Supersonic Pre-Transitional Disturbances in Boundary Layers on Porous Surfaces

The effect of wall permeability on the response of pre-transitional supersonic boundary layers subject to low-amplitude, free-stream vortical disturbances is investigated via asymptotic methods and numerically. A porous wall with regularly-spaced cylindrical pores couples the pressure and wall-normal velocity fluctuations at the wall when the spanwise diffusion is negligible, thereby reducing the growth of low-frequency laminar streaks. Highly-oblique Tollmien-Schlichting waves that develop further downstream are instead enhanced by the porous set. This finding is confirmed by a triple-deck analysis, which shows that the onset of Tollmien-Schlichting waves shifts upstream over a permeable wall. For boundary layers on concave permeable walls, the initial amplitude of the Görtler vortices is attenuated, while their growth rate is initially enhanced and then attenuated further downstream.