On transition in supersonic and hypersonic boundary layers

The roles of the first and second modes of instability in high-speed boundary-layer transition are quantified. Linear stability theory and the N-factor criterion are used to predict transition location. The effect of heat transfer is also studied; the results demonstrate that the transition reversal phenomenon can be explained by the opposite effect of cooling on the first and second modes of instability. A heating strip can be used to stabilize first-mode waves at supersonic speeds. Compressibility is destabilizing at free-stream Mach numbers of 2-3.5. The predicted transition location is due to the oblique first modes of instability, up to free-stream Mach numbers between 6 and 6.5. At higher Mach numbers, the predicted transition location is due to a combination of two-dimensional (2-D) first and second modes of instability.