Sound Radiation by Moving Surfaces and the Green’s Functions Technique

Michel Roger

Ecole Centrale de Lyon

Abstract

The present chapter is dealing with some fundamentals of sound radiation from rigid moving bodies or bodies in a flow. The theo­retical background of the analogy is reminded in a first part. Ac­cording to Ffowcs Williams & Hawkings’ formulation the problem of sound generation by unsteady flows in the presence of solid sur­faces is restated as a problem of linear acoustics with equivalent moving sources. Therefore the solving procedure is based on stan­dard Green’s function technique. This procedure is detailed in the second part as a necessary background and source motion is con­sidered a key feature of the radiation. Aspects inherent to the wave operator and specific aspects of acoustic sources on the one hand, and source physics and source motion on the other hand, are ad­dressed separately for the sake of physical understanding. In the third part formal developments and dimensional analysis of Ffowcs Williams & Hawkings’ equation are proposed, both to highlight the flow features involved in sound generation and to point out the ef­fects of motion. Some introductory topics have been presented in chapter 1 and are re-addressed for specific purposes.

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