Problems in flow acoustics

The research field "flow acoustics" is defined, and problems of current interest are discussed. The physical interpretation of fluid-mechanical sound sources for an acoustic medium, both at rest and in motion, is addressed. In the first case, it has been possible to relate the sound pressure field for low Mach numbers to sound sources that depend linearly on velocity; progress has been achieved by means of the causality correlation method with the application of laser-Doppler velocity measurements. In the theory of the acoustic medium in motion, it has been found that vortices downstream from a nozzle discharge can function as sound sinks; for a unidirectional mean flow, conditions for the sound sources are developed on the basis of the causality principle and the boundedness of the flow quantities. Experimental and theoretical results for flowfield oscillations in a high-velocity duct flow with sudden duct enlargement are discussed. Finally, some points concerning the influence of shear flow on sound propagation are described.