Guillermo Terrones

Through the computation of the most-unstable modes, we perform a systematic analysis of the linear Rayleigh-Taylor instability at a spherical interface separating two different homogeneous regions of incompressible viscous fluids under the action of a radially directed acceleration over the entire parameter space. Using the growth rate as the dependent variable, the parameter space is spanned by the spherical harmonic degree n and three dimensionless variables: the Atwood number A, the viscosity ratio s, and the dimensionless variable B=(a(R rho 2)(2)/mu(2)(2))R-1/3, where a(R), rho(2), and mu(2) are the local radial acceleration at the interface and the density and viscosity of the denser overlying fluid, respectively. To understand the effect of the various parameters on the instability behavior and to identify similarities and differences between the planar and spherical configurations, we compare the most-unstable growth rates alpha P* (planar) and alpha S* (spherical) under homologous driving conditions. For all A, when s << 1, the planar configuration is more unstable than the spherical (alpha P*>alpha S*) within the interval 0 < B < infinity. However, as s increases to O(1), there is a region for small values of B where alpha S*>alpha P*, whereas for larger values of B, alpha P*>alpha S* once again. When s similar to 2, the maximum of alpha S* for the n = 1 mode is greater than alpha S* for any other mode (n >= 2). For s similar to O(10), alpha S*>alpha P* for all A within 0 < B < infinity. We find that the instability behavior between the planar and spherical systems departs from each other for s greater than or similar to 2 and diverges considerably for s >> 1. In the limit when s -> infinity, the planar configuration reduces to the trivial solution alpha P*equivalent to 0 for all B and A, whereas alpha S* has a non-zero limiting value for the n = 1 mode but vanishes for all the other modes (n >= 2). We derive an equation for alpha S* in this limit and obtain closed form solutions for the maximum of alpha S* and the value of B at which this occurs. Finally, we compare the most-unstable growth rates between the exact dispersion relation and three different approximations to highlight their strengths and weaknesses.

We develop and apply an explosively driven two-shockwave tool in material damage experiments on Sn. The two shockwave tool allows the variation of the first shockwave amplitude over range 18.5 to 26.4 GPa, with a time interval variation between the first and second shock of 5 to 7 mu s. Simulations imply that the second shock amplitude can be varied as well and we briefly describe how to achieve such a variation. Our interest is to measure ejecta masses from twice shocked metals. In our application of the two-shockwave tool, we observed second shock ejected areal masses of about 4 +/- 1 mg/cm(2), a value we attribute to unstable Richtmyer-Meshkov impulse phenomena. We also observed an additional mass ejection process caused by the abrupt recompression of the local spallation or cavitation of the twice shocked Sn. (C) 2014 AIP Publishing LLC.

We present the development of an explosively driven physics tool to generate two mostly uniaxial shockwaves. The tool is being used to extend single shockwave ejecta models to account for a second shockwave a few microseconds later. We explore techniques to vary the amplitude of both the first and second shockwaves, and we apply the tool experimentally at the Los Alamos National Laboratory Proton Radiography (pRad)facility. The tools have been applied to Sn with perturbations of wavelength lambda = 550 mu m, and various amplitudes that give wavenumber amplitude products of kh is an element of {3/4, 1/2, 1/4, 1/8}, where h is the perturbation amplitude, and k = 2 pi/lambda is the wavenumber. The pRad data suggest the development of a second shock ejecta model based on unstable Richtmyer-Meshkov physics.

We describe a simple algebraic model for the particulate spray that is ejected from a shocked metal surface based on the nonlinear evolution of the Richtmyer-Meshkov instability (RMI). The RMI is a shock-driven hydrodynamic instability at a material interface in which the dense and tenuous fluids penetrate each other as spikes and bubbles, respectively. In our model, the ejecta areal density is determined by the product of the post-shock metal density and the saturated bubble amplitude, which depends on both the amplitude and wavelength of the initial surface imperfections of the metal. The maximum ejecta velocity is determined by the ever-growing spikes, which are accelerated relative to the RMI growth rate by the spatial harmonics that sharpen them. The model is formulated to fit new hydrodynamics and molecular dynamics simulations of the RMI and validated by existing ejecta experiments over a wide range of material properties, shock strengths, and surface perturbations. The results are also contrasted with existing ejecta source models.

We present experimental results supporting physics-based ejecta model development, where our main assumption is that ejecta form as a special limiting case of a Richtmyer–Meshkov (RM) instability at a metal–vacuum interface. From this assumption, we test established theory of unstable spike and bubble growth rates, rates that link to the wavelength and amplitudes of surface perturbations. We evaluate the rate theory through novel application of modern laser Doppler velocimetry (LDV) techniques, where we coincidentally measure bubble and spike velocities from explosively shocked solid and liquid metals with a single LDV probe. We also explore the relationship of ejecta formation from a solid material to the plastic flow stress it experiences at high-strain rates ( $1{0}^{7} ~{\mathrm{s} }^{\ensuremath{-} 1} $ ) and high strains (700 %) as the fundamental link to the onset of ejecta formation. Our experimental observations allow us to approximate the strength of Cu at high strains and strain rates, revealing a unique diagnostic method for use at these extreme conditions.