One sight on sound generation and absorption by vortices

Vortices generate sound and absorb sound. A. Powell and M. S. Howe deduced the equations and showed that the exterior product of vortex and local velocity took part in sound generation and absorption. However, it is difficult to image its physical phenomena. In this paper, as one example, the interaction of sound and vortices at the throat of a cavity with grazing flow is investigated by CFD simulation. As the results, it is found that sound absorbing regions and sound generating regions are locally distributed and moving. It is also found that when counterclockwise vortices turn to the left, they absorb sound and when they turn to the right, they generate sound. Based on these results, 'Floating Vertical Bar Model (FVB Model)' is proposed to image the sound generation and absorption phenomena, in which a vortex is simulated as a bar floating vertically on the water surface, because a vortex has coherent structure along its axis like a bar. When a vortex (a bar) is accelerated perpendicular to its axis, pressure fluctuation (water wave) is easily generated. And when sound wave (water wave) shakes a vortex (a bar), sound energy is absorbed by a vortex (a bar) motion.