Volume 2013, Issue 1 425031
Research Article
Open Access

Pullback Attractors for Nonautonomous 2D-Navier-Stokes Models with Variable Delays

Xiaoli Liu

Xiaoli Liu

School of Mathematics and Statistics, Lanzhou University, Lanzhou 730000, China lzu.edu.cn

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Yejuan Wang

Corresponding Author

Yejuan Wang

School of Mathematics and Statistics, Lanzhou University, Lanzhou 730000, China lzu.edu.cn

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First published: 17 April 2013
Citations: 1
Academic Editor: de Dai

Abstract

Using a method based on the concept of the Kuratowski measure of the noncompactness of a bounded set as well as some new estimates of the equicontinuity of the solutions, we prove the existence of a unique pullback attractor in higher regularity space for the multivalued process associated with the nonautonomous 2D-Navier-Stokes model with delays and without the uniqueness of solutions.

1. Introduction

It is well known that the Navier-Stokes equations are very important in the understanding of fluids motion and turbulence. These equations have been studied extensively over the last decades (see [13], and the references cited therein). Recently, Caraballo and Real [4] considered global attractors for functional Navier-Stokes models with the uniqueness of solutions and for the delay, so that a wide range of hereditary characteristics (constant or variable delay, distributed delay, etc.) can be treated in a unified way. Very recently, Marín-Rubio and Real [5] used the theory of multivalued dynamical system to establish the existence of attractors for the 2D-Navier-Stokes model with delays, when the forcing term containing the delay is sublinear and only continuous.

For the study of asymptotic behavior for functional partial differential equations without the uniqueness of solutions, as far as we know, not many papers have been published. However, some results in the finite dimensional context can be found in [6, 7] (see also [810] for some preliminary and interesting results on the structure of the attractors for ordinary differential delay systems).

The pullback attractor is a possible approach to define an “attractor” for the nonautonomous dynamical systems, the long time behavior of nonautonomous dynamical systems is an interesting and challenging problem; see, for example, [1119], and so forth. The purpose of our current paper is to study existence of pullback attractors for the following functional Navier-Stokes problem:
()
where Ω2 is an open bounded set with regular boundary Γ, ν > 0 is the kinematic viscosity, u is the velocity field of the fluid, p is the pressure, τ is the initial time, g is a nondelayed external force field, f is another external force term and contains some memory effects during a fixed interval of time of length h > 0, ρ is an adequate given delay function, and ϕ the initial datum on the interval [−h, 0].

Using the technique of measure of noncompactness, noting that all norms on finite dimensional spaces are equivalent, we apply the new method to check the pullback ω-limit compactness given in [20] and then get the existence of the pullback attractors in CV.

We consider the following usual abstract spaces:
()
where H =   the closure of 𝒱 in (L2(Ω)) 2 with norm |·| and inner product (·, ·), where for u, v ∈ (L2(Ω)) 2,
()
where V =   the closure of 𝒱 in with norm ∥·∥ and associated scalar product ((·, ·)), where for ,
()
Note that VHHV, where the injections are dense and compact. We will use ∥·∥* for the norm in V and 〈·, ·〉 for the duality pairing between V and V.
Define the trilinear form b on V × V × V by
()

Now, let us establish some assumptions for (1).

We assume that the given delay function satisfies ρC1(; [0, h]), and there exists a constant ρ* satisfying
()
Furthermore, we suppose that f and g satisfy the following assumptions:
  • (H1)

    f(·, v) : H is measurable for all vH,

  • (H2)

    f(t, ·) : HH is continuous for all t,

  • (H3)

    there exist positive constants k1, k2 such that for any vH,

    ()

  • (H4)

    there exists a fixed δ0 > 0 such that for any δ ∈ (0, δ0), the external force satisfies

    ()

Set A : VV as 〈Au, v〉 = ((u, v)), B : V × VV by 〈B(u, v), w〉 = b(u, v, w), for all u, v, wV. Denote by P the corresponding orthogonal projection P : (L2(Ω)) 2H. We further set A : = −PΔ. The Stokes operator A is self-adjoint and positive from D(A) = V∩(H2(Ω)) 2 to H. The inverse operator is compact. Excluding the pressure, the system (1) can be written in the form
()

2. Preliminaries

Let X be a complete metric space with metric dX(·, ·), and denote by 𝒫(X) the class of nonempty subsets of X. As usual, let us denote by the Hausdorff semidistance between A and B, which are defined by
()
where dist X(a, B) = inf bBdX(a, b). Finally, denote by 𝒩(A, r) the open neighborhood {yX∣dist X(y, A) < r} of radius r > 0 of a subset A of a Banach space X.

