Abstract
Our aim in this paper is to study the existence of pullback attractors for the 3D NavierStokesVoigt equations with delays. The forcing term containing the delay is sublinear 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 multivalued dynamical system.
Keywords:
3D NavierStokesVoigt equations; pullback attractors; delay terms; multivalued process1 Introduction
Let be an open, bounded and connected set. We consider the following problem for threedimensional NavierStokesVoigt (NSV) equations with delays in continuous and sublinear operators:
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 threedimensional NavierStokes (NS) equation. In the past decades, many authors [16] investigated intensively the classical threedimensional 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 KleinVoigt viscoelastic incompressible fluid. Oskolkov [7] was the first to introduce the system which gives an approximate description of the KelvinVoigt fluid (see, e.g., [8,9]). In 2010, Levant et al.[10] investigated numerically the statistical properties of the NavierStokesVoigt model. Kalantarov and Titi [11] studied a global attractor of a semigroup generated by equation (1.1) for the autonomous case. Recently, Luengo et al.[12] obtained asymptotic compactness by using the energy method, and they further got the existence of pullback attractors for threedimensional nonautonomous NSV equations.
Let us recall some related results in the literature. Yue and Zhong [13] studied the long time behavior of the threedimensional 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 nonautonomous NSV equations using the energy method. As we know from [15], delay terms appear naturally. In recent years, Caraballo and Real [1618] developed a fruitful theory of existence, uniqueness, stability of solutions and global attractors for NavierStokes 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 [19] and by MarínRubio and Real [20]. Gal and Medjo [21] proved the existence of uniform global attractors for a NavierStokesVoigt model with memory. As commented before, in comparison with threedimensional NavierStokes equations, there is no regularizing effect. Our result of this paper is to establish the existence of pullback attractors for threedimensional NSV equations in when the external forcing term and the function is continuous with respect to u. Another difficulty is to obtain that the multivalued processes are asymptotically compact. In 2007, Kapustyan and Valero [22] presented a method suitable for verifying the asymptotic compactness. The authors [20] applied this method to 2D NavierStokes equations with delays in continuous and sublinear operators. We shall apply the energy method to prove that our multivalued 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).
2 Basic theory of pullback attractors
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 multivalued processes. On the other hand, since our model is nonautonomous, we should use the related results for classical multivalued processes in [23,24], but which are not completely adapted to our model.
Let be a metric space, and let be the class of nonempty subsets of X. As usual, we denote by the Hausdorff semidistance in X between and , i.e.,
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 multivalued dynamical system associated with (1.1).
Definition 2.1 A multivalued process U is a family of mappings for any pair of real numbers such that
If the above relation is not only an inclusion but also an equality, we say that the multivalued process is strict. For example, the relation generalized by 3D nonautonomous NSV equation (see, e.g., [12]) is an equality, while the relation generalized by 3D NS equations (see, e.g., [22]) is strict.
Definition 2.2 Suppose that is a family of sets. A multivalued process U is said to be asymptotically compact if for any , any sequences with , , and , the sequence is relatively compact in X.
Lemma 2.1If a multivalued processUisasymptotically compact, then the setsare nonempty compact subsets ofX, where
Furthermore, attracts in a pullback sense toat timet, i.e.,
Indeed, it is the minimal closed set with this property.
Definition 2.3 The family of subsets is said to be pullbackabsorbing with respect to a multivalued process U if for every and all bounded subset B of X, there exists a time such that
Lemma 2.2Let the family of setsbe pullbackabsorbing for the multivalued processU, and letUbeasymptotically compact. Then, for any bounded setsBofX, it holds that
Definition 2.4 Suppose that U is a multivalued process. A family is said to be a pullback attractor for a multivalued process U if the set is compact for any and attracts at time t to any bounded set in a pullback sense, i.e.,
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[25]
Assume thatUis a multivalued process, andUisasymptotically compact and a familyis pullbackabsorbing forU. Then, for anyand any bounded subsetBofX, the set
is a nonempty compact subset contained in, which attracts toBin a pullback sense. In fact, defined above is the minimal closed set with this property.
Furthermore, for any bounded setB, the setis a pullback attractor. From Definitions 2.3 and 2.4, it is easy to see that.
If there exists a timesuch thatis bounded, for boundedB, then
As we know, for the singlevalued processes, the continuity of processes provides invariance; while in the multivalued processes, the upper semicontinuity (defined below) provides negatively invariance of the omega limit sets and the attractor. The following is the definition of the upper semicontinuity of the multivalued processes (see in [22]).
