The effects of nonuniform flows on sound generation and propagation are examined. When the mean flow gradients, the Mach number and frequency are not too small, it is not, in general, possible to separate the processes of sound generation and propagation. The aeroacoustics of unsteady nonuniform flows is therefore closely connected to the field of unsteady aerodynamics. Sound generation and propagation for potential and vortical mean flows are analyzed using theories based on the rapid distortion approximation wherein Euler’s equations are linearized about a nonlinear mean flow. For mean potential flows, two sets of results are presented. For a single lifting airfoil, it is shown that the so-called quadrupole effects become very significant. For a cascade of loaded blades, the results show strong interference between the acoustic and unsteady aerodynamic loading. Finally, for vortical flows, the coupling between the mean flow vorticity and the unsteady part of the flow strongly affects the nature, intensity and propagation of the sound waves.

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