Stochastic Navier-Stokes Equations with Artificial Compressibility in Random Durations
Abstract
The existence and uniqueness of adapted solutions to the backward stochastic Navier-Stokes equation with artificial compressibility in two-dimensional bounded domains are shown by Minty-Browder monotonicity argument, finite-dimensional projections, and truncations. Continuity of the solutions with respect to terminal conditions is given, and the convergence of the system to an incompressible flow is also established.
1. Introduction
The Navier-Stokes equation (NSE for short), named in honor of Navier and Stokes, who were responsible for its formulation, is an acknowledged model for equation of motion for Newtonian fluid. It is closely connected to the theory of hydrodynamic turbulence, the time dependent chaotic behavior seen in many fluid flows.
The well-posedness of the Navier-Stokes equation has been studied extensively by Ladyzhenskaya [1], Constantin and Foias [2], and Temam [3], among others. Although some ingenious approaches have been made, the problem has not been fully understood. The nonlinearity, part of the cause of turbulence, made the problem extraordinarily difficult. In hope of taking advantage of the noise, randomness has been introduced into the system and some pioneer work has been done by Flandoli and Gatarek [4], Mikulevicius and Rozovsky [5], Menaldi and Sritharan [6], and others. Although the introduction of randomness is not very successful in overcoming the difficulty, it provides a more realistic model than deterministic Navier-Stokes equations and is interesting in itself.
The usual method of proving existence and uniqueness of solutions by fixed point arguments does not apply to the stochastic system on hand since the drift coefficient in the backward stochastic Navier-Stokes equation is nonlinear, non-Lipschitz and unbounded. The drift coefficient is monotone on bounded L4(G) balls in V, which was first observed by Menaldi and Sritharan [6]. The method of monotonicity is used in this paper to prove the existence of solutions to BSNSEs. The proof of the uniqueness and continuity of solutions also relies on the monotonicity assumption of the coefficients. Existence and uniqueness of solutions are shown to hold under the boundedness on the terminal values.
The structure of the paper is as follows. The functional setup of the paper is introduced and several frequently used inequalities are listed in Section 2. The a priori estimates for the solutions of projected BSNSEs are given under different assumptions of the terminal conditions and external body force in Section 3. The existence and uniqueness of solutions of projected BSNSEs are shown in Section 4. Also the existence of solutions of BSNSEs under suitable assumptions is shown by Minty-Browder monotonicity argument. The uniqueness of the solution under the assumption that terminal condition is uniformly bounded in H1 sense is given in Section 5. The continuity of solutions and the convergence as ɛ approaches zero are also studied.
2. Preliminaries
Denote by (·, ·) the inner product of 𝕃2, the inner product of , ℍ−1 the dual space of , and 〈·, ·〉 the duality pairing between and ℍ−1. Let |·| be the norm of 𝕃2 and let ∥·∥ be the norm of . Without causing any confusion, we also use the same notations to denote the norms of L2 and . For any x∈𝕃2 and y∈, there exists x′∈ℍ−1, such that (x, y) = 〈x′, y〉. Then the mapping x↦x′ is linear, injective, compact and continuous. A similar result holds for H−1 and L2.
Suppose that (Ω, ℱ, P) is a complete probability space. Let 𝕎(t) be an 𝕃2-valued ℚ-Wiener process, where ℚ is a trace class operator on 𝕃2. Let be a complete orthonormal system in 𝕃2 such that there exists a nondecreasing sequence of positive numbers , lim j→∞λj = ∞ and −Δej = λjej for all j. Let ℚek = qkek with , and {bk(t)} be a sequence of independent standard Brownian motions in ℝ. Then Wiener process 𝕎(t) is taken as 𝕎(t)=.
Let Q be a trace class operator on L2. Similarly, we can define a complete orthonormal system , a nondecreasing sequence of positive numbers such that −Δej = κjej, and positive numbers such that and . Let W(t)=. Then W(t) is an L2-valued Q-Wiener process. From now on, let {ℱt} be the natural filtration of {𝕎(t)} and {W(t)}, augmented by all the P-null sets of ℱ. A complete definition of Hilbert space-valued Wiener processes can be found in [15].
