Position Paper for the Boundary Layer Receptivity Panel

Abstract

The prediction of boundary layer transition was initially approached by asking where the laminar boundary layer first becomes unstable to what were then thought of as temporally growing instability waves (commonly referred to as Tollmien-Schlichting waves for inviscidly stable flows). However, it eventually became clear that the transition point (or better, the transition Reynolds number) is strongly affected by the specific details of the free-stream disturbance environment, and some of the research got redirected toward understanding just how it was that instability waves got generated by that environment, which was subsequently termed the receptivity phenomenon by Morkovin (1969). It was natural for experimentalist to study this phenomenon by imposing controlled, small amplitude, harmonic disturbances (i.e., disturbances of a single frequency, say ω) on thier flows, which then generated a spatially growing shear wave of the type referred to simply as an amplifying wave by plasma physicists who, incidentally, recognized very early on that these spatially growing waves are intimately connected to temporally growing instabilities.