Random Dynamics of the Stochastic Boussinesq Equations Driven by Lévy Noises
Abstract
This paper is devoted to the investigation of random dynamics of the stochastic Boussinesq equations driven by Lévy noise. Some fundamental properties of a subordinator Lévy process and the stochastic integral with respect to a Lévy process are discussed, and then the existence, uniqueness, regularity, and the random dynamical system generated by the stochastic Boussinesq equations are established. Finally, some discussions on the global weak solution of the stochastic Boussinesq equations driven by general Lévy noise are also presented.
1. Introduction
Dynamical systems driven by non-Gaussian processes, such as Lévy processes, have attracted a lot of attention recently. Ordinary differential equations driven by Lévy processes have been summarized in [1]. Peszat and Zabczyk [2] have presented a basic framework for partial differential equations driven by Lévy processes.
The Navier–Stokes fluid equations are often coupled with other equations, especially, with the scalar transport equations for fluid density, salinity, or temperature. These coupled equations model a variety of phenomena arising in environmental, geophysical, and climate systems. The related Boussinesq fluid equations [3–5] under Gaussian fluctuations have been recently studied, for example, existence and uniqueness of solutions [6], stochastic flow, dynamical impact under random dynamical boundary conditions [7, 8], and large deviation principles [9, 10], among others.
This paper is organized as follows. In Section 2, we first present some properties of the subordinator Lévy process Y(t), then review some fundamental properties of the stochastic integral with respect to Lévy process Y(t). Section 3 is devoted to the existence, uniqueness, regularity, and the cocycle property of the stochastic Boussinesq equations. Finally, some discussions on the global weak solution of stochastic Boussinesq equations driven by general Lévy noise are also presented in Section 4.
2. Preliminaries
In this section, we introduce some operators and fraction spaces and then present some properties of the subordinator Lévy process Y(t) and the stochastic integral with respect to Lévy process Y(t).
In order to reformulate the stochastic Boussinesq equations (1) as an abstract stochastic evolution, we introduce the following function spaces.
Let Hs,q(D) denote the closure of 𝒱 with respect to the ℍs,q-norm, Vs,q(D) denote the closure of 𝒱 with respect to the 𝕍s,q-norm, and V′ be the dual space of Vs,q(D). In particular, we denote by H1,2 and V1,2 H and V, respectively.
In order to apply the technique of the reproducing Kernel Hilbert space, it is better to introduce the definition γ-radonifying.
Definition 1 (see [13].)Let K and X be Banach spaces, a bounded linear operator L : K → X is called γ-radonifying if and only if L(γK) is σ-additive, where γK is the canonical cylindrical finitely additive set-valued function (also called a Gaussian distribution) on K.
The following is our standing assumption:
Assumption 1. Space E ⊂ H∩𝕃4 is a Hilbert space such that for some δ ∈ (0,1/2),
Remark 2. Under the above assumption, we have the facts that E ⊂ H and the Banach space U is taken as H∩𝕃4 (see [11, 14, 15] for more details and related results). In fact, space E is the reproducing kernel Hilbert space of noise W(t) on H∩𝕃4.
It is well known that subordinators form the subclass of increasing Lévy processes, which can be thought of as a random model of time evolution (see [16]). We will present some properties of the subordinator Lévy process Y(t), t ≥ 0, then review briefly the stochastic integral with respect to Lévy process Y(t).
Definition 3 (see [1], [2], [11].)Let E be a Banach space, and let Y = (Y(t), t ≥ 0) be an E-valued stochastic process defined on a probability space (Ω, ℱ, ℙ). Stochastic process Y is called a Lévy process if
- (L1)
Y(0) = 0, a.s.;
- (L2)
process Y has independent and stationary increments; and
- (L3)
process Y is stochastically continuous, that is, for all δ > 0 and for all s ≥ 0,
()
A subordinator Lévy process is an increasing one-dimensional Lévy process.
For p > 0, Sub(p) denotes the set of all subordinator Lévy processes Z, whose intensity measure ρ satisfies the condition .
Next, we recall some basic definitions and properties for general random dynamical systems, which are taken from [7]. Let (H, d) be a complete separable metric space and (Ω, ℱ, ℙ) be a probability space.
