The development of contemporary views of flow-tone generation

The evolution of theoretical understanding of the sources and characteristics of flow tones in the 20th century is traced to the present view that noise originates from hydrodynamic instabilities. The instabilities are reinforced by the geometry of the flow channel and result in vortices which are the noise sources over a wide range of Re. The two types of flow tones, identified in extensive investigations, are shear-flow surface interaction with simple fluid path feedback, and acoustic resonance or flow entrainment on solid surface. The surface is necessary for stabilizing the oscillatory phenomena. The survey covers work on subsonic and choked jet tones and cavity tones. It is shown that the tones are stimulated by a trip mechanism, i.e., pressure or velocity fluctuations, which cause secondary pressure or velocity fluctuations. The fluctuation is transmitted upstream to a boundary region, where the process of reinforcing the downstream disturbance is reinitiated.