Progress in experiments of converging Richtmyer-Meshkov instability

Abstract

When a corrugated interface separating two fluids of different properties is subjected to an impulsive acceleration, such as a shock wave, perturbations initially presented on the interface will grow with time and finally develop into turbulent mixing. This type of hydrodynamic instability is generally referred to as Richtmyer-Meshkov (RM) instability. RM instability is a significant topic in fluid mechanics, and has become increasingly attractive in recent years due to its academic significance in vortex dynamics and compressive turbulence as well as important applications in the fields of inertial confinement fusion, supersonic combustion and supernova explosion. For example in inertial confinement fusion, the implosion of the spherically converging shock wave induces intensive mixing between the outer ablator and the inner fuel, and hence greatly decreases the fusion yield. Over the past decades, the RM instability induced by a planar shock wave has been intensively studied through experiments, simulations and theories. Previous studies show that the initial perturbation on the density inhomogeneity subjected to a planar shock wave experiences successively linear growth, nonlinear growth and turbulent mixing. The pressure disturbance and the baroclinic vorticity are found to be the main factors responsible for the perturbation development. However, in most realities, RM instability occurs in a convergent geometry, and the geometry convergence nature would significantly complicate the instability development. So far, the RM instability triggered by a converging shock has seldom been investigated, and the instability development pattern in convergent geometry as well as the influences of the convergence effects, such as the Bell-Plesset (BP) and Rayleigh- Taylor (RT) effects, on the instability development remains unclear. To perform a converging experiment, one has to overcome two crucial challenges in RM instability experiments: producing a stable converging shock wave and generating a well-controlled initial interface in laboratory conditions, and these are very difficult to realize using the current experimental methods. In this paper, we review the converging RM instability experiments in two types of shock tube facilities, i.e., the coaxial-structure converging shock tube and the conventional converging shock tube. Various interface formation techniques developed based on these shock tube test sections of different structures are also discussed. Special attention is paid to the influences of the typical convergence effects, including the BP effect, the RT effect and the compressibility, on the instability development. Finally, based on the existing studies, we propose the following three issues for future experimental study: (1) RM instability induced by high Mach number shock waves; (2) developments of three-dimensional interfaces subjected to an impulsive impact; (3) interaction of a rippled shock with an unperturbed or a perturbed density interface.