Study On Richtmyer-Meshkov Instability Induced By Cylindrical Convergent Shock At A Perturbed Gas-layer

If a shock wave passes across an interface between two different fluids,initial perturbations on the interface grow continuously with time and later induce a flow transition to turbulent mixing.This type of interfacial instability is usually referred to as Richtmyer-Meshkov instability(RMI).The study of RMI is significant for academic researches such as shock dynamics,vortex dynamics and compressible turbulence,and also is of great meaning for realistic problems including supernova explosions,scramjet engine and inertial confinement fusion(ICF).In recent decades,RMI has been systematically studied through theoretical analysis,experiment and numerical simulation.So far,underlying mechanisms of planar shock-induced RMI have been well understood,and numerous theoretical models have been established to characterize the perturbation growth of planar RMI.However,convergent shock-induced RMI(particularly convergent RMI at two or more interfaces),which includes initial setting more relevant to ICF,has been rarely investigated.This thesis reports experimental and numerical studies on convergent RMI at a gas layer with two interfaces.Experimentally,an existing soap-film technique is extended to generate gas layers with controllable shapes,and a novel semi-annular shock tube is adopted to produce cylindrically convergent shocks.The flow field is recorded by a high-speed camera combined with a schelieren system.Also,high-resolution simulations with a compressible multi-component flow solver are performed to obtain detailed flow field,with which an in-depth analysis on flow mechanisms is accessible.The content of this thesis is as follows:1.The evolution of an SF6 layer with a sinusoidal outer interface and a circular inner interface surrounded by air subjected to a cylindrical convergent shock is experimentally studied.Results show that the thicker the gas layer,the weaker the interfacecoupling effect,and the slower the evolution of the outer interface.Induced by the distorted transmitted shock and interface coupling,the inner interface exhibits a slow perturbation growth which can be largely suppressed by reducing the layer thickness.After reshock,the inner perturbation increases linearly at a growth rate regardless of the initial layer thickness as well as the outer perturbation amplitude and wavelength,and the growth rate can be well predicted by the model of Mikaelian(Physica D,vol.36,1989,pp.343-357).After the linear stage,the growth rate decreases continually,and finally the perturbation freezes at a constant amplitude caused by the successive stagnation of spikes and bubbles.The convergent geometry constraint as well as the very weak compressibility at late stages are responsible for this instability freeze-out.2.The RMI of a helium layer with a sinusoidal outer interface and a circular inner interface surrounded by air is experimentally studied.It is observed that the inner interface of the shocked light gas layer remains nearly undisturbed during the experimental time even after the reshock,which is distinct from the counterpart in the heavy gas layer.This can be ascribed to the quick decay of perturbation amplitude of the transmitted shock inside the helium layer and the Rayleigh-Taylor stabilization on the inner surface(light/heavy)caused by flow deceleration.The outer interface first experiences an ’accelerated’ phase inversion owing to the geometry convergence and later suffers a continuous deformation.Compared with a sole heavy/light interface,the wave influence(interface coupling)inhibits(promotes)the instability growth of the outer interface.3.The evolution of an SF6 layer with sinusoidal inner and outer interfaces surrounded by air subjected to a cylindrical convergent shock is studied both experimentally and numerically.Results show that the development of the outer interface is evidently affected by the outgoing rarefaction wave generated at the inner interface,and such an influence relies on both the layer thickness and the phase difference between the inner and outer interfaces.The development of the inner interface impacted by a rippled convergent shock is found to be insensitive(sensitive)to the layer thickness for in-phase(anti-phase)layers.Particularly,for anti-phase layers of different thicknesses,the inner interface presents distinctly different morphologies at late stages.A new theoretical model considering the effects of baroclinic vorticity,geometric convergence,nonuniform impact of a rippled shock and the startup process is developed,which reasonably predicts the present experimental and numerical results.