Mechanisms of nozzle lip noise and experimental investigation by causality correlation approach Free

Excess jet noise, arising from upstream flux unsteadiness and pressure fluctuations near the nozzle lip, may be of important consequence to the limiting performance of multi‐element jet noise suppressor nozzles. The relative significance of the excess noise increases as Mach number is reduced. Important parameters include the shape of the nozzle approach cone, the velocity ratio across the lip, the nature and thickness of the boundary layer at the exit plane (laminar or turbulent), the lip thickness and the levels of nozzle‐borne turbulence. In our experiments, nozzles of 2‐cm diameter were instrumented with specially constructed pressure transducers implanted into the nozzle surfaces. Considerable effort was expended to devise a transducer/pinhole configuration with frequency response to 100 kHz. Our experiments yield the distribution of surface dipole strength over the nozzle lips, by cross‐correlation of the pressure transducer signals with sound signals recorded in the farfield, for various angles and axial positions of the nozzle transducer [as is Siddon, J. Acoust. Soc. Am. 53, 619–633 (1973)]. Results suggest that an optimally contoured nozzle with laminar boundary layer generates insignificant lip noise, but certain types of internal flow disturbance will enhance the sideline radiation at high frequencies. The lip dipole strength is concentrated within one or two wavelengths of the exit plane, and has a circumferential distribution varying roughly as cosφ squared. [Work supported by the General Electric Company, Aircraft Engine Group, Evendale, OH.]