Definition 2.1. A quaternion of ℱt-Adapted processes (uɛ, Zɛ, pɛ, Zɛ) is called a solution of backward Navier-Stokes equation (2.6) if it satisfies the integral form of the system
- (a)
uɛ∈∩;
- (b)
Zɛ∈;
- (c)
pɛ∈∩;
- (d)
Zɛ∈.
The following simple results are frequently used and given as lemmas. Readers may refer to Temam [3] for similar proofs.
Lemma 2.2. For any and p ∈ L2, one has
-
1 = ∑i,j∫G∂iuj∂iwjdx ==,
-
2 〈(u · ∇)v, w〉 = ∑i,j∫Gui(∂ivj)wjdx,
-
3 〈(u · ∇)v, w〉 = −〈(∇·u)w, v〉 − 〈(u · ∇)w, v〉,
-
4 =,
-
5 〈−∇p, u〉 = − ∑i∫G∂ip uidx=∫Gp∂iuidx = 〈p, ∇·u〉.
Remark 2.3. Sometimes is denoted by .
Lemma 2.4. The following results hold for any real-valued smooth functions ϕ and ψ with compact support in ℝ2:
Proposition 2.5. For any u and v in and w ∈ 𝕃4, one has
Below is a backward version of the Gronwall inequality used frequently in this paper, and the proof is straightforward.
Lemma 2.6. Suppose that g(t), α(t), β(t), and γ(t) are integrable functions, and β(t), γ(t) are nonnegative functions. For 0 ≤ t ≤ T, if
3. A Priori Estimates
The purpose of the this paper is to show the existence and uniqueness of the randomly stopped backward stochastic Navier-Stokes equation (2.6). We employ Galerkin′s method by defining orthogonal projections , where span{e1, e2, …, eN}, for all N ∈ ℕ. An important result is that the Galerkin-type approximations converge weakly to the solution of the Navier-Stokes equation.
Proposition 3.1. Let , , and f ∈ L2(0, T; ℍ−1). Then for any solution of system (3.2), the following is true:
Proof. Applying the Itô formula to to get
Similarly, making use of (3.4), it follows that ∈.
Proposition 3.2. Let , , and f ∈ L2(0, T; ℍ−1), for all n ∈ ℕ and n ≥ 2. The following is true for any solution of system (3.2):
Proof. Let us prove it by the method of mathematical induction. Similar to Proposition 3.1, it is easy to obtain the result for n = 2. Suppose that it is true for all m ≤ n − 1. Let us show that the proposition holds for m = n.
An application of the Itô formula to yields
4. Existence of Solutions
The following lemma states the monotonicity of drift coefficients. The proof involves Proposition 2.5 and is straightforward.
Lemma 4.1. Assume and w ∈ 𝕃4. The following inequalities are true:
- (a)
,
- (b)
+,
- (c)
+.
- (d)
+ C∥v∥2 | u − v|2 ≥ (ν/2)∥u−v∥2.
Corollary 4.2. For any u and v ∈ 𝕃4, let
The proposition below is used in the proof of the existence, and we provide a brief proof. Readers may refer to [14, 16] for a similar and detailed proof.
Proposition 4.3. Let , , and f ∈ L2(0, T; ℍ−1). Then the projected system (3.2) admits a unique adapted solution in
Proof. For every M ∈ ℕ, let LM be a Lipschitz C∞ function which has the following property:
From now on, let us assume the external body force to be an operator and denote it by F. We also assume the following coercivity and monotonicity hypotheses in this paper. Such an approach is commonly used in studying the stochastic Euler equations so that a dissipative effect arises. Also they are standard hypotheses in the theory of stochastic PDEs in infinite dimensional spaces (see Chow [15], Kallianpur and Xiong [17], Prévôt and Röckner [18]).
Assumption A. (A.1) F: →ℍ−1 is a continuous operator.
