The dynamics of shock accelerated light and heavy gas cylinders

Experiments have been carried out in which a cylindrical volume of a gas, that is either lighter or heavier than its surroundings, is impulsively accelerated by a weak shock wave. Laminar jets of helium or sulphur hexafluoride (SF₆) are used to produce the cylinders, and planar laser-induced fluorescence is used to visualize the flow. It is found that the vorticity deposited on the boundary of the SF₆ cylinder by the interaction with the shock wave, separates from the heavy gas to form a pair of vortices, which subsequently wrap the SF₆ around them. This process is quite different from what is observed in the light gas experiments, which showed a small amount of helium to remain with the vorticity, eventually becoming part of the vortex cores. Centrifugal forces combined with differences in the rates of the diffusion of vorticity in the two gases are given as possible reasons for these differences. Measurement of the initial downstream velocity for a heavy gas cylinder is found to agree well with a theory based on two simple models. But, because diffusion causes the light gas jet density to be significantly greater than that of pure helium, the theory overpredicts the measured velocity of the light gas experiments. The final translational velocities for both light and heavy gas experiments are not accurately predicted by the model, and measurements of the vortex spacing are found to be significantly larger than those indicated by this theory. These differences are likely caused by the theory's inability to accurately describe the viscous nonuniform flow.