Effects of initial perturbation spectra on converging Richtmyer–Meshkov turbulence

This work experimentally investigates Richtmyer–Meshkov (RM) turbulence driven by a cylindrical converging shock using time-resolved planar laser-induced fluorescence. An automatically retractable plate is developed to generate membraneless yet sharp interfaces characterised by random short-wavelength perturbations and controllable long-wavelength components. Five initial interfaces are examined: one with random short-wavelength perturbations, and four with these random perturbations superimposed with single-mode long-wavelength perturbations of controlled amplitude and wavenumber. The shocked interface in the converging configuration exhibits strongly unsteady motion: it initially moves inwards uniformly, then decelerates, moves outwards following reshock, and finally decelerates again with noticeable oscillations. The interface initially develops with a well-defined morphology, but quickly transitions to turbulent mixing after reshock. Both reshock and Rayleigh–Taylor instability, unique to converging RM flows, significantly influence the mixing layer evolution. The mixing width exhibits significantly higher post-reshock growth rates compared with the planar counterpart. Scalar mixedness analysis indicates that the random-perturbation case achieves significantly higher homogenisation efficiency than single-mode cases, whereas the mean scalar dissipation rate follows a similar temporal pattern regardless of initial morphology. The initial perturbation spectrum strongly influences the timing of turbulent transition: the high-wavenumber single-mode case reaches the empirical threshold earliest, followed by the random-perturbation case, while the low-wavenumber single-mode cases exhibit substantially delayed or incomplete transition. Scalar energy spectra further highlight that initial perturbations critically affect the efficiency of turbulent cascade and the small-scale energy distribution.