Sound generated from the interruption of a steady flow by a supersonically moving aerofoil

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This paper examines a flow–aerofoil interaction problem that we believe is likely to be an important issue in advanced aircraft propulsion systems involving supersonic propellers. They are potentially noisy and it is important to identify the mechanism by which they generate noise so that they can be optimized for acceptable operation. One of the most potent sources of noise lies in the possibility that a second stage propeller blade cuts through the core of the tip vortex shed from a first stage blade. Then suddenly the streaming core flow must adjust to the boundary constraints of that second stage blade, and the adjustment will inevitably involve compressive waves that escape as sound. How strong these sound waves are, how long their life is and where they go to are important questions, the answers to some of which are obtained in this paper. We examine here what we think is a canonical problem and determine the level and directionality of the sound generated by the interruption of the axial vortex-core flow by a supersonic blade. The principal sound is launched in the Mach-wave direction, where the pressure pulse has an amplitude that decreases much more slowly than it would from spherical spreading. This pressure pulse can reach a distant observer with a very large amplitude, 160 dB or higher being typical. The peak sound pressures are found to be independent of blade speed at high supersonic tip velocity, while the energy radiated in the pulse, because of its reducing duration, attenuates as the supersonic speed increases. This aspect gives grounds for believing that the higher the speed, the quieter will be the stage interaction sound of a contrarotating supersonic propeller.