Critical layer in a shear flow
= Critical layer in a shear flow
{title2=$U(y_c)=c_0$}
A critical layer is a region where the leading wave phase speed coincides with the local shear-flow <velocity> and an outer approximation degenerates. Small viscosity or other corrections must then be retained on a distinguished transverse scale. The width depends on the order of vanishing of $U-c_0$ and on the governing mode equation; simple-shear and stationary-profile layers have different balances.