Aeroacoustics of Three-Stream High-Speed Jets from Coaxial and Asymmetric Nozzles

Abstract

The development of three-stream variable cycle turbofan engines provides an opportunity for noise reduction by optimizing the exhaust conditions of the secondary and tertiary streams. This includes adjustment of the velocity ratios and, more importantly, reshaping of the nozzle. The paper reviews an experimental study of subscale three-stream nozzles, rapid-prototyped with high accuracy and operating at high specific thrust. Far-field noise surveys were collected using a 24-microphone array. For low-bypass configurations, applicable to supersonic aircraft, a coaxial configuration offers no significant noise benefit compared to the single-stream exhaust. Configurations with offset secondary or tertiary streams offer significant noise reduction in the direction of the thicker flow. For bypass ratio around 0.5, reductions of 5.1 dB in overall sound pressure level and 4.2 dB in effective perceived noise level were attained.