Geometric and functional mixing by 2D stationary incompressible flows

Abstract

We study quantitative mixing and deformation of sets and curves for a class of two-dimensional autonomous Hamiltonian flows with finitely many critical points satisfying local conditions that allow finite-order degeneracy. Variation of the period across neighboring trajectories generates transverse shear, providing a common mechanism for scalar mixing, set deformation, and curve stretching. First, for $H^1$ initial data supported away from equilibria and infinite-period trajectories, in regions where the period gradient is bounded away from zero, we establish sharp $(1+t)^{-1}$ decay in $H^{-1}$ towards the time average of the initial data along each periodic trajectory. Second, under the same geometric conditions, we prove matching upper and lower bounds of order $(1+t)^{-1}$ for an orbit-relative geometric mixing scale of transported Lipschitz subdomains whose closures are not invariant under the flow. This scale measures how closely the transported subdomain covers the union of trajectories meeting its initial position. Third, for Lipschitz curves separated from infinite-period trajectories, we derive an explicit first-order large-time expansion of their length with a remainder bounded uniformly in time. In particular, their length grows at most linearly. Counterexamples illustrate how the stated conclusions can fail when selected nondegeneracy or separation assumptions are removed. The analysis combines coordinates adapted to the periodic trajectories with quantitative estimates and asymptotic expansions for the flow Jacobian. Numerical simulations for cellular and radial flows illustrate the functional and geometric mixing rates and the evolution of curve length.