Correlation of Flowfield and Acoustic Field Measurements in High-Speed Jets

The present study demonstrates the capabilities of an optical deflectometry system to provide flowfield turbulence measurements that can be successfully correlated with acoustic field microphone measurements. Results from supersonic fully expanded jets are presented that document the coherence spectra between the flowfield and the acoustic field. The method is applied to jets with subsonic and supersonic convection speed, and shows a strong directionality of the emitted noise in both cases. Similar measurements performed in shock-containing jets with microphones in the forward arc reveal enhanced coherence at the dominant frequency of the broadband shock associated noise. Measurements were also performed with the optical sensors scanning radially outside of the jet in an attempt to obtain a measurement of the spectra in the acoustic near field. These results show an exponential decay of the signal magnitude characteristic of a region where hydrodynamic pressure fluctuations are dominant. The high level of skewness in these signals supports the concept that the crackle observed in the far field originates in the jet flowfield. Further radially, acoustic pressure fluctuations are successfully measured and correlated to the acoustic far field, thus demonstrating another application of this nonintrusive measurement technique in jet acoustics diagnostics.