Definition 1. A family of mappings  U(t, τ) : X𝒫(X),   tτ,   τ is called to be a multivalued process (MVP in short) if it satisfies

  • (1)

    U(τ, τ)x = {x},   for all τ,   xX;

  • (2)

    U(t, s)U(s, τ)x = U(t, τ)x,   for all tsτ,   τ,   xX.

Let 𝒟 be a nonempty class of parameterized sets 𝒟 = {D(t)} t𝒫(X).

Definition 2. Let {U(t, τ)} be a multivalued process on X. One says that {U(t, τ)} is

  • (1)

    pullback 𝒟-dissipative, if there exists a family 𝒬 = {Q(t)} t𝒟, so that for any = {B(t)} t𝒟 and each t, there exists a t0 = t0(, t) ∈ + such that

    ()

  • (2)

    pullback 𝒟-limit-set compact with respect to each t, if for any = {B(t)} t𝒟 and ε > 0, there exists a t1 = t1(, t, ε) ∈ + such that

    ()

where k is the Kuratowski measure of noncompactness.

Definition 3. A family of nonempty compact subsets 𝒜 = {A(t)} t𝒫(X) is called to be a pullback 𝒟-attractor for the multivalued process {U(t, τ)}, if it satisfies

  • (1)

    𝒜 = {A(t)} t is invariant; that is,

    ()

  • (2)

    𝒜 is pullback 𝒟-attracting; that is, for every 𝒟 and any fixed t,

    ()

Let X, Y be two Banach spaces, and let X*, Y* be their dual spaces, respectively. We also assume that X is a dense subspace of Y, the injection i : XY is continuous, and its adjoint i* : Y*X* is densely injective.

Theorem 4 (see [21], [22].)Let X, Y be two Banach spaces satisfy the previous assumptions, and let {U(t, τ)} be a multivalued process on X and Y, respectively. Assume that {U(t, τ)} is upper semicontinuous or weak upper semicontinuous on Y. If for fixed tτ,   τ, U(t, τ) maps compact subsets of X into bounded subsets of 𝒫(X), then U(t, τ) is norm-to-weak upper semicontinuous on X.

By slightly modifying the arguments of Theorem 3.4 and Remark 3.9 in [21], we have the following.

Theorem 5. Let X be a Banach space, and let {U(t, τ)} be a multivalued process on X. Also let U(t, τ)x be norm-to-weak upper semicontinuous in x for fixed tτ, τ; that is, if xnx, then for any ynU(t, τ)xn, there exist a subsequence and a yU(t, τ)x such that (weak convergence). Then the multivalued process {U(t, τ)} possesses a pullback 𝒟-attractor 𝒜 = {A(t)} t in X given by

()
if and only if {U(t, τ)} is pullback 𝒟-dissipative and pullback 𝒟-limit-set compact with respect to each t, where 𝒬 = {Q(t)} t𝒟 is pullback 𝒟-absorbing for the multivalued process {U(t, τ)}.

A multivalued process {U(t, τ)} is said to be pullback 𝒟-asymptotically upper-semicompact in X if for each fixed t, any = {B(t)} t𝒟, any sequence {Tn} with Tn → +, {xn} with xnB(tTn), and any {yn} with ynU(t, tTn)xn; this last sequence {yn} is relatively compact in X.

Remark 6. Let {U(t, τ)} be a multivalued process on X. Then {U(t, τ)} is pullback 𝒟-asymptotically upper-semicompact if and only if {U(t, τ)} is pullback 𝒟-limit-set compact; see [21].

Let X be a Banach space, and let h > 0 be a given positive number (the delay time). Denote by CX the Banach space C([−h, 0]; X) endowed with the norm
()
Let us consider a class of sets parameterized in time, 𝒟 = {D(t)} t𝒫(CX). To study the pullback 𝒟-limit-set compactness of the multivalued process on CX, we need the following result from [20].

