⎪ ⎪ ⎪ μ2 , ψ 2 (t, 0, x, .) = w(t, x) ⎪ ⎪ ⎪ ⎪ ⎪ ⎩ y(t, x) = μ1 , ψ 1 (t, 0, x, .) .

(16)

As μ1 and μ2 are positive, the functional 1 2 2 (ψ 1 , ψ 2 ) → (ψ 1 , ψ 2 )μ,x = [ (μ1 |ψ 1 | + μ2 |ψ 2 | )dξdz] 2 is a semi-norm. We have: 2 Proposition 5. The functional Eψx = 12 (ψ 1 , ψ 2 )μ,x veriﬁes on any solution (ψ 1 , ψ 2 ) of (16) : 1 ∞ dEψ x 2 =− ξ μ1 |ψ 1 | dξ dz dt (17) 1 0 ∞0 1 2 − ξ μ2 |ψ 2 | dξ dz + w y. e 0 0 Proof.

1

∞

ξ μi |ψ i |2 dξ dz dEψ x /dt = − 0 0 i 1 1 1 1 P ∂z u dz − ∂z P u dz − e 0 e 0 1 ∞

1 2 =− ξ μi |ψ i | dξ dz − P (t, 1, x) u(t, 1, x) e 0 0 i 1 + P (t, 0, x) u(t, 0, x). e The two ﬁrst terms of the right member of (17) are negative: they express the instantaneous dissipation of

As a consequence of (17), the coupling of (16) with another passive dynamic model leads to a globally dissipative system provided that the energy functional is chosen such that the transfer between the two subsystems is balanced. This is highlighted in the sequel. We consider the aeroacoustic model studied in Mazet [2005]. For simplicity we mainly consider the non convective case with rectangular domain ]0, Z[×]0, X[ and boundary control at z = Z: ⎧ 0 ∇ u u ⎪ ⎪ ∂ + c =f ⎪ t 0 T ⎪ ρ ρ ∇ 0 ⎪ ⎪ ⎪ ⎨ ux (t, z, 0) = ux (t, z, X) = uz (t, 0, x) = 0 (18) ⎪ ⎪ (t, Z, x) = φ y(t, x) u ⎪ z ⎪ ⎪ ⎪ ⎪ ⎩ w(t, x) = c0 ρ0 ρ(t, Z, x), where u and ρ are the velocity and the density, f is the perturbation source, c0 is the velocity of wave fronts inside the ﬂuid medium and φ is the porosity coeﬃcient of the porous material. The acoustic energy of (u, ρ)T is classically given by the functional: 2 1 Em = (u, ρ)T L2 . 2 We can deﬁne a global energy for (16,18) as: X ρ E= e(x)Eψ x dx + 0 Em . φ 0 We have: Theorem 6. The system (16,18) is dissipative in the sense: X 1 ∞ dE =− e(x) ξ μ1 |ψ 1 |2 dξ dz dx dt 0 0 0 X 1 ∞ 2 − e(x) ξ μ2 |ψ 2 | dξ dz dx 0 0

0

0

on any solution (u, ρ, ψ)T of (16, 18). Proof. From (18) and: X 1 0 ∇ u u dEm /dt = − c0 · dz dx T ρ ρ ∇ 0 0 0 X 1 (∂x ρux + ∂z ρuz + ∂x ux ρ + ∂z uz ρ) dz dx = −c0 0 0 1 X = −c0 ( [ρux ]X dz + [ρuz ]Z x=0 z=0 dx) 0 0 X = −c0 ρ(t, Z, x) uz (t, Z, x) dx. 0

Note that this global energy dissipation due to wave absorption at the boundary, is not accessible under the frequential or convolution forms (1) and (5). In this

In the convective case (Mazet [2005]), similar results can be obtained up to suitable technical adaptations. 5. DIFFUSIVE REALIZATION OF THE IMPEDANCE OPERATOR In order to build state realizations of the boundary control deﬁned by (1,3,4), it can be judicious to start from the symbol of the input-output operator w → y, denoted Q(∂t ) in the sequel. From the numerical point of view, the realization of this operator is more accurate and cheaper than the simulation of the wave propagation inside the medium and so is more adapted for example to control purposes. Let us now compute the symbol of Q(∂t ). The equations of the porous medium (1,3) can be written: ∂z X = −e(x) iω AX (19) X(0, ω) = [v, 0]T , with: 0 ρeﬀ (iω) X = P ,A= . χeﬀ (iω) 0 uˆ Diagonalization of A leads to: √ χeﬀ ρeﬀ √ 0 A=M M −1 , 0 − χeﬀ ρeﬀ

The presence of complex poles of Q(p) (see ﬁg. 1) does not allow us to use the same contour γ as in section 3 (note in particular that Q(p) ∼ Kp1 tanh(K2 p)). For the realization ∞

t ), we so consider a contour γ of the form: of Q(∂ π

γ(ξ) = |ξ| ei sign(ξ)( 2 +α) , with α ∈]0, π2 ] (α will be taken equal to numerical results of section 6).

