Shocked-gas experiments before and after shock arrival. Vorticity, a measure of the intensity of gas swirling, is a key feature of the complex fluid flow produced by the Richtmyer-Meshkov instability (and other fluid instabilities). Vorticity is produced by the interaction of the incident shock wave's pressure gradient with the density gradient at the boundary between the sulfur hexafluoride and the surrounding air. The maximum vorticity is produced when the pressure gradient is perpendicular to the density gradient. The schematic compares the vorticity produced in shock-wave experiments with a curtain versus a column of gas. The first shocked-gas experiments of this type at Los Alamos used gas curtains. The directions of the incident shock wave's pressure gradient and velocity are shown by the oppositely directed arrows on the shock front. (a) For a gas curtain, the density gradient points into the curtain at each of its two interfaces. The vorticity distribution just after the shock wave strikes the curtain can be roughly approximated as a row of small, equally spaced vortices of equal size. (b) For the gas column, the density gradient points radially inward. The large vortices at the sides of the shocked column show that much larger values of vorticity are produced for a gas column, which more severely test a code's ability to model complex fluid flow.