The Richtmyer-Meshkov instability occurs when a shock wave impulsively accelerates a perturbed, corrugated interface between two fluids of different densities, during which vorticity is deposited through the baroclinic torque term $(\nabla \rho \times \nabla p)/\rho^{2}$, leading to the growth of interface perturbations.
The animation shows a cylinder of denser gas (white) in still air (black) before being hit with a Mach 1.2 shock wave
1 Nishihara K., Wouchuk J. G., Matsuoka C., Ishizaki R. and Zhakhovsky V. V. 2010. Richtmyer-Meshkov instability: theory of linear and nonlinear evolution. Phil. Trans. R. Soc. A.
2 Santhosh K. Shankar, Soshi Kawai, Sanjiva K. Lele; Two-dimensional viscous flow simulation of a shock accelerated heavy gas cylinder. Physics of Fluids 1 February 2011; 23 (2): 024102.