Pullback attractors for three-dimensional Navier-Stokes-Voigt equations with delays
© Li and Qin; licensee Springer 2013
Received: 25 April 2013
Accepted: 6 August 2013
Published: 27 August 2013
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© Li and Qin; licensee Springer 2013
Received: 25 April 2013
Accepted: 6 August 2013
Published: 27 August 2013
Our aim in this paper is to study the existence of pullback attractors for the 3D Navier-Stokes-Voigt equations with delays. The forcing term containing the delay is sub-linear and continuous with respect to u. Since the solution of the model is not unique, which is caused by the continuity assumption, we establish the existence of pullback attractors for our problem by using the theory of multi-valued dynamical system.
Here is the velocity vector field, ν is a positive constant, α is a characterizing parameter of the elasticity of the fluid, p is the pressure, g is the external force term which contains memory effects during a fixed interval of time of length , is an adequate given delay function, is the initial velocity field at the initial time , φ is the initial datum on the interval .
Equation (1.1) with becomes the classical three-dimensional Navier-Stokes (NS) equation. In the past decades, many authors [1–6] investigated intensively the classical three-dimensional incompressible NS equation. For the sake of direct numerical simulations for NS equations, the NSV model of viscoelastic incompressible fluid has been proposed as a regularization of NS equations.
Equation (1.1) governs the motion of a Klein-Voigt viscoelastic incompressible fluid. Oskolkov  was the first to introduce the system which gives an approximate description of the Kelvin-Voigt fluid (see, e.g., [8, 9]). In 2010, Levant et al.  investigated numerically the statistical properties of the Navier-Stokes-Voigt model. Kalantarov and Titi  studied a global attractor of a semigroup generated by equation (1.1) for the autonomous case. Recently, Luengo et al.  obtained asymptotic compactness by using the energy method, and they further got the existence of pullback attractors for three-dimensional non-autonomous NSV equations.
Let us recall some related results in the literature. Yue and Zhong  studied the long time behavior of the three-dimensional NSV model of viscoelastic incompressible fluid for system (1.1) by using a useful decomposition method. The authors in [12, 14] deduced the existence of -pullback attractors for 3D non-autonomous NSV equations using the energy method. As we know from , delay terms appear naturally. In recent years, Caraballo and Real [16–18] developed a fruitful theory of existence, uniqueness, stability of solutions and global attractors for Navier-Stokes models including some hereditary characteristics such as constant, variable delay, distributed delay, etc. However, our present problem has no uniqueness of solutions. To overcome the difficulty, we may cite the results by Ball  and by Marín-Rubio and Real . Gal and Medjo  proved the existence of uniform global attractors for a Navier-Stokes-Voigt model with memory. As commented before, in comparison with three-dimensional Navier-Stokes equations, there is no regularizing effect. Our result of this paper is to establish the existence of pullback attractors for three-dimensional NSV equations in when the external forcing term and the function is continuous with respect to u. Another difficulty is to obtain that the multi-valued processes are asymptotically compact. In 2007, Kapustyan and Valero  presented a method suitable for verifying the asymptotic compactness. The authors  applied this method to 2D Navier-Stokes equations with delays in continuous and sub-linear operators. We shall apply the energy method to prove that our multi-valued processes are asymptotically compact by making some minor modifications caused by the term in (1.1).
This paper is organized as follows. In Section 2, we recall briefly some results on the abstract theory of pullback attractors. In Section 3, we introduce some abstract spaces necessary for the variational statement of the problem and give the proof of the global existence of solutions. In Section 4, we consider the asymptotic behavior of problem (1.1).
By using the framework of evolution processes, thanks to [20, 23, 24], we now briefly recall some theories of pullback attractors and the related results. On the one hand, we have to overcome some difficulties caused by multi-valued processes. On the other hand, since our model is non-autonomous, we should use the related results for classical multi-valued processes in [23, 24], but which are not completely adapted to our model.
where denotes the distance between two points x and y in X.
We now formulate an abstract result in order to establish the existence of pullback attractors for the multi-valued dynamical system associated with (1.1).
