Beamformed Flow-Acoustic Correlations in High-Speed Jets

An experimental study of simultaneous multi-point measurements in the flow and acoustic fields of a Mach 1.75 cold air jet is presented. A series of four optical deflectometer (OD) probes were used for the flow field measurements, and eight microphones arranged on a circular arc recorded the far-field pressure in the direction of peak emission. In addition, some of the OD probes were positioned in the near acoustic field of the jet. The correlation methodology involves calculating the delay-and-sum beamformer outputs of the OD probes and microphones, then computing the coherence between the two outputs. With the OD 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 in the direction of peak emission. As the OD probe moves transversely away from the jet, the coherence with the far-acoustic field first drops and then increases significantly to levels on the order of 0.3. The drop is associated with the probe moving into the hydrodynamic pressure field that does not radiate to the far field; the subsequent increase signifies the probe being located in the acoustic pressure field. In the vicinity of the jet exit, the peak coherence between the OD signal and microphone beamformer coincides with the physical location of the OD probe. However, as the shear layer thickens downstream, the peak coherence generally lags the probe location. It is argued that this is caused by the refraction of the acoustic rays by the mean velocity and speed-of-sound gradients. The effect is particularly severe when the OD probe is on the jet centerline past the end of the potential core. This underscores the need to include refraction effects in calculating the beamformer output of the microphone array.