Abstract
A direct correlation between the flow and noise of a jet—between the “cause” and the “effect”—has been measured. Two kinds of correlation have been tried, namely: (1) the broad-band turbulence signal (hot wire) with the broad-band acoustic signal (microphone); (2) the narrow-band filtered turbulence signal with the narrow-band filtered acoustic signal. The data have been analyzed via the Proudman form of Lighthill's integral for aerodynamic noise. Either type of correlation yields the relative intensity and spectrum of the noise originating from unit volume of the jet; this leads to the variation of noise power per unit length of the jet and the over-all spectrum. The peak normalized board-band correlation of 1%–2% roughly implies that a turbulence “eddy” centered at the hot wire contributes only 1%–2% of the rms sound pressure at the microphone. The total effective number of uncorrelated noise-producing “eddies” is thus of order (1/0.02)2, or some 2500 on an equal strength basis. [Work supported by Air Force Office of Scientific Research.]
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