Shock tube experiments using differential interferometry have been carried out to investigate compressible subsonic flows around and behind flat plates and cylinders. Pictures taken with an IMACON image converter camera with framing speeds of up to 2.106 frames per second and streak records show that the trailing edge vortex separation induces pressure waves moving upstream. Each vortex separation is related to the formation of a pressure wave. The flow around the bodies is therefore influenced by the shedding frequency and is possibly coupled with the wake. The interaction of the wake with the flow around the bodies is a phenomenon of fundamental importance which has been neglected in gas dynamics up to now. After a symmetrical onset, the process of vortex separation and pressure wave generation develops into an asymmetrical wake with alternating separation. The experiments have shown that the dynamics of vortices close to the trailing edge is practically inviscid and incompressible. In contrast to this, the pressure waves moving upstream must be treated as a problem of purely compressible nature.
"Vortices And Pressure Waves At Trailing Edges", Proc. SPIE 0491, 16th Intl Congress on High Speed Photography and Photonics, (1 February 1985); doi: 10.1117/12.968016; https://doi.org/10.1117/12.968016