The generation of sound by aerodynamic sources in an inhomogeneous steady flow

Abstract

This paper examines the effect of steady flow on the generation of sound by aerodynamic sources located in the neighbourhood of a scattering body. The analysis is facilitated by the use of a reverse-flow reciprocal theorem which is valid for a certain class of inhomogeneous flow problems. In the case of acoustic ‘bremsstrahlung’, in which sound is generated during the passage of a silent fluid inhomogeneity, such as an entropy spot, past an obstacle in the flow, an elegant discussion of the interaction can be given in terms of a renormalized Green's function. Here the effect of the obstacle is equivalent to a local distortion of the geometry of the three-dimensional space occupied by the medium, the extent of which governs the properties of the acoustic radiation. This is illustrated by means of a detailed analysis of the sound generated during the unsteady convection of a source of constant strength through a neck in a duct.