Study on Aerodynamic Noise induced by Shock Waves in Supersonic Flows and Development of a Method for Reducing the Noise

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1. The oscillations of the normal shock waves in supersonic nozzles and supersonic diffusers were investigated using a supersonic wind tunnels, and a strong correlation were found among the shock oscillation, the static pressure fluctuation on the wall, and the generated aerodynamic noise.2. Small pressure disturbances which are generated and propagated upstream from downstream portion of the shock wave were observed, and it was found that these disturbances induce the shock oscillation. At the downstream portion, the boundary layer behaves unsteadily and violently. Also, vortices generated by the shock oscillation were observed to be convected downstream. Based on these results, the shock oscillation was explained to be sustained by the upstream-propagating disturbances and the downstream-convected vortices.3. The response of a normal shock wave to a white small pressure disturbance having the same power spectral density in the supersonic diffuses was analyzed, and it was shown that the shock displacement spectral density distributions obtained by the analysis agree very well with those measured in the present and previous experimental data.4. Theoretical and experimental investigations were performed on the structures of correctly-expanded and under-expanded supersonic laminar and turbulent jets, and the distributions of the Pivot impact pressure and flow turbulence in the jets, the length of the potential core, the condition of the generation of shock waves and their locations in the jets were made clear.5. The flow mechanism on the distributions of the Pitot impact pressure along the center line of the jet was well explained, and the correlation between the aerodynamic noise level and the correctly and under expanded supersonic jets.