A comparison of boundary layer receptivity mechanisms

The process by which external disturbances generate instability waves in boundary layers is referred to as receptivity. Previous work has shown that the receptivity is concentrated in regions of the boundary layer where strong non-parallel flow effects are present; we examine two typical cases. First the receptivity of the Blasius boundary layer to a variety of free-stream disturbances is investigated. Convected gusts of all orientations produce receptivity levels similar to those for a parallel acoustic wave. At low Mach numbers, oblique acoustic waves produce the strongest receptivity, due to diffraction by the leading edge. The receptivity to acoustic waves is reexamined for the case of a semi-infinite plate in a channel, where a periodic dependence on frequency is found. Secondly, we investigate the receptivity produced by acoustic waves interacting with porous suction surfaces in subsonic flow. Here receptivity occurs due to the rapid adjustment in the mean flow, and also due to local scattering of the acoustic wave by the non-zero admittance of the porous surface. For discrete suction strips, the receptivity is minimized by choosing the strip width equal to the Tollmien-Schlichting wavelength. For the case of a junction between a distributed suction surface and a hard wall, the receptivity can be decreased by introducing a smooth variation in wall properties over a distance comparable to several instability wavelengths. The implications of receptivity for the prediction of boundary layer transition are also discussed.