Exploring noise sources using simultaneous acoustic measurements and realtime flow visualizations in jets

Work was carried out as part of an ongoing effort to relate instantaneous large-amplitude features of the far- eld acoustic to the dynamic evolution and interaction of large-scale structures within the mixing layer of ideally expanded, high-speed, high-Reynolds-number jets. It is believed that such information is essential for a better understanding of jet noise sources and jet aeroacoustic modeling and control. The acoustic measurements were taken with a four-microphone inline array placed 30 deg to the jet axis. Conditional sampling of the data from the microphones was used to create characteristic waveforms for the large-amplitude, far- eld sound pressure peaks. The frequency content of these phase-averaged waveforms compare very well with those of the overall acoustic far eld at 30 deg. A vast majority of the large-amplitude sound events originated between four and nine jet exit diameters downstream of the nozzle. The acoustic measurements were taken simultaneously with real-time ow visualizations to determine the mechanisms that were responsible for the creation of individual far- eld acoustic peaks, and these results have identi ed three noise-generationmechanisms: intense cross-mixing-layer interaction, the tearing of large turbulent structures, and the rollup of large turbulent structures. The simultaneous data also showed that large structures entrain more ambient air into the jet and the mixing layer extends farther into the jet core during intense noise production than during periods of relative quiet. Nomenclature a = wavelet scale b = wavelet translation c = speed of sound D = jet exit diameter SrD = Strouhal number based on jet exit diameter s = radial distance from the jet centerline to the microphone array uc = convective velocity of large structureswithin the jet mixing layer x0 = downstream distance of the front microphone within array ± = space between microphoneswithin the array = standard deviation of the sound pressure dat