A novel air-brake concept for next generation, low-noise civil aircraft is introduced. Deployment of such devices in clean airframe configuration reduces 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 degrees. 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 physical phenomenon, generating a white-noise signature that is about 15 dB louder than a stable swirling flow.

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