Space-time Description of Supersonic Jets with Thermal Non-uniformity

The supersonic jet plumes that exhaust from the engines of tactical aircraft produce intense noise signatures that expose the Navy personnel working on the deck of aircraft carriers to dangerously high levels of noise that often results in hearing damage. Reducing the noise radiated by these supersonic plumes is of interest to the Department of Defense and is the primary motivation of this research. Fundamentally, jet noise reduction is achieved by manipulating the nozzle boundary condition to produce changes in the turbulence development and decrease the acoustic efficiency of coherent structures. The research presented here focuses on a novel jet noise reduction technique involving a centered thermal non-uniformity that alters the base flow by introducing a temperature-driven centerline velocity deficit into a perfectly expanded Mach 1.5 jet. The results indicate $2pm0.5$ dB reductions in peak narrowband spectral sound pressure levels upstream of peak directivity directions for the non-uniform jet compared to a thermally uniform baseline, even for static thrust matched conditions. This reduction is hypothesized to be related to perturbations induced by the thermal non-uniformity that convect inside the irrotational core and reduce the correlation length scales of turbulence at locations far downstream. This hypothesis was evaluated by studying the coherent turbulence via its convective hydrodynamic footprint in the near-field. An indirect investigation of the near-field using a far-field-informed model of the wavenumber-frequency spectra indicate a reduction in the energy contained in the tail of the wavenumber spectra amplitude, suggesting a reduction in the size of large scale structures. A direct evaluation of the spatio-temporal behavior of the near-field was performed using temporally resolved schlieren images. Space-time correlations of the frequency-filtered near-field identified high frequency acoustic waves radiated by compactly coherent turbulent structures and low frequency Mach waves produced by large scale instabilities. In the thermally non-uniform case these features and their sources were found to be decorrelated at downstream regions. These results provide strong evidence that a centered thermal non-uniformity reduces the radiated noise compared to a uniform baseline by shortening the correlation length scales of coherent structures in regions far from the nozzle exhaust.