Definition 4. (Ω, ℱ, ℙ, (θ) t∈ℝ) is called a metric dynamical system if the mapping θ : ℝ × Ω → Ω is (ℬ(ℝ) × ℱ, ℱ) measurable, θ0 = I, θs+t = θs∘θt for all t, s ∈ R, and θtℙ = ℙ for all t ∈ ℝ.
Definition 5. A random dynamical system (RDS) with time T on (H, d) over {θt} on (Ω, ℱ, ℙ, (θt) t∈R) is a (ℬ(R+) × 𝔉 × ℬ(H), B(H))-measurable map:
- (i)
S(0, ω) = Id (identity on H) for any ω ∈ Ω,
- (ii)
(Cocycle property) S(t + s, ω) = S(t, θsω)∘S(s, ω) for all s, t ∈ T and ω ∈ Ω.
An RDS is said to be continuous or differentiable if for all t ∈ T, and an arbitrary outside outside ℙ-nullset B ⊂ Ω, w ∈ B the map S(t, ω) : H → H is continuous or differentiable, respectively.
Assume that the bounded linear operator A generates a C0-semigroup S = (etA) t≥0 on a Hilbert space E and Y defined on a filtered probability space (Ω, ℱ, (ℱ) t≥0, ℙ) is a subordinator Lévy process taking values in a Hilbert space U.
Definition 6. Let x ∈ E be a square integrable -measurable random variable in E. A predicable process X : [t0, ∞) × Ω → E is called a mild solution of the Langevin equation (24) with initial data (t0, x) if it is an adapted E-valued stochastic process and satisfies
It is well known that the Ornstein-Uhlenbeck process X(t), t ≥ 0, has some important integrability. Here we need the Banach space to be of type p, for p ∈ (1,2]. First we recall the definition briefly (see [14] for more details).
Definition 7 (see [14].)For p ∈ (1,2], the Banach space E is called as type p, if and only if there exists a constant Kp(E) > 0 such that for any finite sequence of symmetric independent identically distribution random variables ε1, …, εn : Ω → [−1,1], n ∈ ℕ, and any finite sequence x1, …, xn from E, satisfying
Moreover, if there exists a constant Lp(E) > 0 such that for every E-valued martingale , N ∈ ℕ, satisfying
Lemma 8 (see [11], Corollary 8.1, Proposition 8.4.)Assume that p ∈ (1,2], Z is a subordinator Lévy process from the class Sub(p), E is a separable type p-Banach, U is a separable Hilbert space U, E ⊂ U, and W = (W(t)) t≥0 is a U-valued Wiener process.
Define the U-valued Lévy process as
We have the following existence and regularity results, which have been studied in [2, 11].
Theorem 9. Assume that E = U, S = S(t), t ≥ 0 is the C0 semigroup generated by the bounded linear operator A in the space E. Then, if one of the following conditions is satisfied:
- (i)
p ∈ (0,1] or
- (ii)
p ∈ (1,2] and the Banach space E is of separable martingale type p-Banach space,
Proof. As S = S(t), t ≥ 0, is a C0-group in the separable martingale type p-Banach space E, the Hilbert space H is the reproducing kernel Hilbert space of W(1), and the embedding operator i : H↪E satisfies the γ-radonifying property. The proof of Theorem 9 is just a simple application of Theorems 4.1 and 4.4 in [11].
3. Cocycle Property of the Stochastic Boussinesq Equations
In this section, we will show the existence, uniqueness, regularity, and the cocycle property of the stochastic Boussinesq equations (11).
Lemma 10 (see [7], Lemma 2.3.)If U, V, W ∈ 𝕍, then
Lemma 11 (see [7], Lemma 2.4.)There exists a constant cB > 0 such that if u ∈ V1, θ, η ∈ V2, ϕ = (u, θ), then
- (1)
, u ∈ V, v ∈ D(A), w ∈ H,
- (2)
, u ∈ V, v ∈ D(A), w ∈ V,
- (3)
, u ∈ V, v ∈ V,
- (4)
, u ∈ V, θ ∈ V, w ∈ V,
- (5)
, u ∈ H, θ ∈ D(A), w ∈ V.
Definition 12. An H-valued (ℱt) t≥0 adapted and H4,2(D)-valued cádlág process u(t) (t ≥ 0) is considered as a solution to (11), if for each T > 0,
Lemma 13. Assume that z ∈ L4(0, T; 𝕃4(D)), g ∈ L2(0, T, V′), and v0 ∈ H. Then there exists a unique v ∈ ℋ1,2(0, T) such that
Proof. It can be shown by the same approach as the one in Proposition 8.7 in [11].
