Pullback Attractors for Nonautonomous 2D-Navier-Stokes Models with Variable Delays
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 [1–3], 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 [8–10] for some preliminary and interesting results on the structure of the attractors for ordinary differential delay systems).
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.
Now, let us establish some assumptions for (1).
- (H1)
f(·, v) : ℝ → H is measurable for all v ∈ H,
- (H2)
f(t, ·) : H → H is continuous for all t ∈ ℝ,
- (H3)
there exist positive constants k1, k2 such that for any v ∈ H,
() - (H4)
there exists a fixed δ0 > 0 such that for any δ ∈ (0, δ0), the external force satisfies
()
2. Preliminaries
Definition 1. A family of mappings U(t, τ) : X → 𝒫(X), t⩾τ, τ ∈ ℝ is called to be a multivalued process (MVP in short) if it satisfies
- (1)
U(τ, τ)x = {x}, for all τ ∈ ℝ, x ∈ X;
- (2)
U(t, s)U(s, τ)x = U(t, τ)x, for all t⩾s⩾τ, τ ∈ ℝ, x ∈ X.
Let 𝒟 be a nonempty class of parameterized sets 𝒟 = {D(t)} t∈ℝ ⊂ 𝒫(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)} t∈ℝ ∈ 𝒟, so that for any ℬ = {B(t)} t∈ℝ ∈ 𝒟 and each t ∈ ℝ, there exists a t0 = t0(ℬ, t) ∈ ℝ+ such that
() - (2)
pullback 𝒟-limit-set compact with respect to each t ∈ ℝ, if for any ℬ = {B(t)} t∈ℝ ∈ 𝒟 and ε > 0, there exists a t1 = t1(ℬ, t, ε) ∈ ℝ+ such that
()
Definition 3. A family of nonempty compact subsets 𝒜 = {A(t)} t∈ℝ ⊂ 𝒫(X) is called to be a pullback 𝒟-attractor for the multivalued process {U(t, τ)}, if it satisfies
- (1)
𝒜 = {A(t)} t∈ℝ is invariant; that is,
() - (2)
𝒜 is pullback 𝒟-attracting; that is, for every ℬ ∈ 𝒟 and any fixed t ∈ ℝ,
()
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 : X↪Y 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 t⩾τ, τ ∈ ℝ, 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 t⩾τ, τ ∈ ℝ; that is, if xn → x, then for any yn ∈ U(t, τ)xn, there exist a subsequence and a y ∈ U(t, τ)x such that (weak convergence). Then the multivalued process {U(t, τ)} possesses a pullback 𝒟-attractor 𝒜 = {A(t)} t∈ℝ in X given by
A multivalued process {U(t, τ)} is said to be pullback 𝒟-asymptotically upper-semicompact in X if for each fixed t ∈ ℝ, any ℬ = {B(t)} t∈ℝ ∈ 𝒟, any sequence {Tn} with Tn → +∞, {xn} with xn ∈ B(t − Tn), and any {yn} with yn ∈ U(t, t − Tn)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].
Theorem 7. Let {U(t, τ)} be a multivalued process on CX. Suppose that for each t ∈ ℝ, any and ε > 0, there exist τ0 = τ0(t, ℬ, 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 s⩾τ0, ut(·) ∈ U(t, t − s)B(t − s), θ1, θ2 ∈ [−h, 0] with |θ2 − θ1 | < δ,
() - (3)
for all s⩾τ0, ut(·) ∈ U(t, t − s)B(t − s),
()
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 ϕ ∈ CH, , and assume that f : ℝ × H → H satisfies the hypotheses (H1)–(H3). Then, for each τ ∈ ℝ,
Given T > τ and u : [τ − h, 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.
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
Proof. By the energy inequality and the Poincaré inequality, we have
Integrating between τ and t (⩾τ), we have
Let r = s − ρ(s); note that ρ(s)∈[0, h] and 1/(1 − ρ′(s)) ⩽ 1/(1 − ρ*) for all s ∈ ℝ. Hence,
Finally, we will obtain the bound on the term . It follows from (28) that
By slightly modifying the proof of Lemma 1.1 in [23], we have the following result.
Lemma 10. Let t ∈ ℝ 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
Now we state and prove the main result in this section.
Theorem 11. Suppose in addition to the hypotheses in Lemma 9, assume that
Proof. We take the inner product of (9) with Au(t), we obtain
Let t ∈ ℝ be given arbitrarily and taking τ such that t⩾τ + h + 1. In order to apply Lemma 10, in view of (24), now we firstly obtain
We denote by ℛ the set of all functions r : ℝ → (0, +∞) such that
Denote by R(t) the nonnegative number given for each t ∈ ℝ by
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 g ∈ C(ℝ; 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 ,
Let u = u1 + u2, where u1 = Pmu and u2 = (I − Pm)u. We decompose (9) as follows:
(1) For every fixed t ∈ ℝ, any and ε > 0, we observe that for any T⩾t − s with s⩾0,
(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 < θ1 − θ2 < 1. Hence
(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
Let . Then there exist sequences sn ∈ ℝ+, sn → +∞ (n → ∞), xn ∈ Q(t − sn), and yn ∈ U(t, t − sn)xn such that
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
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.