Mechanisms of jet crackle

Abstract eng:
Crackle is a particularly intense and annoying characteristic of the noise radiated by high-speed (supersonic) jets. Pressure traces show evidence of relatively strong compression in the sound field and more extreme positive than negative pressure fluctuations thus leading to positive skewness. To study its mechanisms, a turbulent free shear flow and its near acoustic field is directly simulated without modeling approximations. The simulations confirm that the ‘footprint’ of pressure skewness originates close to or within the turbulence. A supersonic flow which includes weak gas dynamic nonlinearities over a wavy-wall, with characteristics set to match scales of the turbulence, suggest a basic mechanism that potentially underlies the generation of crackle’s near-field pressure fluctuations.

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