It can be shown that the symbol Q(p) = Q(p) satisﬁes the p hypothesis of theorem 3. From section 2, we then get the following state-representation of Q(∂t ): ∂t ψ = −γψ + w (21) y = ν, ∂t ψ = − γ ν ψ dξ + ν dξ w, t ) and γ is a where ν is the measure associated to Q(∂ suitable contour.

for the

x 10

0.8

0.6

0.4

0.2

0

−0.2

−0.4

−0.6

−0.8

−1 −12000

−10000

−8000

D

where M is the transition matrix; using the change of unknowns Y = M −1 X, we get the diagonal system of ODEs: ∂z Y = −e iω DY with limit conditions deduced from (19). After simple computations, we obtain: ⎧ A(ω)/B(ω, z) + B(ω, z) ⎪ ⎪ P(ω, z) = P(ω, 0) ⎪ ⎨ A(ω) + 1 ⎪ χeﬀ (iω) A(ω)/B(ω, z) − B(ω, z) ⎪ ⎪ , ⎩ uˆ(ω, z) = P (ω, 0) ρeﬀ (iω) A(ω) + 1 (20) where √ A(ω) = e2eiω χeff (iω) ρeff (iω) , √ B(ω, z) = e−eiωz χeff (iω) ρeff (iω) . Finally, the input-output correspondence w → y = Q(iω) w (which summarizes the frequency behavior of the porous medium) can be deduced by taking z = 0 and replacing iω by p ∈ C in (20): χeﬀ (p) tanh e(x) p χeﬀ (p) ρeﬀ (p) . Q(p) = ρeﬀ (p)

5π 180

4

1

Imaginary part

sense, the diﬀusive model (16) is conform to the physical interpretation of such media.

−6000 Real part

−4000

−2000

0

Fig. 1. Poles of Q(p) (e = 1)

6. SOME NUMERICAL RESULTS As described in section 2, we can perform converging approximations of (16) and (21) by using a discretization (ξ l )l=1,L of ξ and standard quadratures. We then get approximate dynamic realizations of the form: X˙ = A X + Bw with X(t) ∈ RL , y = CX such that y y in a suitable sense (Montseny [2005]). The frequency responses of the approximations of H1 (∂t ), t ) respectively obtained with L = 15, 20 H2 (∂t ) and Q(∂ and 150 are given in ﬁgures 2, 3, 4. In the case of H1 (∂t ) t )), 4 decades from 102 to 106 and H2 (∂t ) (respectively Q(∂ (respectively 3 decades from 102 to 105 ) are covered by the ξ-discretization. The numerical values of parameters are: Λ = Λ = 0.1 10−3 m, ρ0 = 1.2 kg.m−3 , P0 = 105 Pa μ = 1.8 10−5 kg.m−1 .s−1 , γ = 1.4, α∞ = 1.3, e = 2 10−2 m. Note that by increasing the number of ξ l , the oscillations t ) will be better approxiof the frequency response of Q(∂ mated. For illustration, the evolution of P obtained from simulation of (16) is given in ﬁgure 5. The explicit scheme used for this simulation and whose stability is proved in Casenave [2008], is written:

t=0.0465 ms

t=0.0655 ms

t=0.0845 ms

2

2

2

1.5

1.5

1.5

1.5

1

1

1

1

0.5

0.5

0.5

0.5

0

0 0

1

2

0 0

t=0.1225 ms

1

2

0 0

1

t=0.1415 ms

2

0

t=0.1605 ms

2

2

2

1.5

1.5

1.5

1.5

1

1

1

1

0.5

0.5

0.5

0.5

0 0

1

Fig. 5. Evolution of P =

2

l bl2 ψ 2 (ξ l )

0 0

1

2

1

2

t=0.1795 ms

2

0

0 0

1

2

0

1

2

(N.B: the unit of length for the z-axis is 10−2 m)

−4

4

10

10 analytic approximation

−5

magnitude

10

magnitude

t=0.1035 ms

2

−6

10

−7

analytic approximation

2

10

0

10

10

−8

10

−2

1

10

2

10

3

10

4

10

5

10

6

10

10

7

10

1

10

2

10

3

10

4

10

5

10

6

10

7

10

frequency (rad/s)

frequency (rad/s) 0

−82

phase (deg)

phase (deg)

−20 −40 −60 −80 −100 1 10

2

10

3

10

4

10

5

10

6

10

7

10

frequency (rad/s)

Fig. 2. Frequency response of operator H1 (∂t ) ⎧

⎪ ψ n+1 (x, ξ l ) = al1 ψ n−1 (x, ξ l ) − bl1 G21 bj2 ψ n2 (x, ξ j ) ⎪ 1 1 ⎪ ⎪ ⎪ j ⎪