Theorem 7. Let {U(t, τ)} be a multivalued process on CX. Suppose that for each t, any and ε > 0, there exist τ0 = τ0(t, , ε) > 0, a finite dimensional subspace X1 of X, and a δ > 0 such that

  • (1)

    for each fixed θ ∈ [−h, 0],

    ()

  • (2)

    for all sτ0,  ut(·) ∈ U(t, ts)B(ts),  θ1, θ2 ∈ [−h, 0] with |θ2θ1 | < δ,

    ()

  • (3)

    for all sτ0,  ut(·) ∈ U(t, ts)B(ts),

    ()

where P : XX1 is the canonical projector. Then {U(t, τ)} is pullback 𝒟-limit-set compact in CX with respect to each t.

3. Existence of an Absorbing Family of Sets in CV

By the classical Faedo-Galerkin scheme and compactness method, analogous to the arguments in [5], we have the following.

Theorem 8. Let one consider ϕCH, , and assume that f : × HH satisfies the hypotheses (H1)–(H3). Then, for each τ,

  • (a)

    there exists a weak solution u to problem (9) satisfying

    ()

  • (b)

    if ϕCV, then there exists a strong solution u to problem (9); that is,

    ()

Given T > τ and u : [τh, T) → H, for each t ∈ [τ, T), we denote by ut the function defined on [−h, 0] by the relation ut(s) = u(t + s), s ∈ [−h, 0]. We also denote CH = C([−h, 0]; H) and CV = C([−h, 0]; V). Let C be the arbitrary positive constants, which may be different from line to line and even in the same line.

Thanks to Theorem 8, we can define a multivalued process (CV, {U(·, ·)}) as
()

We first need a priori estimates for the solution u of (9) in the space CH and a necessary bound on the term , which will be very useful in our analysis; it relates the absorption property for the multivalued process {U(t, τ)} on CV.

Lemma 9. In addition to the assumptions (H1)–(H4), assume that

()
holds true. Then
()
()
provided that α > 0 is small enough.

Proof. By the energy inequality and the Poincaré inequality, we have

()
We fixed two positive parameters ε1 and ε2 to be chosen later on. Then by (H3) and Young’s inequality, we can deduce that
()
Therefore,
()
Let α > 0 to be determined later on. Then it follows that
()

Integrating between τ and t  (⩾τ), we have

()

Let r = sρ(s); note that ρ(s)∈[0, h] and 1/(1 − ρ(s)) ⩽ 1/(1 − ρ*) for all s. Hence,

()
Combining (30) and (31) together, we get
()
Let ε1 = νλ1/4 and using (23), so we can choose positive constants α and ε2 small enough such that and α < δ0 (where δ0 is given in the assumption (H4)). Then, it follows that
()
Setting now t + θ instead of t (where θ ∈ [−h, 0]), multiplying by eα(t+θ), it holds
()
Note that , thus the conclusion (24) follows immediately from (34).

Finally, we will obtain the bound on the term . It follows from (28) that

()
Integrating from t − 1 to t, we have
()
Similar to the arguments of (31), we can deduce that
()
Recall that ε1 = νλ1/4 and . By (24) and (36)-(37), we have (25) as desired, and thus the proof of this lemma is completed.

By slightly modifying the proof of Lemma  1.1 in [23], we have the following result.

Lemma 10. Let t be given arbitrarily. Let g, h, and y be three positive locally integrable functions on (−, t] such that y is locally integrable on (−, t], which satisfy that

()
where a1, a2, and a3 are positive constants. Then
()

Now we state and prove the main result in this section.

Theorem 11. Suppose in addition to the hypotheses in Lemma 9, assume that

()
holds true. Then the multivalued process {U(t, τ)} on CV is pullback 𝒟-dissipative.

Proof. We take the inner product of (9) with Au(t), we obtain

()
Now we evaluate the terms, using (H3) and Young’s inequality, and we arrive to
()
Next,
()
Thanks to (41)–(43) and the fact that for φD(A), we can deduce that
()
and consequently,
()
Since ε1 = νλ1/4 and , it is easy to see that (νλ1/2) − α > 0. Then
()

Let t be given arbitrarily and taking τ such that tτ + h + 1. In order to apply Lemma 10, in view of (24), now we firstly obtain

()
Then, it follows from (24) and (25) that
()
Combining (25) and (47)-(48) together, by Lemma 10, we can conclude that
()
where
()
Therefore, if we take τ such that tτ + 1 + 2h, then similar to the above mentioned, we get
()

We denote by the set of all functions r : → (0, +) such that

()
and denote by the class of all families 𝒟 = {D(t)} t𝒫(CV) such that , for some r𝒟, where 𝒫(CV) denotes the family of all nonempty subsets of CV and denotes the closed ball in CV centered at zero with radius r𝒟(t).

