Mach-number dependence of acoustic source properties in high speed jets - Part I: Ensemble statistics of acoustically active regions

We present ensemble statistics connecting the near-field and far-field of a cold circular jet at Mach numbers 0.6, 0.85 and 1. The far-field data consist of pressure measurements, specifically in the cone dominated by noise generated by coherent structures. The near-field data consists of pressure measurements just outside the shear layer, and of 10 kHz TR-PIV measurements in various windows of the near-field. We extract diagnostic time-series from the PIV data, providing spatial and temporal resolution. Band-pass filtering is performed by using continuous wavelet transforms. We calculate frequency-specific cross-correlations between near-and far-field data, thereby locating and characterizing the near-field regions most active in the production of coherent noise. The topological diagnostic Q in particular seems to be active in a very narrow range of axial, transverse and spectral values. Possible changes in active regions below, at and above Ma = 0.85 are noted for further study.