Far-field acoustic investigation into chevron nozzle mechanisms and trends

Chevron nozzles currently offer one of the most feasible methods of reducing jet exhaust noise in medium to high-bypass turbofan engines. Tests were conducted in the University of Cincinnati Nozzle Acoustic Test Facility, simulating a separate flow exhaust system to provide insight into some of the basic mechanisms and trends of this emerging technology. For this study, a baseline inner nozzle and three chevron nozzles were tested over a wide range of operating conditions, including dual and single How. Chevrons with varying numbers of lobes and levels of penetration were selected for this study to provide insight into the impact of these geometric parameters on the noise level. Spectral and directivity results from heated, coaxial flow tests showed that the chevron nozzles are most effective at lower frequencies and at aft directivity angles. Reductions in overall sound pressure level (SPL) ranging from 3 to 6 dB were documented. Calculations of perceived noise level directivity also showed 4-6 dB reduction at aft angles. The data also illustrated clear and consistent trends with respect to the chevron geometric parameters. Specifically, the chevron penetration was determined to be a primary factor in controlling the tradeoff between low-frequency reduction and high-frequency SPL increases. Although slight differences were observed with varying chevron lobe numbers at a fixed penetration, it appears that the effect is less significant than the penetration. Finally, the data indicated clear dependence of the chevron benefit on the velocity difference between the inner and outer streams.