Screech receptivity control using exit lip surface roughness for under-expanded jet noise reduction

Abstract

This study presents an experimental investigation on altering the receptivity of the screech phenomenon toward the goal of jet noise reduction. The screech receptivity refers to the propensity to sustain the screech feedback loop through the reflection/scattering of feedback waves at the jet exit lip and their coupling with hydrodynamic instabilities near the jet exit. The receptivity is modified by attaching sandpapers of different roughnesses on the exit lip of pipes and nozzles. First, the pipe exit lip thickness is varied to determine variations in screech staging, frequency, and amplitude. As lip thickness varies, the screech mode staging varies with minimal changes in frequency, whereas screech amplitude depends on screech mode. The jet noise reduces as the lip surface roughness increases due to screech amplitude reduction or elimination because of the diffuse reflection of screech tone feedback waves by the lip surface. The coarse sandpaper on the exit lip eliminated the screech, while others reduced screech amplitude at high under-expansion levels. The coarse sandpaper reduced more than 50% of the maximum acoustic power observed with the plain lip at the corresponding under-expansion level. The surface roughness does not affect the screech tone at all the under-expansion levels or screech frequencies. The effect begins at a particular frequency (cutoff) and continues for frequencies lower than the cutoff frequency. The cutoff frequency increases with the lip surface roughness and varies with the lip thickness. By properly designing the surface roughness of the lip, the effect becomes a better passive technique for jet noise control.