Vortex Sound

Physical arguments are developed to show how aerodynamic sound is generated as a result of the movement of vortices, or of vorticity, in an unsteady fluid flow. In a very simple model the aeolian tone is attributed to the dipolelike flow caused by the stretching of vortex rings. There is no net stretching of vortex rings in a free flow, but each moving element of vorticity still causes a local dipole-like flow, so that equal and opposite elements act together to form quadrupoles. Mathematical developments follow. The simple aeolian tone model is shown to be valid for low Strouhal numbers, the general result also incorporating the effect of the fluctuating Bernoulli pressure at the surface of the body. The free flow quadrupole model is shown to be exact, and also to agree with Lighthill's result. Using vorticity as a common basis, the compressible flow is compared to the corresponding incompressible one, the hydrodynamic field and the relationship to classical field theory being discussed. (Author).