Study on mode-coupling effects in diverging Richtmyer–Meshkov instability

Mode-coupling effects in diverging Richtmyer–Meshkov (RM) instability are experimentally investigated using a special shock tube designed based on shock dynamics theory. Dual- and tri-mode discontinuous air/SF6 interfaces, precisely generated using a soap-film technique, are examined. High-speed schlieren images show that mode coupling significantly influences the evolution of constituent modes, with the coupling strength strongly dependent on the initial modal spectra. A robust and efficient vortex method is developed to simulate interface deformation and mode growth across all experimental cases. By incorporating point deletion and adaptive desingularization, the method allows for robust tracking of strongly nonlinear interface evolution and achieves reasonable agreement with the present experiments, as well as those in planar and converging geometries, highlighting its potential as a versatile tool for multi-mode RM instability analysis. Comparative analysis under identical initial conditions among three geometric configurations reveals contrasting geometric effects: geometric expansion suppresses mode coupling, while contraction enhances it. Additionally, interface deceleration in curved geometries induces the Rayleigh–Taylor stabilization effect that progressively strengthens, which together with geometric expansion significantly weakens mode coupling in diverging configurations. Suppression of mixing-width growth due to mode coupling is consistently observed in multi-mode RM instability across all geometries considered.