Excess Broadband Noise Observed with Overexpanded Jets

Results of an experiment on the characteristics of an excess noise occurring with convergent - divergent nozzles in the overexpanded regime are presented in this paper. Data are obtained with five convergent - divergent nozzles and a convergent nozzle, all having the same exit diameter. The results clearly establish that, relative to the convergent case and at same pressure ratios, the convergent - divergent nozzles are noisier in the low Mach number range of the overexpanded regime. This is evidenced from the directivity patterns as well as the overall radiated sound power. The excess noise is broadband in nature and is found to be more pronounced with nozzles having a higher design Mach number or larger half angle of the divergent section. It appears to occur when a shock resides within the divergent section and results from random unsteady motion of the shock. [PUBLICATION ABSTRACT]