Abstract
The character of jet noise undergoes a marked change above choking, the noise due to turbulent mixing being dominated by a powerful whistle or screech whose wavelength is related to the regular shock wave spacing. The mechanism in two-dimensional flow is further examined (by the aid of a dynamic Schlieren apparatus), verifying the suggested mechanism and showing the similarity to that in axially symmetric flow where discontinuities in frequency, partly analogous to edge tones, occur. The resultant sound emitted as the periodic eddy system traverses the regular shock wave pattern is highly directional, producing a powerful beam at doubled frequency normal to the jet and an intense beam at eddy frequency in the upstream direction adjacent to the jet, resulting in fluctuations in jet velocity direction at the orifice which initiate new stream disturbances.
A gain criterion for the self-maintained cycle is given, enabling certain qualitative deductions concerning the intensity to be made, and use will be made of this in considering methods of reducing the noise level.