Aero-Acoustics of Drag Generating Swirling Exhaust Flows

Aircraft on approach in high-drag, high-lift configurations create inherently noisy flow structures. For flaps, slats, and undercarriage, the strong correlation between overall noise and drag suggests that future quiet aircraft will need to generate drag at low noise levels. This paper presents a novel noise-reduction concept based on the idea that appreciable pressure drag can be generated by a relatively quiet swirling exhaust flow. A first aeroacoustic assessment of ram-pressure-driven swirling exhaust flows and their associated vortex breakdown instability is presented. The technical approach combines 1) an in-depth aerodynamic analysis, 2) qualitative acoustic source descriptions via plausibility arguments, and 3) detailed quantitative phased microphone-array measurements of a model-scale engine nacelle with stationary swirl vanes at a full-scale approach Mach number of 0.17. The analysis shows an acoustic signature composed of 1) quadrupole-type turbulent mixing noise in the swirling core flow and 2) scattering noise from vane boundary layers and turbulent eddies of the burst vortex structure near the nacelle, pylon, and vane centerbody trailing edges. The highest stable swirl-angle setting yields a nacelle-area-based drag coefficient of 0.83 with a full-scale overall sound pressure level of about 40 dBA at the International Civil Aviation Organization approach certification point.