Positive Lyapunov exponents and mixing in stochastic fluid flow. Part I

In this three-part lecture series, we will present a series of works by Bedrossian, Blumenthal and Punshon-Smith on the chaotic mixing and enhanced dissipation properties of a passive tracer subject to the motion of an ergodic Markovian flow of spatially regular incompressible velocity fields on a compact domain. Such flows crucially include the motion of a velocity field solving the stochastic Navier-Stokes equations in T2 (or T3 with hyperviscosity), but can be more general or much simpler in character. We will show how tools from the smooth ergodic theory of random dynamical systems can be developed to show that these flows "generically'' give rise to ergodic flows of volume preserving diffeomorphisms (Lagrangian flow) that have a positive Lyapunov exponent and are almost surely exponentially mixing. Moreover, when diffusion is added, they exhibit optimal enhanced L2 dissipation, whereby the interaction between advection and diffusion causes a passive tracer to decay on much faster time scales than the heat equation. This enhancement time scale is optimal in the class of Lipschitz velocity fields and provides the first examples of flows with such properties beyond the uniformly hyperbolic setting.