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 decreasing distributed vortex region in a two-dimensional incompressible fluid. In particular, an initial small-but-finite amplitude, 3-fold, ''edge''-located hole or depletion perturbation of a monopole evolves into 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 magnitude. Near-inviscid simulations are made with surgery-regularized contour 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 results. Our numerical results are in agreement with small-but-finite amplitude 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 Vlasov plasmas. (C) 1996 American Institute of Physics.