On the successive stages of flow noise generation

Abstract

The physics of flow noise generation is discussed and a qualitative survey of the successive stages of noise formation is presented in terms of the energy transport equations of acoustic waves. Initially, an acoustic wave, whose energy transport characteristics are assumed known, is subject to dissipative thermodynamic processes. The resultant frictional heat input into the fluid causes a continuous steepening of the entropy gradients and an eventual collapse of the disturbance, thus inducing mixing of the fluid strata. From the thermodynamic aspect of these phenomena, the mixing processes may be viewed as consisting of two parts. First, an expansion of the fluid takes place, due to the elevated pressure at the end of the frictional heat input. This expansion process, which brings the pressure of the fluid back to its ambient surroundings, is itself a source of an ideal, acoustic secondary wave, since its thermodynamic nature is purely nondissipative. Subsequently, the fluid attains thermodynamic equilibrium by means of an isobaric contraction process. By an analysis of the wave energy terms, it is shown that it is the frictional collapse and its resultant sudden expansion, which constitute the main acoustic flow noise sources.