The sound‐producing oscillations of round underexpanded jets impinging on normal plates

The results of acoustical and optical experiments in which ‘‘moderately’’ underexpanded sonic round jets impinge on flat plates normal to the jet axis are presented and analyzed. Periodic unstable oscillations of the jet flow, with the resultant radiation of sound of discrete frequencies, occur over a wide variation of control parameters, namely, pressure ratio, plate size, and spacing of the plate from the jet nozzle. For ‘‘small’’ plates, the principal oscillations with λ/D about 4 (λ=acoustic wavelength, D=nozzle diameter) occur when the standoff shock wave lies in a pressure recovery region of the periodic cellular structure of the choked jet and is, therefore, highly unstable; then the oscillations have key characteristics in common with the high‐harmonic excitation of Hartmann’s acoustic air‐jet generator. An analogous feedback mechanism in the standoff zone is suggested in which pressure waves reflected from the plate trigger the motion of the unstable shock wave. For ‘‘large’’ plates, acoustic fee...