Denote by R(t) the nonnegative number given for each t by

()
and consider the family of closed balls 𝒬 = {Q(t)} t in CV defined by
()
It is straightforward to check that , and moreover, by (51) and (52), the family of 𝒬 is pullback 𝒟-absorbing for the multivalued process {U(t, τ)} on CV.

The proof of Theorem 11 is completed.

4. Existence of the Pullback Attractors in CV

Theorem 12. Suppose in addition to the hypotheses in Theorem 11 that gC(; H). Then there exists a unique pullback 𝒟-attractor for the multivalued process {U(t, τ)} in CV.

Proof. Since A−1 is a continuous compact operator in H, by the classical spectral theory, there exist a sequence ,

()
and a family of elements of D(A) which are orthonormal in H such that
()
Let Vm = span {w1, …, wm} in V and Pm : VVm be an orthogonal projector.

Let u = u1 + u2, where u1 = Pmu and u2 = (IPm)u. We decompose (9) as follows:

()
()
We divide the proof into three steps.

(1) For every fixed t, any and ε > 0, we observe that for any Tts with s⩾0,

()
Taking the inner product in H of (57) with Au2 = A(IPm)u, we get
()
By (H3) and Young’s inequality, we have
()
To estimate (B(u(T), u(T)), Au2(T)), we recall some inequalities [19]:
()
and thus
()
Note that , and set L = 1 + log  (λm+1/λ1). Then by Young’s inequality, we can deduce that
()
By (60)–(64) and Poincaré inequality, we obtain
()
Applying the Gronwall’s lemma in the interval [ts, t + θ], it yields
()
Let ε > 0 be given arbitrarily. Note that gC(; H), then we can take m + 1 large enough such that for any fixed η > 0,
()
()
Combining (67) and (68) together, we can get for m + 1 large enough,
()
On the other hand, thanks to Lemma 9 and Theorem 11, we can deduce that when m + 1 and s are large enough,
()
Thanks to (69) and (70), it follows from (66) that when m + 1 and s are large enough,
()

(2) Now we consider the ordinary functional differential system (58) and check the condition (2) in Theorem 7. Note that . Without generality, we assume that θ1, θ2 ∈ [−h, 0] with 0 < θ1θ2 < 1. Hence

()
Notice that
()
Then, it follows from (H3), (H4), and (24) that
()
Since gC(; H) and t is fixed,
()
Equations (74)–(75) imply that the condition (2) in Theorem 7 is proved.

(3) Invoking Theorem 7, in view of the previous arguments and Theorem 11, we can see that the multivalued process {U(t, τ)} is pullback 𝒟-limit-set compact and pullback 𝒟-dissipative in CV.

In order to get the existence of pullback 𝒟-attractors, by the proof of Theorem 3.2 in [21], now we only need to show the negative invariance of , where

()
and is a pullback 𝒟-absorbing set of {U(t, τ)} in CV.

Let . Then there exist sequences sn+,  sn → +  (n), xnQ(tsn), and ynU(t, tsn)xn such that

()
On the other hand, for n sufficiently large,
()
Then by the pullback 𝒟-limit-set compactness of the multivalued process {U(t, τ)}, there is a subsequence of , which we still relabel as such that and
()
Clearly, .

We observe that yn is bounded in CV for n sufficiently large. Then by slightly modifying the proof of the existence of solutions (see [16] for details), in view of Theorem 2.11 in [21], we can see that

()
This together with (77)–(79), we can deduce that , and thus the proof of Theorem 12 is finished.

Acknowledgments

This research was supported by the National Natural Science Foundation of China under Grant no. 10801066 and the Fundamental Research Funds for the Central Universities under Grant no. lzujbky-2011-47 and no. lzujbky-2012-k26. The Project was sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

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