The structure of concentrated vortex cores

Abstract In a vortex we often observe an approximately axially symmetric core of spiralling fluid in which the vorticity is high. A review is given here of recent work on the structure of such vortex cores. It begins with a discussion of the equations of motion and the appropriate boundary conditions, and continues with a description in general terms of the vigorous but highly responsive character of the flow, of the effects of compressibility and turbulence and of the phenomenon of energy separation. In many vortex cores, including those of trailing vortices far downstream behind a body, and those of leading-edge vortices above slender wings, the variations of velocity and pressure in the axial direction are small compared with those in the radial direction. Such cores, called quasi-cylindrical, are described in some detail. When this simplifying feature is absent, as in some naturally occurring vortices and some vortex-tube flows, only rough calculations can be made and the more rigorous work is confined to similarity solutions and expansions based on these. Finally, a discussion is given of the phenomena of stagnation in the axial flow, spiral instability and finite transition, any of which may destroy the typical core of high vorticity or prevent its maintenance.