Amplification of acoustic perturbations during reflection from critical layer in supersonic flows

The paper is concerned with the propagation of acoustic perturbations in a medium moving at a variable velocity. It is shown that on reflection from the critical layer (i.e., the layer where the flow velocity is equal to the phase velocity of the perturbations), the perturbation amplitude increases if the change of flow velocity on scales of the order of the perturbation wavelength exceeds the velocity of sound.