Correlations of near-And far-field acoustic data in a high-speed jet

The cross-correlations of near- and far-field pressure in a M=0.6 cold jet are oscillatory, which implies some phaselocking. A simple model and synthetic signals show that the locking may be related to the steep attenuation with distance from the near-field sensors. In the near-field, wavelet-based nonlinear filtering separates fluctuations that are recognizable at x/D = 3 and 6 ('matched') from the others ('residue'). This decomposition yields distinct signatures in the crosscorrelations. The corresponding time lags and frequencies in the cross-correlations point to several categories of sources and their locations. The interpretation is consistent with the eddy pairing or breakdown (i.e. loss of identity) as the sources of noise for short lags, but the longer time lags require another explanation.