Experimental And Theoretical Study On Converging Richtmyer-meshkov Instability

When an interface separating two different fluids is impulsively accelerated by a shock wave, the perturbations initially deposited on the interface will grow with time continuously due to the generated baroclinic vorticity (i.e., misalignment of pressure and density gradients), and finally a transition to turbulent mixing occurs. This shock-induced interfacial instability is often referred to as the Richtmyer-Meshkov instability(RMI). RMI plays an important role in a wide range of applications. For example, in inertial confined fusion (ICF), the ablative material on the capsule surface is heated by numerous high-energy laser beams or x rays, forming an inward-moving converging shock. The strong converging shock would impact the multi-layer interfaces inside the capsule, causing intense RMI growth on each interface. The coupling effect of the de-veloping interfaces complicates the converging RMI in ICF significantly. In order to understand the physical mechanisms in converging RMI, in this thesis, we perform a systematic experimental study on the evolution of a single sinusoidal interface and a gas shell (inner and outer interfaces) subjected to a cylindrically converging shock in a ver-tical diaphragmless converging shock tube and a semi-annular converging shock tube,respectively.. Theoretical analysis is further presented to facilitate the understanding of converging RMI.In experiments, we have firstly studied the development of a single-mode air/SF6 interface subjected to a cylindrical shock in a vertical coaxial diaphragmless shock tube using the planar laser sheet imaging technique. The SF6 gas was seeded with Ti02 micro-particles before each experimental run. The evolutions of sinusoidal interfaces with different initial configurations (different wave numbers and amplitudes) after the shock impact were captured, and the dependence of initial configurations on the am-plitude growthwas analyzed. Based on the special structure of the semi-annular shock tube, we designed a removable "drawer-like" interface formation device, in which gas shells with designed-shaped inner and outer surfaces could be readily generated. We obtained the first shock-tube experiments of the development of perturbed gas shell subjected to a cylindrically converging shock. Moreover, the double-layered interfaces of different initial perturbation combinations (outer perturbed, inner perturbed and dou-ble perturbed interfaces) were studied, and the coupling effect of the inner and outer perturbations was emphasized.In the theoretical study, we have considered the compressibility effect on the per-turbation development in the converging RMIbased on the previous theoretical work.The individual contribution of each convergence effect (i.e., the Bell-Plesset (BP) effec-t, the Rayleigh-Taylor (RT) effect) on the perturbation growth was quantified. For the case of double-interface evolution subjected to a converging shock, we proposed a new circulation model corresponding to a perturbed interface interacting with a distorted interface based on the S-Z model.In conclusion, we have performed experimental and theoretical investigations on converging RMI based on the well-designed converging shock tube facilities and ad-vanced flow diagnostics. On one hand, we have obtained the developments of the single-mode air/SF6 interface with different initial parameters subjected to a cylindrical shock in the vertical coaxial diaphragmless shock tube. The effects of wave numbers and initial amplitudes on the evolution of single-mode interface were discussed. In addi-tion, we have found three important converging effects, including the BP effect, the RT effect and the compressible effect, and quantitatively analyzed the individual contribu-tion of each effects on the amplitude growth rate. On the other hand, we have obtained the first shock-tube experiments of the development of perturbed gas shell subjected to a cylindrically converging shock. We proposed a new circulation model corresponding to a perturbed interface interacting with a distorted interface in cylindrical geometry based on the S-Z model, which provided good explanation to the experimental growth rates.