A Novel Turbomachinery Air-Brake Concept for Quiet Aircraft

Abstract

A novel air-brake concept for next-generation, low-noise civil aircraft is introduced. Deployment of such devices in clean airframe configuration can potentially reduce aircraft source noise and noise propagation to the ground. The generation of swirling outflow from a duct, such as an aircraft engine, is demonstrated to have high drag and low noise. The simplest configuration is a ram pressure-driven duct with stationary swirl vanes, a so-called swirl tube. A detailed aerodynamic design is performed using first principles based modeling and high-fidelity numerical simulations. The swirl-drag-noise relationship is quantified through scale-model aerodynamic and aeroacoustic wind tunnel tests. The maximum measured stable flow drag coefficient is 0.83 at exit swirl angles close to 50 deg. The acoustic signature, extrapolated to full-scale, is found to be well below the background noise of a well-populated area. Vortex breakdown is found to be the aerodynamically and acoustically limiting phenomenon, generating a white-noise signature that is about 15 dB louder than a stable swirling flow.