Definition 2.5 Let U be a multivalued process on X. It is said to be upper semicontinuous if for all , the mapping is upper semicontinuous 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[22]
Assume that a multivalued processUand a family. If, in addition, isasymptotically compact and upper semicontinuous, then the familyis negatively invariant, i.e.,
whereBis a bounded setBofX. The familyis also negatively invariant and the familydefined in Lemma 2.3 is also negatively invariant.
Lemma 2.5[22]
Given a universe, if a multivalued processUisasymptotically compact, then, for anyand for any, the omega limit setis a nonempty compact set ofXthat attracts toat timetin a pullback sense. Indeed, it is the minimal closed set with this property. If, in addition, the multivalued processUis upper semicontinuous, thenis negatively invariant.
From the above lemmas, we obtain the following results which are rather similar to Theorem 3 in [20]. We only sketch it here.
Lemma 2.6Suppose thatis a universe andis pullbackabsorbing for a multivalued processU, which is alsoasymptotically compact. Then the results in Lemma 2.5 hold. Furthermore, the family, where, and the following results hold:
(1) For each, the setdefined above is compact.
(3) SupposeUis upper semicontinuous, thenis negatively invariant.
(5) Assume, the minimal family of closed setsattracts pullback to elements of.
(6) Assume, eachis closed and the universeis inclusionclosed, thenand it is the only family ofwhich satisfies the above properties (1), (2) and (3).
(7) Ifcontains the families of fixed bounded sets, thendefined in Lemma 2.3 is the minimal pullback attractor of bounded sets, andfor each. In addition, if there exists somesuch thatis bounded, thenfor all.
3 Introduction to some abstract spaces and the existence of solutions
We first recall some notations about the function spaces which will be used later to discuss the regularity of pullback attracting sets. Let us consider the following abstract space:
The symbols H, V denote the closures of in , , respectively. In other words, H= the closure of in with the norm and the inner product , where for ,
V= the closure of in with the norm associated to the inner product , where for ,
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.
Define for all , where P is the orthoprojector from onto H. Considering the properties of the operator A, we have as
We define
for every function u, v, , and the operator as
Obviously, is a continuous trilinear form such that
which yields
Moreover, b and B satisfy the following:
Now, we make some assumptions. The given delay function ρ satisfies , and there is a constant independent of t satisfying
Moreover, we assume that satisfies the following assumptions:
(H3) There exist two functions . The above and for all , for , such that , , .
As to the initial datum, we assume
Next, we shall consider the solution of (1.1).
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 [12], we have .
Now, we define a functional , where is a differentiable and nonnegative strictly increasing function given by . We have
From the above analysis, taking into account for all , we obtain that . Hence, it is clear that u is a weak solution to (1.1) if , for all , and satisfies the energy equality
in the distributional sense on .
Theorem 3.1Suppose that (H1)(H4) hold. Then there exists a global solution u to (3.2).
Proof We shall prove the result by the FaedoGalerkin 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.
Denote , where , , are unknown real functions satisfying the finitedimensional problem
with . From [12], we can obtain the local wellposedness of this finitedimensional delay problem. The following provides estimates which imply that the solutions are well defined in the whole .
By (3.4), we obtain
Multiplying (3.5) by , summing from to , and using the properties of the operator b, we easily get
Observing (H3) and using the Young inequality, we have
Considering the above inequality in (3.6) and observing that
we easily get
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 [12].
By the compactness of the injection of V into H, using the above estimates and the AscoliArzelà theorem, we deduce that there exist a subsequence (we relabel the same) and for any with in . Recalling (3.9) and the above analysis, we have
By the properties of operator A and (3.10), we deduce that weakly in . Reasoning as in [26] on page 76, we deduce that weakly in for any .
On the other hand, observing that by (3.11), (H2) and the hypothesis on ρ in (3.1), for any time t, we conclude for any
and as is bounded in the space obtained from (3.9), by (H3), we deduce
where . Thus we can easily derive from (3.12) and (3.13)
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. □
Remark 3.1 We can obtain the uniqueness if there are additional assumptions on the forcing term g. For example, if we suppose that g satisfies (H1), (H3), for an arbitrary and , then for the solutions u, v,
then we obtain the uniqueness of solutions.
4 Existence of pullback attractors
In this section, we discuss the existence of pullback attractors for the 3D NavierStokesVoight equations with delays in continuous and sublinear operators. At first, we propose the assumptions for g given in Section 3:
(H1^{′}) For all , is measurable.
(H2^{′}) For all , is continuous.
(H3^{′}) There are two nonnegative functions with for some and such that for any ,
To construct a multivalued 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 .
By Theorem 3.1, we know there exists a solution to problem (3.2) although we have no discussion on the uniqueness of solutions to problem (3.2). We may define two strict processes, as
Considering the regularity of the problem, the asymptotic behavior of the two processes shall be the same, as we shall see in what follows.
