Molecular Dynamics Simulation Of Interfacial Instability At Microscale

When a material interface between two fluids of two different density is subjected to an external force or acceleration(the acceleration pointing from light to heavy fluid),initial perturbations on the interface grow continuously with time and eventually in-duce a flow transition to turbulent mixing.This type of interfacial instability is usually known as the Rayleigh-Taylor(RT)instability.If the fluid interface is subjected to an instantaneous impact,such as a shock wave,the Richtmyer-Meshkov(RM)instability occurs.The RT and RM instabilities play important roles in inertial confinement fusion(ICF),weapon implosion and other engineering applications.For example in ICF,RT and RM instabilities induce intensive mixing between the outer ablator and the inner fuel,which greatly reduces the energy gain of ICF.This is one of the major reasons impeding the emerging gain or ignition of ICF.It is therefore highly desirable to study these two instabilities.Most of previous studies on RT and RM instabilities focus on the development of interface instability at macro-scale.However,this is not the case for ICF,where the RT and RM instabilities develop at a micro-scale.In this work,we perform numerical simulations of microscopic RM and RT instabilities via molecular dynamics(MD)method,focusing on the developments of single-and dual-mode inter-facial instabilities.The main research contents are as follows:1.Detailed development process of the microscopic RM instability is obtained by MD simulation.It is found that the interface evolution at micro-scale is generally similar to that at macro-scale.However,due to the influence of non-equilibrium processes such as diffusion,dissipation and heat conduction,the microscopic RM instability presents novel interface evolution structure and new perturbation growth pattern.2.Viscosity plays an important role in the development of microscopic RM insta-bility.In order to characterize the effect of viscosity,a new Reynolds number is defined according to the characteristic quantities of RM instability flow.The compressible lin-ear theory considering viscous dissipation can reasonably predict the linear growth rate,and based on this,a viscous compressible nonlinear model is proposed,which can well predict the whole amplitude growth of microscopic RM instability from the early linear to later nonlinear stages.3.For single-mode RT instability,the early-stage perturbation growth is obviously restrained by viscosity,but the late-satge asymptotic growth rate suffers a negligible influence of viscosity.Based on this finding,a smooth matching model matching the viscous linear model and the potential flow nonlinear theoretical model is proposed,which can reasonably predict the growths of bubble and spike from early to late stages.4.At micro-scale,the instability of dual-mode RT instability presents new mode coupling and competition rules for the weaker nonlinearity:the basic modes grow in-dependently;the growths of new generated harmonics after saturation are only affected by the low-order basic modes.5.As compared to the RT instability under a constant acceleration field,the evo-lution of RT instability is slower with the acceleration of higher-order-time function.