Nonlinear growth of dynamical overstabilities in blast waves

The numerical gasdynamics code ZEUS-2D is used to directly model the dynamical overstabilities in blast waves. The linear analysis is confirmed by perturbing a blast wave with a low-amplitude eigenfunction of the overstability. The amplitude of the perturbations is increased in order to determine the nonlinear behavior of the overstabilities. The overstability is found to saturate due to weak transverse shocks in the shell. Transverse velocities in the dense shell reach the postshock sound speed, and high-density regions with sizes of the order of the shell thickness form. Transverse oscillations continue even after saturation. This confirms and explains the damping of the overstability experimentally discovered by Grun et al. (1991).