SHOCK ASSOCIATED NOISE FROM CHOKED JETS

Shock associated noise is an important part of choked jet noise. Sound field and spectra thereof were investigated experimentally by utilizing the heterodyne analysis technique, so that both the screech and the wide-band noise can be recorded and studied. The sound pressures and spectra vary widely with the stagnation pressure, but the overall sound pressure level at 90°to the jet shows good regularity, it is nearly a constant between pressure ratios 3 and 8.5, given by 97 + 201og d, dB, d being nozzle diameter in mm., and empirical formulae are given to facilitate the SPL prediction on the whole range of pressures. The wide-band shock associated noise was found to be predominant in the range of pressure ratio between 2.5 and 7, and screech only up to slightly more than 5. For pressure ratios less than 2 or more than 8.5, the jet noise is entirely dominated by turbulent noise. New formulae for screech frequency and peak frequency of wide-band shock associated noise are given, which agree well with experiments. The existance of non-harmonic components in screech is explained. Interference effect seems to exist in the upstream direction between shock associated noise and turbulent noise, but not in the 90°direction. This phenomenon is explained by considering the phase relations of the noise.