Lemma 14 (see [2], Proposition 10.1.)Let u : [0, T] → B be a continuous function whose left derivative
In order to apply the Yosida approximation for the solution of (11), we need to introduce some definitions of dissipative mapping (operator) (see [17] for details).
Definition 15. Let (B, | · |B) be a separable Banach space, B* be the dual space of B. The subdifferential ∂ | x|B of norm | · |B at x ∈ B is defined by the formula
A mapping F : D(F) ⊂ B → B is said to be dissipative, if for any x, y ∈ D(F), there exists z* ∈ ∂ | x − y|B such that
A dissipative mapping F : D(F) ⊂ B → B is called an m-dissipative mapping or maximal dissipative if the image of I − λF is equal to the whole space B for some λ > 0 (and then for any λ > 0), that is,
Assume that F is an m-dissipative mapping. Then its resolvent Jα and respectively the Yosida approximations Fα, α > 0, are defined by
Lemma 16 (see [2], Proposition 10.2.)Let F : D(F) → B be an m-dissipative mapping on B. Then
- (1)
for all α > 0 and x, y ∈ B, ;
- (2)
the mapping Fα, α > 0, are dissipative and Lipschitz continuous:
() -
Moreover, |Fα(x)|B≤|F(x)|B, for all x ∈ D(F); and
- (3)
lim α→0Fα(x) = x, for all .
The following theorem is one of the main results of this paper, which will be proved by applying the well-known Yosida approach.
Theorem 17. For every u0 ∈ H, under Assumption 1, the stochastic Boussinesq system (11) admits a unique cádlág mild solution u(t).
Proof. Denote ZA(ω) to be the stationary solution of Langevin equation (24). Let V = U − ZA. Then (11) is converted into the following evolution equation with random coefficients:
Now consider the following random approximate equation:
Next we will show that
For the sake of simplification, we just present the estimations when η = 0, and the remaining estimates can be obtained by the similar arguments for η ≠ 0.
Let Yα be the solution of the integral equation:
For α > 0 and β > 0, direct computation implies
Hence, it follows that
By Gronwall inequality, we have
By Lemma 14,
Similarly, by Lemma 16, for t ∈ [0, T],
Next, we are going to show that the solution Yα of the Yosida approximations equation is a mild solution:
Theorem 18. For any U0 ∈ H, the map φ : 𝕋 × Ω × H → H defined by the solution of stochastic Boussinesq equation (11) as U(t) = Φ(t, ϑt(ω))U0 has the cocycle property; that is, the solution of stochastic Boussinesq equation (11) generates a random dynamical system (Ω, ℱ, ℙ, (ϑt) t≥0, Φ).
Proof. From Theorem 17, stochastic Boussinesq equation (11) admits a unique solution V(t, Z(ω)(t), x). Define the map
- (i)
By the similar argument of Theorem 17, every solution Yα(t) of the Yosida approximation equation (50) is measurable. Notice that Yα(t) → Y(t) uniformly as α → 0. Hence, the limit function Y(t) is also measurable. Thus, the mapping Φ is measurable.
- (ii)
Obviously, Φ(0, ω) = I.
- (iii)
It suffices to verify that the cocycle property holds for the mapping Φ, that is,
()
In fact, recalling that ZA(ω)(s) = ZA(θsω)(0), it follows that
Moreover,
Since
Thus,
Therefore, we obtain
The uniqueness of the solution implies that almost surely V1(t) = V2(t) holds, that is,
Thus, the cocycle property for the mapping Φ holds.
By the definition of random dynamical systems [18], the solution mapping of the stochastic Boussinesq equation (11) generates a random dynamical system Φ. Thus, the proof of Theorem 18 is complete.
4. Discussion
In Section 3, we have studied the long-time behavior of stochastic Boussinesq equations (1) driven by subordinator Lévy noise and have shown the cocycle property of random dynamical systems generated by the mild solution of stochastic Boussinesq equation (1). To prove the existence of random attractor, it suffices to show the existence of random absorbing set and the compactness of random dynamical system Φ, we refer the similar argument to [13].
Acknowledgments
The authors would like to thank an anonymous referee for valuable comments. Supported by the NSF of China (no. 10971225, no. 11028102, no. 11271013, and no. 91130003), the SRF for ROCS, SEM, and the NSF Grant 1025422.