Choked jet edge tones, continued again: Scattering of evanescent waves as the sound source

Abstract

The early suggestion of monopole sources due to the high-impedance supersonic jet switching above and below the edge resulted in excessive acoustic radiation estimates. It is now proposed that the sound is better attributable to the growing then decaying ‘‘evanescent wave’’ of the sinuous jet instability, estimated by Rayleigh’s formula applied to a plane just outside the jet flow. The wave amplitude, growth and decay rates (both taken as a cosine form), wavelength, and convection speed (Mach number=0.6 for pressure ratio 3.07) are all estimated from schlieren and shadowgraph photographs. For the nozzle aspect ratio of 4.8, the two limiting cases calculated are (i) two-dimensional and (ii) acoustically equivalent line source. The preferred downstream radiation calculated in both cases is consistent with experiment, while the amplitudes calculated appear somewhat high, (ii) being within difficult measurement error (3–4 dB). [Work supported by Texas Advanced Research Program.]