  Supersonic pre-transitional boundary layers flowing over porous flat and
concave surfaces are studied using numerical and asymptotic methods. The porous
wall is composed of thin equally-spaced cylindrical microcavities. The flow is
perturbed by small-amplitude, free-stream vortical disturbances of the
convected gust type. From the proximity of the leading edge, these external
agents generate the compressible Klebanoff modes, i.e. low-frequency
disturbances of the kinematic and thermal kind that grow algebraically
downstream. For Klebanoff modes with a spanwise wavelength comparable with the
boundary-layer thickness, the porous surface has a negligible effect on their
growth. When the spanwise wavelength is instead larger than the boundary-layer
thickness, these disturbances are effectively attenuated by the porous surface.
For a specified set of frequency and wavelengths, the Klebanoff modes evolve
into oblique Tollmien-Schlichting waves through a leading-edge-adjustment
receptivity mechanism. The wavenumber of these waves is only slightly modified
over the porous surface, while the growth rate increases, thus confirming
previous experimental results. An asymptotic analysis based on the triple-deck
theory confirms these numerical findings. When the wall is concave, the
amplitude of the Klebanoff modes is enhanced by the wall curvature and is
attenuated by the wall porosity during the initial development.
