Nicholas A. Denissen

Estimation of Metal Strength at Very High Rates Using Free-Surface Richtmyer–Meshkov Instabilities

Recently, Richtmyer–Meshkov Instabilities (RMI) have been proposed for studying the average strength at strain rates up to at least 107/s. RMI experiments involve shocking a metal interface that has initial sinusoidal perturbations. The perturbations invert and grow subsequent to shock and may arrest because of strength effects. In this work we present new RMI experiments and data on a copper target that had five regions with different perturbation amplitudes on the free surface opposite the shock. We estimate the high-rate, low-pressure copper strength by comparing experimental data with Lagrangian numerical simulations. From a detailed computational study we find that mesh convergence must be carefully addressed to accurately compare with experiments, and numerical viscosity has a strong influence on convergence. We also find that modeling the as-built perturbation geometry rather than the nominal makes a significant difference. Because of the confounding effect of tensile damage on total spike growth, which has previously been used as the metric for estimating strength, we instead use a new strength metric: the peak velocity during spike growth. This new metric also allows us to analyze a broader set of experimental results that are sensitive to strength because some larger initial perturbations grow unstably to failure and so do not have a finite total spike growth.

Simulations of the Tilted Rig Experiment Using the xRage and FLAG Hydrocodes

The tilted rig experiment is a derivative of the rocket rig experiment designed to study mixing of fluids by the Rayleigh–Taylor instability. In this experiment, a tank containing two fluids of different densities is accelerated downwards between two parallel guide rods by a rocket motor. The rocket rig is inclined by a few degrees off the vertical to force a two-dimensional Rayleigh–Taylor instability. Thus, the tilted rig experiment can help calibrate two-dimensional mixing models. Simulations of the tilted rig experiments using two of Los Alamos National Laboratory’s hydrocodes are reported. Both codes, xRAGE and FLAG, are multidimensional, multimaterial, massively parallel, hydrodynamics codes that solve the Euler equations. xRAGE operates in an Eulerian framework, while FLAG operates in an Arbitrary Lagrangian–Eulerian (ALE) framework, with a Lagrange step followed by mesh relaxation and remapping. Direct comparisons between simulations and experimental results are reported, as well as report the behavior of the variable-density turbulence models implemented in the codes.© 2012 ASME

Modeling Turbulent Rayleigh-Taylor Mixing with Dynamic Interfaces

Variable-density turbulent mixing is found in a wide variety of applications, and modeling these effects is a continuing challenge. Reynolds–Averaged Navier–Stokes models remain the most common design tool in a wide variety of fields. This paper extends validation of RANS models for variable-density turbulence to a two-dimensional Rayleigh–Taylor test case. The combined effects of bulk fluid motion and turbulence model behavior are discussed and several metrics are shown to demonstrate the ability of four-equation turbulence model to describe this class of flows.

Application of a Novel Approach for Turbulence Model Initialization: Preliminary Results

Progress on the implementation of a novel approach to initialization for RANS simulations of interfacial instability induced turbulent mixing is demonstrated on various problems. The strategy consist of using an analytical model to compute the instability evolution from the quiescent state, and make use of its prediction to generate initial conditions for the turbulence model. Explicitly, an incompressible inviscid model for Rayleigh-Taylor and Richtmyer-Meshkov instabilities continuously updates the turbulence model variables values in the mixing layer, until the Reynolds number suggests that the flow has become turbulent. Implementation of this procedure is made in three steps: first, the instability model is run alone while the interface is evolved by the hydrocode hosting the turbulence model; second the turbulence model is started and the turbulence variables updated in accordance with the instability growth model predictions; finally, the Reynolds number suggests that the turbulent mixing regime is reached, causing the instability model to stop and the turbulence model to continue alone. The initialization methodology is tested on canonical Rayleigh-Taylor and Richtmyer-Meshkov problems, on the tilted rig test problem, and on the chevron problem. Overall, this new approach to turbulence model initialization show promising results.