In order to simplify the calculation form, we introduce a function . For any , we set
From (4.1), we can find that
Lemma 4.1Suppose that (H1′)(H3′) hold, for any initial datumand any, it holds
Proof Let u be a solution of (3.2), so . Multiplying (1.1) by and using the energy equality and the Poincaré inequality, we have
where we have denoted
Considering
and integrating (4.5) from τ to t, we deduce
where we set
Observing the above estimates, we easily deduce
Applying the Poincaré inequality and the Gronwall inequality to (4.8), we deduce that (4.4) holds. This finishes the proof of this lemma. □
Next, we shall prove that the processes and defined above are pullbackabsorbing. To obtain this, we propose the assumptions
(H4′)
and the relation among constants , d defined above, and α in (1.1) satisfies
(H5′)
(H6^{′})
where the function is given by (4.1).
Before proving that the two multivalued processes possess pullbackabsorbing sets, we introduce the definition of the two natural tempered universes which shall play the key role for our main purpose.
Definition 4.1 Suppose that is the collection of the sets of all functions such that
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 inclusionclosed, 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 pullbackabsorbing sets for the two processes mentioned above.
Lemma 4.2Suppose that (H1^{′})(H6^{′}) hold and the constantsα, d, , satisfyand.
(1) Then, for anyand any family, there exitssuch that any initial datumand anyfor anysatisfy that, where the positive continuous functionis given by
(2) Letbe includedwhich is given by
Then the setand ispullback absorbing for the process. Therefore, ispullback absorbing for the process.
Proof Since the proof is a consequence of the definition of , we only sketch it here. From Lemma 4.1, (H5^{′}) and (H6^{′}) , we have
We complete the proof of (1). Estimate (2) is a consequence of (1). □
For the two processes and , they possess pullbackabsorbing sets. In order to apply Lemma 2.3 to obtain the existence of pullback attractors, it is necessary to prove that the two multivalued processes are asymptotically compact. This will be done in the following lemma.
Lemma 4.3Suppose that the assumptions in Lemma 4.2 hold. The two processesandareasymptotically compact.
Proof For any , a sequence with and a sequence with with , we shall prove that the sequence is relatively compact in . By the properties concerning operator b mentioned in Section 3, we deduce that
It is easy to get the above estimate which is independent of n. The sequences of and possess their subsequence, relabeled the same in suitable spaces such that there exist and satisfying
According to the assumptions on a function g and analogously as in Theorem 3.1, we deduce that strongly in V a.e. .
By the Lebesgue theorem and the uniform estimate of in , we deduce that the function converges to the function strongly. Therefore, for any , we have and
The uniform estimate of in implies that the sequence is equicontinuous in for any . In addition, the sequence is bounded, which is independent of n in . Using the AscoliArzelà theorem, we can obtain
From the uniform boundedness of the sequence in , for any , we can also obtain , weakly in V.
By the analogous argument, for any compact sequence and , we obtain that the sequence is convergent to weakly in V. To achieve our result in Lemma 4.3, we only need to prove
The proof is slightly different from Proposition 6 in [20] or in [22]. 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 .
From the weak convergence of in V, we get
Therefore we have to check that
From the energy equality, for any , we obtain
where or . For any , define the continuous functions and as
By (4.14), it is clear that J and are nonincreasing functions. By the convergence (4.10), for any , we have that . Using the same analysis method as in [22], 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 4.4Suppose that (H1′)(H6′) hold. The two processesandare semicontinuous and thatandhave compact values in their respective topologies.
Proof In fact, the upper semicontinuity 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 energyprocedure 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.
Theorem 4.1Suppose that (H1′)(H6′) hold. For any, then there exist global pullback attractorsandfor the processin the universe of fixed bounded sets and in, respectively. Moreover, they are unique in the sense of Lemma 2.5 and negatively and strictly invariant forUrespectively, and the following holds:
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.
Theorem 4.2Suppose that the assumptions in Theorem 4.1 hold. For any, there exist global pullback attractorsandfor the processin the universes of fixed bounded sets and in. They are unique in the sense of Lemma 2.5 and negatively and strictly invariant forU, respectively, and we have. Moreover, the relationship between the attractors forand foris as follows:
whereis the continuous mapping defined by.
Proof The proof is rather similar to that of Theorem 5 in [20]. Since the regularity is different from [20], we only sketch the proof of (4.15) here.
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. □
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
Haiyan carried out the main part of this manuscript. Yuming participated in the discussion and corrected the main theorems. All authors read and approved the final manuscript.
Acknowledgements
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. CUSFDHD2013068).
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