Vorticity and the theory of aerodynamic sound

Lighthill strongly advocated the use of vortex methods in most areas of flu id mechanics with the notable exception of the theory of aerodynamic sound. But it is straightforward to transform his famous 'acoustic analogy' to ma ke vorticity rather than Reynolds stress the ultimate 'source' of sound in homentropic flows. 'Vortex sound' theory becomes especially useful in appli cations involving acoustically compact flow-structure interactions, where i t actually emerges as an extension of Kelvin's theory of 'vortex impulse', a notion that Lighthill regarded as important enough to be given special tr eatment in undergraduate lectures on fluid mechanics. The 'impulse source' can be recast in a form more suited for numerical or analytical evaluation, and is closely related to the 'compact Green's function'. Convergence diff iculties encountered in the casual application of the acoustic analogy to n on-compact flow-structure interactions are resolved in a natural manner by the methods of vortex sound theory. New illustrations of these methods are given in this paper by consideration of the unsteady development of lift by a starting airfoil, of the production of sound by a 'vortex whistle', and of the infrasound generated when a high-speed train enters the tunnel.