Mechanisms of coupling-induced instabilities in shock-accelerated tandem light square bubbles

This study presents high-resolution numerical simulations of shock-accelerated tandem helium square bubbles to investigate coupling-induced instabilities, focusing on the effects of non-dimensional initial separation distance and Atwood number on interface dynamics and mixing behavior. The simulations are performed using a third-order modal discontinuous Galerkin method for the two-dimensional compressible Euler equations, with shock Mach numbers in the range 1.22≤Ms≤1.42. The results demonstrate that closer initial separations lead to enhanced vortex interaction, earlier jet formation, and stronger baroclinic vorticity generation, culminating in accelerated interface deformation. A simplified theoretical model is developed to elucidate the dependence of vorticity on shock strength, Atwood number, and bubble spacing, showing good agreement with the numerical data. Secondary instabilities such as the Kelvin–Helmholtz instability are identified and quantified through enstrophy evolution and circulation analysis. Furthermore, a normalized mixed mass metric is introduced to assess the interfacial mixing, revealing a significant increase in mixing at smaller separation distances. The results underscore the critical role of geometric configuration and density contrast in governing shock-induced multiphase flow phenomena and provide quantitative benchmarks for Richtmyer–Meshkov instability studies.