The role of nozzle exit-lip surface roughness on jet noise

This paper attempts to describe the jet noise sensitivity to the surroundings by considering the surface roughness of the nozzle exit lip. A thick lip convergent nozzle is used, and its lip surface roughness is altered by attaching sandpapers of various grades ranging from smooth (320, 120 grit) to rough (coarse, 80 grit). Far-field acoustic measurements for different under-expansion levels are carried out by changing the nozzle pressure ratio from 1.5 to 6.5. The results are compared with that of the smooth lip nozzle. The jet noise (screech tone) varies with the surface roughness of the exit lip due to changes in the receptivity caused by the diffuse reflection of upstream acoustic waves by the lip. The screech tone is eliminated with the rough sheets at higher pressure ratios, while screech amplitude is decreased for smooth sandpapers.