Beamformed Flow-Acoustic Correlations in a Supersonic Jet

An experimental study of simultaneous multipoint measurements in the flowfield and acoustic field of a Mach 1.75 cold-air jet is presented. A series of four optical-deflectometer probes measured turbulent fluctuations in or near the jet flow, and eight microphones recorded the far-field pressure in the direction of peak emission. The correlation methodology involves the coherence between the delay-and-sum beamformer outputs of the optical-deflectometer probes and the microphones. This procedure yields results with greater fidelity and higher coherence levels than obtained with individual optical-deflectometer-to-microphone correlations. With the optical-deflectometer probes in the jet shear layer, there is a significant correlation, on the order of 0.1, between the turbulent fluctuations and far-field noise. As the optical-deflectometer probe moves transversely away from the jet, its correlation with the microphone beamformer first drops and then increases. This trend signifies the transition from hydrodynamic to acoustic pressure fields. In the vicinity of the nozzle exit, the peak coherence between the beamformed optical-deflectometer and microphone signals coincides with the physical location of the optical-deflectometer probe. However, as the shear layer thickens downstream, the peak coherence generally lags the probe location, which is a probable result of acoustic refraction by the mean flow. [PUBLICATION ABSTRACT]