Measurement of Source Wave-Packets in High-Speed Jets and Connection to Far-Field Sound

A novel rotating near-fleld microphone array is used to measure multi-point pressure statistics on a conical surface surrounding the jet plume, just outside the turbulent shear layer. The microphone array extends axially to 10 jet diameters, with a maximum radial distance of 2 diameters from the jet centerline. A Green’s function based method is used to project the near-fleld pressure to the acoustic fleld. The diagnostic method is an extension of the approach previously described by Reba et al., 1 relying on measurement of equivalent noise sources described by second order statistics of a scalar quantity (pressure), rather than fourth-order statistics of a vector quantity over a volume as required by approaches based on the Lighthill Acoustic Analogy. Although the source description adopted here is less fundamental than that of Lighthill, it can be measured experimentally with relative ease. The diagnostic method is applied to a Mj = 1:5 ideally expanded jet over a range of temperature ratios, with acoustic Mach numbers from 1 to 2. It is shown that the near-fleld pressure statistics are well-represented by a Gaussian wave-packet model. Model parameters include convection speed, spatial source extent, and streamwise correlation scale. The acoustic fleld re-constructed from the near-fleld data is compared to direct far-fleld measurements and shown to give very good agreement.