Evolution of vortex surfaces in the Richtmyer-Meshkov instability

We study the vortex dynamics in the Richtmyer-Meshkov instability (RMI) using the vortex-surface field (VSF). The VSF is a Lagrangian-based structure-identification method, whose isosurface is a vortex surface consisting of vortex lines. For the RMI with a single-mode interface separating two different fluids, we derive the initial VSF from the vorticity generated by the baroclinic effect after the normal shock wave interacting with the perturbed interface. Then the evolving VSF is calculated using the two-time method from a series of velocity fields obtained from direct numerical simulation. The typical vortex surfaces in the evolution display signature spike and bubble structures, and are slightly different from the isosurfaces of the density and vorticity magnitude before significant vortex reconnection. Based on the evolving vortex surfaces, we elucidate the development of asymmetric spike and bubble and estimate the growth rate of the mixing zone.