NONAXISYMMETRIC INSTABILITIES IN A SLENDER TORUS - 2-DIMENSIONAL AND 3-DIMENSIONAL SIMULATIONS

Nonaxisymmetric instabilities in accretion disks are investigated in terms of the slender, non-self-gravitating, ideal fluid torus model. Nonlinear simulations are carried out in two- and three-dimensions, using Cartesian-grid finite differencing. The fastest growing instability is the principal mode. This mode saturates with the formation of ellipsoidal density distributions, designated as "planets." The simulations provide evidence for nonlinear mode-mode coupling. When several wavenumbers m are unstable, multiple "planets" form that subsequently merge. Sources of numerical error are examined, and the effects are contrasted with those of the physical instability. The same qualitative evolution is seen in both the two- and the three-dimensional simulations, even though strict vertical hydrostatic equilibrium is no longer rigorously maintained after mode saturation.