PROLOGUE

This chapter is concerned with noise suppression. Noise in the context of this book is undesired sound—particularly that generated by a fluid flow. Although tangential to the primary topics of this monograph, noise control is an important and broad topic and deserves, and is addressed in, its own books. Hence, the treatment here is rather cursory and consists simply of introducing the topic and relating noise control to other flow control topics discussed elsewhere in this book. Particularly for cold subsonic flows, small-scale turbulence fluctuations and unsteady flow oscillations, either in free-shear modes or interacting with solid surfaces, provide the primary sources for the flow-induced sound energy. For hot supersonic flows, the interaction of the turbulence large eddies with the flow is the dominant noise source. In either case, therefore, controlling the flow modulates the sound field favorably or adversely. The goal is of course to reduce noise pollution either for human comfort, for military advantage, or for the prevention of violent structural vibrations. Both passive and active control strategies can be employed to suppress flow-induced noise.