⎪ ⎪ n+1 n−1 ⎪ ψ (x, ξ ) = a ψ (x, ξ ) − b G bj1 ψ n1 (x, ξ j ) ⎪ l2 l2 12 l l 2 ⎨ 2 j

n n+1 ⎪ u (x) = b ψ (x, ξ ) ⎪ l1 1 l ⎪ ⎪ ⎪ l ⎪

⎪ ⎪ n+1 ⎪ (x) = bl2 ψ n2 (x, ξ l ), ⎪ ⎩P

−84

−86

−88

−90 1 10

2

10

3

10

4

10

5

10

6

10

7

10

frequency (rad/s)

Fig. 3. Frequency response of operator H2 (∂t ) e−ξj 2Δt − 1 −ξl 2Δt ali = e , bjk = cjk , −ξ j ⎤ ⎡ 0 1 ⎥ ⎢ −1 0 1 ⎥ 1 ⎢ . . . ⎢ .. .. .. ⎥ G21 = G12 = ⎥, ⎢ 2Δz ⎣ −1 0 1 ⎦ −1 0

where c li are some coeﬃcients computed by simple quadrature of μ (ξ )Λl (ξ)dξ, Λl being the classical interpolation (22) function: i l (x, ξ l ), ψ n+1 (x, ξ l ) ∈ with u n+1 (x), P n+1 (x) ∈ RK and ψn+1 −ξ ξ − ξ l−1 ξ 1 2 Λl (ξ) = 1[ξl−1 ,ξl ] (ξ) + l+1 1]ξ ,ξ ] (ξ). CK , where K = 300 is the number of points of discretizaξ l − ξ l−1 ξ l+1 − ξ l l l+1 tion in z, and: The boundary conditions are: l

−6

−3

10

4

analytic approximation −7

3

10

magnitude

x 10

2

−8

10

1 −9

10

1

2

10

3

10

4

10

10

5

10

6

10

frequency (rad/s)

0

phase (deg)

50

−1

0

−50

−2

−100

−3 0

−150 1 10

2

3

10

4

10

5

10

10

0.1

0.2

0.3

0.4

0.5 0.6 time (ms)

0.7

0.8

0.9

1

6

10

frequency (rad/s)

t) Fig. 4. Frequency response of operator Q(∂

Fig. 7. Outputs y obtained by simulation of (16) (- -) and (21) (—). 7. CONCLUSION

u(t, 1) = 0, (23) 4 P (t, 0) = 1 − cos(2π × 1.3 10 t) 1[0,T ] (t) = w. (24) We clearly observe the dissipative and dispersive nature of this propagative model, due to the convolution operators H1 (∂t ) and H2 (∂t ). In ﬁgure 6 we can see at a ﬁxed time t = 0.09 ms, the functions ψ 1 involved in the synthesis of u.

On the basis of the above energy dissipation analysis, we can conclude that the model (1) can be viewed as a dissipative absorbing controller for aeroacoustic waves. Furthermore the quality of the numerical results of the simulation of the boundary operator w → y conﬁrms that the state-realizations (16) and (21) can be used for more deepened investigations on absorbing boundary control. REFERENCES

−3

6

x 10

5

4

3

2

1

0

−1

−2 0

0,5

1

1,5

2

Fig. 6. Functions ψ 1 (t, ., x, ξ l ) l = 1 : L at time t = 0.09 ms We compare in ﬁgure 7 the outputs y := u|z=0 obtained by simulation of (16) and (21) with the input w given by (24). Note that the diﬀerence between the two results is mainly due to the error approximation of z-discretization. As the simulation of (21) doesn’t need any z-discretization, we can consider a high number of ξ l to approximate the operator Q. Then the simulation of (21) is more accurate but remains cheaper than the scheme (22) (whose global dimension is 10500).

C. Casenave, E. Montseny, Time-local dissipative formulation and stable numerical schemes for a class of integrodiﬀerential wave equations, to appear in SIAM Journal on Applied Mathematics. S. Gasser, Etude des propri´et´es acoustiques et m´ecaniques d’un mat´eriau m´etallique poreux a ` base de sph`eres creuses de nickel, PhD Thesis, Grenoble, France, 2003. D. Lafarge, Propagation du son dans les mat´eriaux poreux a structure rigide satur´es par un ﬂuide viscothermique, ` PhD thesis, Universit´e du Maine, 1993. M. Lavrentiev, B. Chabat, M´ethodes de la th´eorie des fonctions d’une variable complexe, MIR, Moscow, Russia, 1977. P.-A. Mazet, Y. Ventribout, Control of Aero-acoustic Propagations with Wall Impedance Boundary Conditions: Application to a Porous Material Model, WAVES 2005, Providence, USA, June 2005. G. Montseny, Repr´esentation Diﬀusive, Hermes-Science, Paris, France, 2005. S. R. Pride, F.D. Morgan, A.F.Gangi, Drag forces of porous-medium acoustics, Phys. Rev. B. 47 (1993), 49644978.