Acoustics and Thrust of Quiet Separate-Flow High-Bypass-Ratio Nozzles.

The NASA Glenn Research Center recently completed an experimental study to reduce the jet noise from modern turbofan engines. The study concentrated on exhaust nozzle designs for high-bypass-ratio engines. These designs modified the core and fan nozzles individually and simultaneously. In comparison with chevrons, tabs appeared to be an inefficient method for reducing jet noise. Data trends show that interaction between fan flow and the core cowl could strongly impact noise and cruise performance irrespective of the mixing device installed. The study demonstrates that modifications of the core nozzle are generally more advantageous than of the fan nozzle. Even greater advantage in noise reduction and associated cruise thrust penalties is demonstrated through simultaneous modification of both nozzles. The best nozzle design had a 0.06% cruise thrust loss and, corrected for takeoff thrust loss, a 2.7-EPNdB reduction for the effective perceived noise level metric. This design simultaneously employed chevrons on the core and fan nozzles. Last, five nozzle configurations with cruise thrust loss of less than 0.5% and noise reductions of over 2.5 EPNdB are identified as candidates for full-scale engine and flight demonstrations. [ABSTRACT FROM AUTHOR]