Impinging jet noise reduction using non-circular jets

Experimental studies in the acoustic far field and flow visualization are carried out on circular and non-circular impinging jets at a nozzle to plate distance ratio of 5, for the nozzle pressure ratios of 1.8, 4, and 6. Spectra and directivity of the overall sound pressure level of circular jets are compared with those of non-circular jets for the same nozzle pressure ratios, at various emission angles. Non-circular jets mitigate the transonic impinging tones at around the critical pressure ratio, compared to the circular jet. Square jet is quieter at low nozzle pressure ratios by 5-8 dB compared to the circular jet. At higher under expansion conditions, the triangular and square jets almost completely eliminate the supersonic impinging tones. Elliptical jet is noisy at a nozzle pressure ratio of 6, even compared to the circular jet. Elliptical minor axis plane is quieter at lower nozzle pressure ratios compared to the major axis plane and the trend is reversed for the higher nozzle pressure ratios. Schlieren flow visualization studies reveal that the shock cell structures are weakened and symmetry of the shock cells are lost for the non-circular jets compared to the circular jet.