Direct measurements of the correlation between axial in-jet velocity fluctuations and far field noise near the axis of a cold jet

Abstract

A “noise source” has been experimentally identified in a cold jet at M = 0·98 by using the causality method proposed by Ribner and Siddon, which includes measuring the correlations between the velocity fluctuations inside the jet (these velocity fluctuations are measured by means of a laser velocimeter) and the far field acoustic pressure. This “noise source” is located in a cylindrical domain about the jet axis, within the transition region of the jet, which is from 5 to 10 D in axial extent. The emitted noise spectrum is at Strouhal numbers below St = 0·5. The “source” produces nearly all of the far field noise measured directly at 20° and 30° to the jet axis. The noise measured is essentially of the “shear noise” type: more precisely the level of the “shear noise” exceeds by at least 17 dB that of the “self noise”.