Linear inviscid damping and enhanced viscous dissipation of shear flows by using the conjugate operator method

Abstract: We study the large time behavior of solutions to two-dimensional Euler and Navier-Stokes equations linearized about shear flows of the mixing layer type in the unbounded channel $\mathbb{T} \times \mathbb{R}$. Under a simple spectral stability assumption on a self-adjoint operator, we prove a local form of the linear inviscid damping that is uniform with respect to small viscosity. We also prove a local form of the enhanced viscous dissipation that takes place at times of order $\nu^{-1/3}$, $\nu$ being the small viscosity. To prove these results, we use a Hamiltonian approach, following the conjugate operator method developed in the study of Schr\"odinger operators, combined with a hypocoercivity argument to handle the viscous case.