Sound sources of the screech tone radiated from circular supersonic jet oscillating in the helical mode

The generation mechanism of the screech tone in the helical oscillation mode is investigated using a series of instantaneous photographs. From the analysis of the photographs, six evanescent sound sources are observed as prominent points along the jet axis. These sound sources move along circular orbits in the planes perpendicular to the jet axis and slightly downstream of the rear edges of each shock cell. The speed of the moving sound sources is supersonic and moving Mach cones generated behind the moving sound sources form the helical-shaped wave fronts of the screech tone. The existence of the moving Mach cones about the jet axis was confirmed by comparing Schlieren photographs and drawings of the envelopes of the moving Mach cones. [Work supported by The Mitsubishi Foundation.]