This paper presents extensive acoustic measurements on jets impinging on surfaces of various surface roughness values. Besides surface roughness, the effects of nozzle-to-plate spacing distance and nozzle pressure ratio are also investigated. Turbulent mixing noise and tonal noise are explained using far-field wall-jet velocity and impingement region temperature fields. The results demonstrate that roughness of the impingement plate widens the staging region of impingement noise. In general, high speed jet impinging on a rough plate generates less noise compared to a smooth plate. When tones are removed from the spectra, it is found that acoustic power monotonically decreases with increasing surface roughness. Thermal imaging in the stagnation region indicates that whenever tones are present, the temperature at the stagnation region is high. Further, sound pressure directivity pattern of impingement noise is constructed by superimposing a wall-jet and a free jet in the appropriate orientations.

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Experimental acoustic investigation of underexpanded free and impinging jet is carried out for various nozzle-plate spacing. The reservoir pressure is slowly increased from atmospheric pressure to 6 bar and than decreased from 6 bar to atmospheric pressure. The free jet acoustic radiation remains same for both paths, but it is observed that for impinging jet the acoustic radiations differ in some regions. The hysteresis effect observed in acoustic characteristics may be due to the presence of hysteresis effects in the recirculation zone of the impinging jet. This variation is significant for nozzle-plate spacing of 2 to 4 times jet diameter. It is also seen that the acoustic staging occurs for low pressure and high pressure for small and large nozzle-plate spacing, respectively.