Experimental Study On Richtmyer-Meshkov Instability Induced By A Curved Shock

The Richtmyer-Meshkov(RM)instability occurs when a shock wave passes through a distorted interface between two fluids of different properties.RM instability plays a crucial role in inertial confinement fusion(ICF),supernova explosion,scramjet and other practical problems,and it also has great significance in academic researches such as flow stability,vortex dynamics and compressible turbulence.The RM insta-bility induced by a curved shock wave is more closely related to practical applications and thus receives increasing attention in recent years.For example,in ICF,as the target pellet is ablated by laser irradiation,a spherical convergent shock wave is immediately produced.Later,the spherical shock wave moves inward and passes through the ma-terial interface inside the target,triggering the RM instability.In supernova explosion,the spherical diverging shock wave produced by the collapse explosion propagates out-wards and can also initiate the RM instability,leading to changes in the volume of the nebula.It has been found that a theoretical model that takes the effects of RM instability into account can better predict the evolution of nebula volume.Therefore,studies on the development of the perturbed interface under the impact of convergent and diver-gent shock waves can help to better understand these problems.In this paper,new ex-perimental methods for generating convergent and divergent shocks are first proposed,based on which a convergent-divergent shock tube is developed.Then,experimental studies on the RM instability at a single-mode interface impacted by cylindrically con-vergent and divergent shocks are carried out in the newly developed shock tube.The main contribution of this paper is given below:1)Based on shock dynamics theory,a new convergent-divergent shock tube with special wall profiles is developed.The curved wall of the shock tube is specially de-signed to modify the shape of the shock wave,with which convergent and divergent shock waves can be successfully generated.The design is verified by numerical and experimental results.In the shock tube,the cylindrical convergent shock is transformed to be planar one(with a higher strength than the original planar shock)before it ar-rives at the geometric center.In this way,the shock reflection at the geometric center is avoided,which allows us to capture a long-term evolution of convergent RM insta-bility.Also,the shock tube is able to produce divergent shocks by transforming the strong planar shock into a divergent one.Several divergent shock waves with differ-ent Mach numbers(within a certain range)are generated using the same profile,which demonstrates the insensitivity of the divergent shock on the wall profile.The pressure characteristics,shock propagation law and the variation of the shock Mach number of the diverging shock wave are also examined.2)High-speed schlieren system is used to capture the cylindrical shock-induced RM instability.Due to the elimination of shock reflection at the geometric center,a long-term growth of the perturbation amplitude is obtained,based on which the influ-ence of nonlinearity and geometric convergence on the instability growth can be an-alyzed in detail.Eliminating the reflected shock wave and the Rayleigh-Taylor(RT)effect induced by the interface deceleration,the perturbation amplitude follows a linear growth with time.The long-term linear growth of perturbation amplitude is mainly as-cribed to the counteraction between the promotion by the Bell-Plesset(BP)effect and the inhibition by nonlinearity.3)Experimental study on the interaction of the diverging shock wave with a single-mode air/SF6 interface is performed in the convergent-divergent shock tube facility.It is found that the instability development differs evidently from the corresponding planar and convergent cases.Specifically,in a divergent geometry,the growth rate of the interface amplitude decays more quickly,and later enters a saturation state(i.e.the instability growth stagnates).This is mainly due to the influence of pure RM instability,geometric divergence and RT stabilization effects.4)Experimental study on the interaction of a divergent shock with higher Mach number and a single-mode interface with large amplitude is performed.First,to examine the influence of large amplitude effect in the divergent RM instability,the interaction of low Mach number divergent shock wave striking a single-mode interfaces with large amplitude is carried out.Second,high Mach number divergent shock waves impacting single-mode interfaces with small amplitude is investigated,exploring the influence of Mach number effect on the divergent RM instability.Finally,the evolution of the single-mode interface with large amplitude impacted by divergent shock waves with high Mach number is considered and the coupling mechanism of high Mach number effect and large amplitude effect on the instability growth is examined.