On the evolution of spherical gas interfaces accelerated by a planar shock wave

Abstract

The interaction of a planar shock wave with a spherical gas interface (SF6 or helium) surrounded by air is studied experimentally and numerically. By means of the high-speed schlieren photography with high time resolutions and the numerical method VAS2D, the detailed flow field structures including the evolution of interfaces and the development of wave patterns are obtained. The sequences of schlieren frames of SF6 show that the refracted shock wave converges inside the volume and causes the shock focusing within the bubble interface, resulting in an outward jet. The SF6 jet is different from the krypton jet studied because of the difference in their acoustic impedances. The pressure perturbation plays an important role in the jet formation. Quantitative data of the evolving interface length, height, and vortex spacing as well as the displacements of the interface and the jet are acquired and compared. The generation and distribution of vorticity are also analyzed numerically and are found to be the dominant factors for the interface deformation and the resulting turbulent mixing. Furthermore, the velocities of the interfaces and shock waves obtained from the x-t diagrams in the experimental and numerical photographs are compared with those predicted from one-dimensional gasdynamics. In this way, the air contaminations of both gases are re-evaluated and it is shown that the contamination by air inside the helium bubble is significant while it has just small influence in the SF6 case, which confirms the findings in literatures.