Varad Karkhanis

A Source Model for Ejecta

We present a Richtmyer–Meshkov instability based model for the ejected mass per unit area produced when a shockwave impinges upon a free surface with specified surface roughness amplitude and wavelength. The model is valid for sinusoidal and non-sinusoidal surface profiles and for multiple shocks, and is readily implemented in hydrodynamic computer codes. We compare the model with microscopic and macroscopic direct numerical simulations and with an extensive Sn experimental data set. The model works well for all surface profiles and captures both the time evolution and total mass for all shapes considered.

The Rayleigh-Taylor Instability driven by an accel-decel-accel profile

We describe numerical simulations of the miscible Rayleigh-Taylor (RT) instability driven by a complex acceleration history, g(t), with initially destabilizing acceleration, g > 0, an intermediate stage of stabilizing deceleration, g 0. Initial perturbations with both single wavenumber and a spectrum of wavenumbers (leading to a turbulent front) have been considered with these acceleration histories. We find in the single-mode case that the instability undergoes a so-called phase inversion during the first acceleration reversal from g > 0 to g 0, the horizontal mean density profile becomes RT-unstable and...

Ejecta Production from Second Shock: Numerical Simulations and Experiments

From detailed numerical simulations and comparison with recent experiments, we explore ejecta production at an interface that is impulsively accelerated by two successive shock waves. The perturbed material interface demarcates the boundary between a metal and vacuum resulting in the formation of ejecta driven by the Richtmyer–Meshkov instability. The numerical simulations were performed with the astrophysical FLASH code, in which the shocked metallic response is conceptually modeled using continuum hydrodynamics. The experimental data were obtained from a two-shockwave, high-explosive tool at Los Alamos National Laboratory capable of generating ejecta from a shocked Sn surface in to a vacuum. In both the simulations and the experiment, linear growth is observed following the first shock event, while the second shock strikes a finite-amplitude interface leading to nonlinear growth. The timing of the second incident shock was varied systematically in our simulations to realize a finite-amplitude re-initialization of the RM instability driving the ejecta. We take advantage of the nonlinear growth following the second shock, to evaluate a recently proposed model for sourcing of mass in ejecta formation that accounts for shape effects through an effective wavelength. In particular, we find the agreement between simulations, experiments and the mass model is improved when such shape effects associated with the interface at the instance of second shock are incorporated. The approach outlined here of combining continuum simulations with validated nonlinear models can aid in the design of future experimental campaigns.

Evolution of the single-mode Rayleigh-Taylor instability under the influence of time-dependent accelerations

From nonlinear models and direct numerical simulations we report on several findings of relevance to the single-mode Rayleigh-Taylor (RT) instability driven by time-varying acceleration histories. The incompressible, direct numerical simulations (DNSs) were performed in two (2D) and three dimensions (3D), and at a range of density ratios of the fluid combinations (characterized by the Atwood number). We investigated several acceleration histories, including acceleration profiles of the general form g(t)∼t^{n}, with n≥0 and acceleration histories reminiscent of the linear electric motor experiments. For the 2D flow, results from numerical simulations compare well with a 2D potential flow model and solutions to a drag-buoyancy model reported as part of this work. When the simulations are extended to three dimensions, bubble and spike growth rates are in agreement with the so-called level 2 and level 3 models of Mikaelian [K. O. Mikaelian, Phys. Rev. E 79, 065303(R) (2009)10.1103/PhysRevE.79.065303], and with corresponding 3D drag-buoyancy model solutions derived in this article. Our generalization of the RT problem to study variable g(t) affords us the opportunity to investigate the appropriate scaling for bubble and spike amplitudes under these conditions. We consider two candidates, the displacement Z and width s^{2}, but find the appropriate scaling is dependent on the density ratios between the fluids-at low density ratios, bubble and spike amplitudes are explained by both s^{2} and Z, while at large density differences the displacement collapses the spike data. Finally, for all the acceleration profiles studied here, spikes enter a free-fall regime at lower Atwood numbers than predicted by all the models.