Use of Thermal Nonuniformity to Reduce Supersonic Jet Noise

It is shown experimentally that thermal nonuniformity can reduce peak supersonic jet noise while keeping static thrust levels equivalent. A thermal nonuniformity is generated by introducing an unheated stream on the centerline of a plenum containing heated air. This axisymmetric arrangement was studied with the hypothesis that the convection and structure of plume turbulence could be changed by perturbations induced by the thermal nonuniformity. The results for ideally expanded and overexpanded jets indicate 2 +/- 0.5 dB reductions in peak narrowband spectral sound pressure levels upstream of peak directivity directions for nonuniform jets compared with the baseline uniform jets, even for a modest temperature difference between the core jet and the unheated stream. The mechanisms for this reduction are examined based on wavenumber-frequency analyses using the far-field acoustic spectra, suggesting that peak spectral energies shift to higher wavenumbers when the thermal nonuniformity is introduced. Convection velocities of radiating structures calculated from the spectral peaks show a reduction of 10% for the design jet condition. These results indicate that temperature-driven velocity deficits may be useful for developing supersonic jet noise reduction strategies, while even greater reductions are thought feasible by increasing the ratio between the heated and unheated stream temperatures.