Direct numerical simulations of Richtmyer–Meshkov instabilities in shocked fluid layers are reported and compared with analytic theory. To investigate new phenomena such as freeze‐out, interface coupling, and feedthrough, several new configurations are simulated on a two‐dimensional hydrocode. The basic system is anA/B/Acombination, whereAis air andBis a finite‐thickness layer of freon, SF6, or helium. The middle layerBhas perturbations either on its upstream or downstream side, or on both sides, in which case the perturbations may be in phase (sinuous) or out of phase (varicose). The evolution of such perturbations under a Mach 1.5 shock is calculated, including the effect of a reshock. Recently reported gas curtain experiments [J. M. Budzinskietal., Phys. Fluids6, 3510 (1994)] are also simulated and the code results are found to agree very well with the experiments. A new gas curtain configuration is also considered, involving an initially sinuous SF6or helium layer and a new pattern, opposite mushrooms, is predicted to emerge. Upon reshock a relatively simple sinuous gas curtain is found to evolve into a highly complex pattern of nested mushrooms. ©1996 American Institute of Physics.