A dynamical approach to the study of instability near Couette flow

Abstract

AbstractIn this paper, we obtain the optimal instability threshold of the Couette flow for Navier–Stokes equations with small viscosity , when the perturbations are in the critical spaces . More precisely, we introduce a new dynamical approach to prove the instability for some perturbation of size with any small , which implies that is the sharp stability threshold. In our method, we prove a transient exponential growth without referring to eigenvalue or pseudo‐spectrum. As an application, for the linearized Euler equations around shear flows that are near the Couette flow, we provide a new tool to prove the existence of growing modes for the corresponding Rayleigh operator and give a precise location of the eigenvalues.