(A.2) There exist positive constants α and β, such that
(A.3) For any u and v in , a constant κ > ν, and a positive constant α,
(A.4) For any u∈ and some positive constant α,
Remark 4.4. Assumption (A.2) is usually called the coercivity condition of the dissipative term and the external body force. Assumption (A.3) is the monotonicity condition of dissipative term and the external body force. The first half of the inequality is used in the proof of the uniqueness in Section 5. The second half of the inequality is used in the proof of the existence in Section 4. Assumption (A.4) is the linear growth condition of the external body force.
Lemma 4.5. Assume u and v ∈ 𝕃4. Then the following inequality is true:
Corollary 4.6. Let u and v ∈ 𝕃4. Define
Remark 4.7. To prove Corollary 4.6, the monotonicity assumption (A.3) is used.
Proposition 4.8. (i) Let and . Then for any solution of system (4.13), the following is true:
(ii) Let and . The following is true for any solution of system (4.13):
Proof. (i) Similar to the proof of Proposition 3.1, utilizing Assumption (A.2), (3.6) becomes
(ii) The proof is similar to (i).
Proposition 4.9. Suppose that and . Then for any solution (uɛ, Zɛ, pɛ, Zɛ) of system (4.13), there exists a constant K0, such that
Proof. The proof involves an application of the Itô formula to , and the second half of the coercivity assumption. We skip the proof since it is similar to Proposition 3.1.
Theorem 4.10. Let and . For system (4.12), there exists a solution (uɛ, Zɛ, pɛ, Zɛ) in
Proof. We have the following steps.
Step 1 (The limits). Clearly, by Proposition 4.8, there exist uɛ, pɛ, Zɛ, and Zɛ, such that
Step 2 (The Itô formula). For convenience, let us denote Nk by N again. Let M ≤ N and . For any v∈ and some constant K, such that ∥v∥ ≤ K uniformly, define
Step 3 (Monotonicity). By Corollary 4.6, we get
5. Uniqueness, Continuity and Convergence of Solutions
5.1. Uniqueness and Continuity
The backward Navier-Stokes equation is well-posed if the regularity of the terminal condition in Proposition 4.9 is imposed. Only the uniqueness and continuity are left to check. Let us first prove the following lemma.
Lemma 5.1. For any u and v in and w ∈ 𝕃4, one has
Proof. By Proposition 2.5,
Theorem 5.2. Let and . System (4.12) admits a unique adapted solution in
Proof. The existence of an adapted solution is shown in Theorem 4.10. Suppose that (uɛ1, Zɛ1, pɛ1, Zɛ1) and (uɛ2, Zɛ2, pɛ2, Zɛ2) are solutions of system (4.12) according to terminal conditions (ξ1, η1) and (ξ2, η2), respectively. The regularity of the solutions is guaranteed by Proposition 4.9. Denote
Remark 5.3. The uniqueness and continuity with weaker terminal conditions, such as when the terminal conditions are uniformly bounded in L2 sense, are still open. The difficulty lies in the nonadaptiveness nature of the backward system. For instance, the function l1 defined in Corollary 4.6 is not ℱt adapted. This is why we defined another function l(t) in the proof of the uniqueness based on the -bound of the solution. Fortunately, l(t) is ℱt adapted and has similar properties as l1(t). One can also show the uniqueness and continuity using Lemma 5.1, without introducing the function l(t).
5.2. The Convergence of the Solution As ɛ Approaches Zero
Theorem 5.4. Assume the conditions in Theorem 4.10(ii). Then as ɛ approaches 0, the first three elements in the solution of (4.12), (uɛ, Zɛ, pɛ), converge to (u, Z, p), the solution of (5.10).
Proof. Similar to Step 1 of the proof of Theorem 4.10, we know that there exist u, p, Z and a sequence of positive numbers such that ɛi → 0, , and in corresponding spaces.
From (4.18) and (4.20), one knows that along a subsequence,
Similar to Steps 2 and 3 in the proof of Theorem 4.10, we are able to show that (u, Z, p) solves (5.10).
Acknowledgments
The author thanks Professor P. Sundar for his helpful discussion and insightful suggestions. The author also thanks the anonymous referee for offering valuable comments and suggestions on the earlier version of the paper.