Direct Correlation of Noise and Flow of a Jet

A direct correlation between the flow and noise of a jet—between the “cause” and the “effect”—has been measured. Two kinds of correlations were explored, namely (1) the broad‐band turbulence signal (hot‐film) with the broad‐band acoustic signal (microphone), and (2) the narrow‐band filtered turbulence signal with the narrow‐band filtered acoustic signal; the latter approach was ultimately adopted. The correlations were analyzed in terms of an extension of Proudman's form of Lighthill's integral for aerodynamic noise; this yielded the relative intensity and spectrum of the noise originating from unit volume of a jet (35 locations) and received at a farfield point (r = 96D, φ = 40°); this in turn led to the relative emission of successive “slices” of a jet versus axial distance X over the measurement range (1D ⩽ × ⩽ 7D). Qualitative agreement was found with Ribner's X° law, and the spectral peaks for each slice were located in frequency essentially as predicted by Powell. The maximum normalized broad‐band correlation of 1%–2% implies roughly that a turbulence “eddy” centered at the hot‐film 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.