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

The present study demonstrates the capabilities of an optical deflectometry (OD) technique to provide flow field turbulence measurements that can be successfully correlated with far-field acoustic field measurements. Results from supersonic fully expanded jets are presented and show variations in the frequency content of the coherence between the flow field and the acoustic field with OD positioning. The method is applied to jets with supersonic convection speed, producing Mach wave radiation, and show the strong directionality of the emitted noise. Similar measurements performed in shock-containing jets with microphones in the forward arc reveal enhanced coherence at the same frequency as the Broad Band Shock Associated Noise. Measurements are also made 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 characteristic of the hydrodynamic region, where an evanescent pressure field dominates the total pressure fluctuation. The high level of skewness in these signals also support the concept that the crackle observed in the far-field originates in the near field. Further radially, acoustic pressure fluctuations are measured and successfully correlated to the acoustic far-field, thus demonstrating another application of this non-intrusive measurement technique in jet acoustics diagnostics.