On the sound sources of the screech tone radiated from a choked circular jet

In this experimental study, the generation of the screech tone in the dominant helical oscillation mode C was investigated by using a series of instantaneous Schlieren photographs and it was observed that the moving sound source for mode C appears to be the prominent point on the helical-shaped coherent vortical structure. This sound source is generated by the interaction between the helical-shaped vortical structure and the oblique shock at the rear edge of the third shock cell. The sound source rotates about the jet axis in the plane perpendicular to the jet axis at a constant supersonic speed and forms a Mach cone. The envelope of the Mach cone widens upstream and downstream as the sound source moves along the circular orbit and forms two helical-shaped wave-fronts of the screech tone propagating in upstream and downstream directions.