Boundary layer and supersonic flow
It may be noticed that in all this we have said little or nothing about the boundary layer, and it was the boundary layer that caused all the trouble in subsonic flow when it came to corners, and it was the boundary layer that was so important in transonic flow when the incipient shock wave was formed, and for which we had to think of such devices as vortex generators.
The truth is that the boundary layer is relatively unimportant in supersonic flow; it is thin and the viscous forces within it are relatively small. This largely accounts for the ability of supersonic flow to turn sharp corners.
Curiously enough, at the even higher speeds which will be mentioned later in this chapter, the boundary layer thickens again, and once more becomes significant. So the supersonic region is especially privileged in this respect, and in many respects the theory of the flow is simpler than over any other range of speeds.