AXISYMMETRIZATION OF AN ISOLATED VORTEX REGION BY SPLITTING AND PARTIAL MERGING OF SATELLITE DEPLETION PERTURBATIONS

In numerical studies, we observe the essentially inviscid approach to axisymmetry of an isolated-and-perturbed monopolar and uniformly decre asing distributed vortex region in a two-dimensional incompressible fl uid. In particular, an initial small-but-finite amplitude, 3-fold, ''e dge''-located hole or depletion perturbation of a monopole evolves int o a 2-fold state. This occurs through the nonlinear processes of hole stretching, splitting ann partial merger. The initial growth rate for this downward cascade is proportional to the initial perturbation magn itude. Near-inviscid simulations are made with surgery-regularized con tour dynamics codes (CDS), using from 5 to 11 contours. Pseudospectral (PS) simulations with varying Newtonian and hyperviscosities, that is considering continuum and dissipation effects, yield consistent resul ts. Our numerical results are in agreement with small-but-finite ampli tude perturbations that were used in recent laboratory experiments on magnetized electron columns. This process may also occur in late time evolutions associated with the ''bump-on-tail'' initial condition in V lasov plasmas. (C) 1996 American Institute of Physics.