Drag Management in High Bypass Turbofan Nozzles for Quiet Approach Applications

Abstract

The feasibility of a drag management device that reduces engine thrust on approach by generating a swirling outflow from the fan (bypass) nozzle is assessed. Deployment of such “engine airbrakesâ€‌ (EABs) can assist in achieving slower and/or steeper and/or aeroacoustically cleaner approach profiles. The current study extends previous work from a ram airdriven nacelle (a socalled “swirl tubeâ€‌) to a “pumpedâ€‌ or “fandrivenâ€‌ configuration and also includes an assessment of a pylon modification to assist a row of vanes in generating a swirling outflow in a more realistic engine environment. Computational fluid dynamics (CFD) simulations and aeroacoustic measurements in an anechoic nozzle test facility are performed to assess the swirlflowdragnoise relationship for EAB designs integrated into two NASA highbypass ratio (HBPR), dualstream nozzles. Aerodynamic designs have been generated at two levels of complexity: (1) a periodically spaced row of swirl vanes in the fan flowpath (the “simpleâ€‌ case), and (2) an asymmetric row of swirl vanes in conjunction with a deflected trailing edge pylon in a more realistic engine geometry (the “installedâ€‌ case). CFD predictions and experimental measurements reveal that swirl angle, drag, and jet noise increase monotonically but approach noise simulations suggest that an optimal EAB deployment may be found by carefully trading any jet noise penalty with a trajectory or aerodynamic configuration change to reduce perceived noise on the ground. Constant speed, steep approach flyover noise predictions for a singleaisle, twinengine tubeandwing aircraft suggest a maximum reduction of 3 dB of peak tonecorrected perceived noise level (PNLT) and up to 1.8 dB effective perceived noise level (EPNL). Approximately 1 dB less maximum benefit on each metric is predicted for a nextgeneration hybrid wing/body aircraft in a similar scenario.