If the above relation is not only an inclusion but also an equality, we say that the multi-valued process is strict. For example, the relation generalized by 3D non-autonomous NSV equation (see, e.g., ) is an equality, while the relation generalized by 3D NS equations (see, e.g., ) is strict.
Definition 2.2 Suppose that is a family of sets. A multi-valued process U is said to be -asymptotically compact if for any , any sequences with , , and , the sequence is relatively compact in X.
Indeed, it is the minimal closed set with this property.
We can see obviously that a pullback attractor does not need to be unique. However, it can be considered unique in the sense of minimal, that is, the minimal closed family with such a property. In this sense, we obtain the following property and the existence of pullback attractors.
Lemma 2.3 
is a nonempty compact subset contained in , which attracts to B in a pullback sense. In fact, defined above is the minimal closed set with this property.
Furthermore, for any bounded set B, the set is a pullback attractor. From Definitions 2.3 and 2.4, it is easy to see that .
As we know, for the single-valued processes, the continuity of processes provides invariance; while in the multi-valued processes, the upper semi-continuity (defined below) provides negatively invariance of the omega limit sets and the attractor. The following is the definition of the upper semi-continuity of the multi-valued processes (see in ).
Definition 2.5 Let U be a multi-valued process on X. It is said to be upper semi-continuous if for all , the mapping is upper semi-continuous from X into , that is to say, given a converging sequence , for some sequence such that for all n, there exists a subsequence of converging in X to an element of .
Lemma 2.4 
where B is a bounded set B of X. The family is also negatively invariant and the family defined in Lemma 2.3 is also negatively invariant.
Lemma 2.5 
Given a universe , if a multi-valued process U is -asymptotically compact, then, for any and for any , the omega limit set is a nonempty compact set of X that attracts to at time t in a pullback sense. Indeed, it is the minimal closed set with this property. If, in addition, the multi-valued process U is upper semi-continuous, then is negatively invariant.
From the above lemmas, we obtain the following results which are rather similar to Theorem 3 in . We only sketch it here.
For each , the set defined above is compact.
attracts pullback to any .
Suppose U is upper semi-continuous, then is negatively invariant.
Assume , the minimal family of closed sets attracts pullback to elements of .
Assume , each is closed and the universe is inclusion-closed, then and it is the only family of which satisfies the above properties (1), (2) and (3).
If contains the families of fixed bounded sets, then defined in Lemma 2.3 is the minimal pullback attractor of bounded sets, and for each . In addition, if there exists some such that is bounded, then for all .
We shall use to denote the norm of . The value of a functional from on an element from V is denoted by brackets . It follows that , and the injections are dense and compact.
Moreover, we assume that satisfies the following assumptions:
(H1) is measurable for all .
(H2) For all , is continuous.
(H3) There exist two functions . The above and for all , for , such that , , .
As to the initial datum, we assume
(H4) , and , where .
Definition 3.1 It is said that u is a weak solution to (1.1) if u belongs to for all such that coincides with in and satisfies equation (3.2) in for all .
If u is a solution of (3.2), then it is easy to get , and for all . From the property of the operator A, we have . On the other hand, reasoning as in , we have .
in the distributional sense on .
Theorem 3.1 Suppose that (H1)-(H4) hold. Then there exists a global solution u to (3.2).
Proof We shall prove the result by the Faedo-Galerkin scheme and compactness method. For convenience and without loss of generality, we set the initial time . As to different value τ, we only proceed by translation.
Consider the Hilbert basis of H formed by the eigenfunctions of the above operator A, i.e., . In fact, these elements allow to define the operator , which is the orthogonal projection of H and V in with their respective norms.
with . From , we can obtain the local well-posedness of this finite-dimensional delay problem. The following provides estimates which imply that the solutions are well defined in the whole .
From the above inequality and the Gronwall inequality, one has that is bounded in , also in , for any . Observing (3.5), we know that the sequence is bounded in for all . The reason is the same as in .
By the properties of operator A and (3.10), we deduce that weakly in . Reasoning as in  on page 76, we deduce that weakly in for any .
From the above discussion, passing to the limit, we prove that u is a global solution of (3.2) in the sense of Definition 3.1. □
then we obtain the uniqueness of solutions.
