Enhanced Dissipation in the Navier-Stokes Equations Near the Poiseuille Flow

We consider solutions to the 2d Navier-Stokes equations on T x R close to the Poiseuille flow, with small viscosity nu > 0. Our first result concerns a semigroup estimate for the linearized problem. Here we show that the x-dependent modes of linear solutions decay on a time-scale proportional to nu(-1/2)vertical bar log nu vertical bar. This effect is often referred to as enhanced dissipation or metastability since it gives a much faster decay than the regular dissipative time-scale nu(-1) (this is also the time-scale on which the x-independent mode naturally decays). We achieve this using an adaptation of the method of hypocoercivity. Our second result concerns the full nonlinear equations. We show that when the perturbation from the Poiseuille flow is initially of size at most nu(3/4+), then it remains so for all time. Moreover, the enhanced dissipation also persists in this scenario, so that the x-dependent modes of the solution are dissipated on a time scale of order nu(-1/2)vertical bar log nu vertical bar. This transition threshold is established by a bootstrap argument using the semigroup estimate and a careful analysis of the nonlinear term in order to deal with the unboundedness of the domain and the Poiseuille flow itself.