Introduction

Numerical schemes for the simulation of viscous rotational flows usually adopt one of two well known frameworks of reference, Eulerian or Lagrangian. Attention is focussed upon the whole of the relevent flow regime in Euler methods, usually by means of a spatially distributed fixed grid or cellular structure upon which to hang such data as the local velocity and fluid properties, updated at each stage of a time stepping procedure. Vortex dynamics on the other hand generally follows the alternative route of Lagrangian modelling in which attention is focussed upon individual particles as they move through the fluid. According to vortex cloud theory all disturbances in incompressible viscous flow can be linked to vorticity creation at solid boundaries, followed by continuous convection and diffusion. A cloud of discrete vortices may thus in principle be able to represent any rotational viscous fluid motion, accuracy depending upon the degree of discretisation and the quality of the convection and diffusion schemes.

The special attraction of this approach for external aerodynamic flows in particular is the removal of any need to consider the rest of the flow regime which of course extends to infinity. In such problems Euler models require the establishment of suitable grids extending sufficiently far out into space to define acceptable peripheral boundary conditions around the target flow regime. For simple body shapes such as cylinders or plates this may be straightforward enough.