In this section, we discuss the existence of pullback attractors for the 3D Navier-Stokes-Voight equations with delays in continuous and sub-linear operators. At first, we propose the assumptions for g given in Section 3:
(H1′) For all , is measurable.
(H2′) For all , is continuous.
To construct a multi-valued process, we introduce symbols and , where as two phase spaces. Let denote the set of global solutions of (1.1) in and the initial datum .
Considering the regularity of the problem, the asymptotic behavior of the two processes shall be the same, as we shall see in what follows.
Applying the Poincaré inequality and the Gronwall inequality to (4.8), we deduce that (4.4) holds. This finishes the proof of this lemma. □
where the function is given by (4.1).
Before proving that the two multi-valued processes possess pullback-absorbing sets, we introduce the definition of the two natural tempered universes which shall play the key role for our main purpose.
Let be the class of all families such that for some . In the same way, let denote the class of all families satisfying for some .
Let be any fixed bounded subset of . Observing that , which is inclusion-closed, by (H4′) and (H5′), we deduce that the family is contained in . With regard to , we use the same method and obtain a similar conclusion if is included in .
The following lemma provides that there exist pullback-absorbing sets for the two processes mentioned above.
Then the set and is -pullback absorbing for the process . Therefore, is -pullback absorbing for the process .
We complete the proof of (1). Estimate (2) is a consequence of (1). □
For the two processes and , they possess pullback-absorbing sets. In order to apply Lemma 2.3 to obtain the existence of pullback attractors, it is necessary to prove that the two multi-valued processes are asymptotically compact. This will be done in the following lemma.
Lemma 4.3 Suppose that the assumptions in Lemma 4.2 hold. The two processes and are -asymptotically compact.
According to the assumptions on a function g and analogously as in Theorem 3.1, we deduce that strongly in V a.e. .
From the uniform boundedness of the sequence in , for any , we can also obtain , weakly in V.
The proof is slightly different from Proposition 6 in  or in . We only sketch it here. We use a contradiction argument. Suppose that it is not true, then there would exist a value ε, a sequence (relabeled the same) , and with satisfying for all . We shall see in V. In order to achieve the last claim, because the sequence is weakly convergent to in V, we only need the convergence of the norms above. In other words, as .
By (4.14), it is clear that J and are non-increasing functions. By the convergence (4.10), for any , we have that . Using the same analysis method as in , we can deduce that for , , which gives (4.13) as desired. □
We can apply the technical method for any family in in Lemma 4.3. Suppose that the assumptions in Lemma 4.3 hold. We can deduce that the processes and are -asymptotically compact and -asymptotically compact.
Lemma 4.4 Suppose that (H1′)-(H6′) hold. The two processes and are semi-continuous and that and have compact values in their respective topologies.
Proof In fact, the upper semi-continuity of the process can be obtained by similar arguments to those used for the Galerkin sequence in Theorem 3.1.
As to the process , applying the same energy-procedure in Lemma 4.3, we shall obtain that in any set of solutions possesses a converging subsequence in this process, whence the assertion in Lemma 4.4 follows. □
According to the results in Section 2, the following two theorems shall be obtained, which are our result in this paper. Observing Lemmas 4.3 and 4.4, and applying Lemma 2.6, we obtain the following theorem.
The above theorem proves that there exist pullback attractors in the framework, while we shall prove that there exist pullback attractors in the framework in the following theorem.
where is the continuous mapping defined by .
By Theorem 3.1, we can conclude that maps into bounded sets in if , and also maps bounded sets from into bounded sets of .
Noting that is the minimal closed set, and using Lemma 2.5 and the above arguments, we deduce that also attracts bounded sets in in a pullback sense. Therefore the inclusion holds.
As to the opposite inclusion of the first identification in (4.15), for any bounded set B, it follows from the continuous injection and the attractor . Thus , whence the opposite inclusion of the first identification in (4.15) holds.
Analogously, it is obvious that the second relation in (4.15) holds. □
This work was supported in part by the NNSF of China (No. 11271066, No. 11031003), by the grant of Shanghai Education Commission (No. 13ZZ048), by Chinese Universities Scientific Fund (No. CUSF-DH-D-2013068).
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.