Sound Radiation by Moving Surfaces and the Green’s Functions Technique
Ecole Centrale de Lyon
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.