WHY DO VORTICES GENERATE SOUND

Emphasizing physical pictures with a minimum of analysis, an introduct ory account is presented as to how vortices generate sound. Based on t he observation that a vortex ting induces the same hydrodynamic (incom pressible) flow as does a dipole sheet of the same shape, simple physi cal arguments for sound generation by vorticity are presented, first i n terms of moving vortex rings of fixed strength and then of fired rin gs of variable strength. These lead to the formal results of the theor y of vortex sound, with the source expressed in terms of the vortex fo rce rho(u (boolean AND) zeta') and of the form introduced by Mohring i n terms of the vortex moment (y (boolean AND) zeta'), (rho is the cons tant fluid density, u the flow velocity, zeta = del (boolean AND) u th e vorticity and y is the flow coordinate). The simple ''Contiguous Met hod'' of finding the contiguous acoustic field surrounding an acoustic ally compact hydrodynamic (incompressible) field is also discussed. So me very simple vortex flows illustrate the various ideas. These are al l for acoustically compact, low Mach number flows of an inviscid fluid except that a simple argument for the effect of viscous dissipation i s given and its relevance to the ''dilatation'' of a vortex is mention ed.