On the Limitations of High Speed Jet Noise Suppression (Keynote Paper)

This paper deals with an experimental investigation on the suppression of high-speed jet noise using air/water microjet injection at the nozzle exit. The far-field acoustic measurements from a high temperature M_j = 1.38 and M_j = 0.9 axisymmetric jet issuing from a converging nozzle show the suppression of screech tones, Mach wave radiation/crackle and mixing noise due to the use of microjets. Estimations of the contributions of different noise sources to the far-field sound are made using the current data supported by observations of previous investigators. It appears that the mixing noise reduction due to elimination of large eddies is found to be about 3–5 dB. Any further reduction of noise may only be accomplished by significant turbulence suppression and thermodynamic changes in the jet.