Distributed exhaust for jet noise reduction

Abstract

Acoustic detection of low flying, jet-powered military air vehicles and takeoff noise levels from commercial jets are often driven by jet mixing noise radiated from the exhaust. Cueing provided by the noise signature results in increased opportunity for a ground-based visual observer against such a target. Significant reductions in detectability and takeoff noise levels can be achieved through reductions in jet noise. Research has been conducted over the past several years to develop innovative, quiet, distributed exhaust nozzle (DEN) concepts, which attempt to achieve revolutionary reductions in jet mixing noise while minimizing propulsion penalties. The DEN’s benefit relies on discharging the exhaust flow through many miniature nozzles rather than one or two large nozzles. Noise suppression from the DEN concept results from a favorable shift in frequency content compared to conventional jets. Significant increases in atmospheric attenuation and decreases in the ear’s sensitivity at these higher frequencies result in much reduced detection ranges and perceived noise levels. This technology is applicable to both subsonic and supersonic aircraft, and particularly to low flying fixed-wing unmanned air vehicles, special operations forces transports, and future commercial transports with especially stringent noise reduction requirements. [Work supported by NASA LaRC.]