In this note, we discuss the problem of large deviations for the stochastic 2D Navier-Stokes equations. We show that the occupation measures of the trajectories of the system satisfy a large deviations principle, provided that the noise acts on all Fourier modes. In the case when the noise is more degenerate and acts on all the determining modes, we obtain an LDP of local type. The proofs use the methods introduced in [13,20] based on a Kifer-type sufficient condition for LDP and a multiplicative ergodic theorem.
AMS subject classifications: 35Q30, 60B12, 60F10Keywords: Navier-Stokes system, white-in-time noise, large deviations principle, occupation measures, coupling
IntroductionIn this note, we review the results of the paper [21], where the large deviations principle (LDP) is studied for the stochastic 2D Navier-Stokes (NS) equations driven by a white-in-time noiseThis system describes the motion of an incompressible fluid in a bounded domain D ⊂ R 2 with a smooth boundary ∂D, where ν > 0 is the kinematic viscosity, u = (u 1 (t, x), u 2 (t, x)) and p = p(t, x) are unknown velocity field and pressure of the fluid, f is the external (random) force, and u,Before stating our results, let us make some comments about this system and recall some previous results on its ergodicity. The system is considered in the usual space of divergence-free vector fields where L = −Π∆ is the Stokes operator, B(u) = Π( u, ∇ u), and Π is the orthogonal projection onto H in L 2 (Leray projector).Structure of the noise. We assume that f is of the formwhere the functions h and η are, respectively, the deterministic and random components of the external force with h ∈ H andHere {b j } is a sequence in R + such that B 0 = ∞ j=1 b 2 j < ∞, {β j } is a sequence of independent standard Brownian motions defined on a filtered probability space (Ω, F, {F t }, P) satisfying the usual conditions (see Definition 2.25 in [14]), and {e j } is an orthonormal basis in H consisting of the eigenfunctions of L with eigenvalues {α j }.Under these conditions, problem (0.3), (0.2) admits a unique solution and defines a Markov family (u t , P u ) parametrised by the initial condition u = u 0 ∈ H. The corresponding Markov semigroups are given bywhere P t (u, Γ) = P u {u t ∈ Γ} is the transition function. Recall that a measure µ ∈ P(H) is stationary if P * t µ = µ for any t > 0. The existence of a stationary measure is a relatively simple question, it is proved by using the classical Bogolyubov-Krylov argument. The uniqueness and the mixing are much more difficult problems which have been extensively studied in recent years. We refer the reader to the papers [7,16,6,17,2,11,22] and the book [18] for a detailed discussion of this topic. In particular, as it is stated in the following theorem, if η is sufficiently non-degenerate, then the family (u t , P u ) admits a unique stationary measure, which is exponentially mixing (see Theorem 3.5.2 in [18]).1 To simplify the notation, we shall assume that ν = 1. 2 P(H) is the set of probability Borel measures...