| Supersonic boundary layer flow over a compression ramp is considered in the limit of large Reynolds numbers and for Mach numbers O(1), using the asymptotic theory of viscous-inviscid interactions. Reversed flow is known to occur near the ramp corner once the ramp angle is sufficiently large. At a second, larger ramp angle the flow is shown to exhibit an absolute instability which is believed to be a precursor of transition to turbulence. The properties of this instability, as well as a convective instability that precedes it are elucidated. Both types of instabilities are associated solely with the back-flow region. It is also shown that a compliant surface can, under certain circumstances control or delay the evolution of these instabilities. A compliant surface can also be subject to Tollmien-Schlichting instabilities and these are also investigated for supersonic flow over a flat, flexible surface. The effect of the material properties of the surface are considered in isolation and in combination, and neutral curves of stability are obtained. For a simple elastic surface, where deflections are proportional to the local pressure difference, the flow remains stable as for a rigid wall. A combined surface, where damping and inertial effects are significant exhibits complicated stability characteristics which are documented here in detail. |
