Richtmyer-Meshkov Instability Of Multi-mode Interfaces Accelerated By A Planar Shock Wave

When an interface that separates two fluids with different densities is impacted by a shock wave,the disturbances initially existing on the interface will develop immediately.With the continuous evolution of the interface structures at various scales and the coupling between different modes,the flow will eventually enter a turbulent mixing state.This is generally referred to as the Richtmyer-Meshkov(RM)instability.RM instability plays an important role in multi-scale scientific and engineering issues,such as supernova explosions,supersonic and hypersonic air-breathing engines and inertial confinement fusion(ICF).In ICF,specifically,RM instability is one of the important reasons for ignition failure.Because the single-mode interface is the most basic perturbation form and is convenient for theoretical analysis,it has received widespread attention.However,a multi-mode interface is ubiquitous in both science and engineering.As a result,it is of great significance to understand the evolution of a multi-mode interface for revealing scientific laws and guiding engineering practice.At present,the studies of multi-mode interfaces are rare and not systematic.This dissertation focuses on the RM instability of a V-shaped interface,which is a typical configuration of a multi-mode interface.The initial interface is generated by the soap film technique,and the flow field is recorded by the high-speed schlieren system.All the experiments are conducted in a planar shock tube.First,the evolution of a V-shaped interface with a bubble configuration in the linear and nonlinear stages is studied.Second,the two-bubble competition in the initial stage of RM instability is investigated.Third,the effects of transverse waves on the RM instability of the multi-mode interface are highlighted.Finally,the RM characteristics of the interfaces with different shapes(single-mode,trapezoid,V-shape)before and after reshock are studied.The main contents are concluded as follows:1.The evolution of the V-shaped interface with a bubble configuration in linear and nonlinear stages is studied experimentally.The evolution characteristics of a multimode interface with sharp corners are revealed.The impulsive model can predict the linear growth rate of the V-shaped interface with small amplitudes,but it needs to be modified for large amplitudes.Due to the existence of high-order modes in the initial interface,the existing nonlinear models underestimate the nonlinear growth rate of the V-shaped interface.Based on the Sohn potential model,a new nonlinear model is proposed,and it gives a satisfactory prediction for the amplitude growth rate.2.The bubble competition of multi-mode interfaces in the initial stage of RM instability is studied experimentally,and the influence of the bubble competition effect and the nonlinear effect on the interface evolution is clarified.For large interfaces(bubbles),the nonlinear effect has a stronger influence on the interface growth than the bubble competition effect,while for small interfaces(bubbles),the conclusion is just the opposite.The effect of the initial bubble size difference on the bubble competition in the streamwise direction is not significant.With the increase of the initial bubble size difference,the difference between the large bubble growth rates in the spanwise and streamwise directions increases.3.The influence of transverse waves on RM instability of multi-mode interfaces is studied experimentally.For small amplitudes,the transverse waves and the shock proximity effects are weak.Thus,the impulsive model gives a good prediction for the linear growth rate.For large amplitudes,the transverse waves and the shock proximity effects cannot be ignored.After considering the shock proximity corrections,the model can predict the linear growth rate well.The transverse waves and the shock proximity effects introduce additional force to the flow,resulting in the reduction of the first mode amplitude and the increase of the high-order mode amplitudes.Consequently,the transverse waves and the shock proximity effects cause the change of the interface shape and flatten the bubble front.From the modes,the increase of the amplitude of the second harmonic mode leads to the flattening of the bubble front.4.Experiments of accurately defined interfaces(sinusoidal,trapezoidal and Vshaped interfaces)impacted by reshock are conducted,and the evolution features of the interface before and after reshock are analyzed.Before reshock,there are some differences between the linear and nonlinear growth behaviors of the heavy/light and light/heavy interfaces.After reshock,the RM behaviors of the interfaces maintain a strong memory of the initial conditions.The growth behaviors of bubble amplitudes determine the growth behaviors of the whole mixing widths.The ranking of the overall mixing width growth rates for different interfaces is V-shape>single-mode>trapezoid.Our experiments verify the two-dimensional single-mode reshock model.