Schlieren photos, for those unaware, are photos that show supersonic flow fields around – typically – models in wind tunnels. As air flows supersonically past a fixed object (or, of course, the other way ’round), shockwaves are set up as the air is shoved to the side. Air being compressible, it is compressed. And thus the density is very briefly increased. As the density of air increases, so does its index of refraction… thus light is “bent,” like looking through a lens. A further description of Schlieren photography is HERE.
I’ve always gotten a kick out of Schlieren photos. The best of ’em seem to me to be better art than much of what’s actually intended to be art. Every now and then NASA (and it’s predecessor the NACA) wind tunnel tested some generic shapes, as well as re-entry vehicles, wings, hypersonic aircraft designs, etc. I figured I’d show a few, what the hell. Here, for example, is a flat-faced cylinder at Mach 6.86, flat face forward:
And here’s one of a sphere at Mach 6.86:
Neat, eh? Better’n a can of Campbell’s Soup, at the very least.