Bulletin of the American Physical Society
15th APS Topical Conference on Shock Compression of Condensed Matter
Volume 52, Number 8
Sunday–Friday, June 24–29, 2007; Kohala Coast, Hawaii
Session D2: Large Scale Computations |
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Chair: Ramon Ravelo, Los Alamos National Laboratory Room: Fairmont Orchid Hotel Amphitheater |
Monday, June 25, 2007 3:45PM - 4:15PM |
D2.00001: Multibillion-atom Molecular Dynamics Simulations of Plasticity, Spall, and Ejecta Invited Speaker: Modern supercomputing platforms, such as the IBM BlueGene/L at Lawrence Livermore National Laboratory and the Roadrunner hybrid supercomputer being built at Los Alamos National Laboratory, are enabling large-scale classical molecular dynamics simulations of phenomena that were unthinkable just a few years ago. Using either the embedded atom method (EAM) description of simple (close-packed) metals, or modified EAM (MEAM) models of more complex solids and alloys with mixed covalent and metallic character, simulations containing billions to trillions of atoms are now practical, reaching volumes in excess of a cubic micron. In order to obtain any new physical insights, however, it is equally important that the analysis of such systems be tractable. This is in fact possible, in large part due to our highly efficient parallel visualization code, which enables the rendering of atomic spheres, Eulerian cells, and other geometric objects in a matter of minutes, even for tens of thousands of processors and billions of atoms. After briefly describing the BlueGene/L and Roadrunner architectures, and the code optimization strategies that were employed, results obtained thus far on BlueGene/L will be reviewed, including: (1) shock compression and release of a defective EAM Cu sample, illustrating the plastic deformation accompanying void collapse as well as the subsequent void growth and linkup upon release; (2) solid-solid martensitic phase transition in shock-compressed MEAM Ga; and (3) Rayleigh-Taylor fluid instability modeled using large-scale direct simulation Monte Carlo (DSMC) simulations. I will also describe our initial experiences utilizing Cell Broadband Engine processors (developed for the Sony PlayStation 3), and planned simulation studies of ejecta and spall failure in polycrystalline metals that will be carried out when the full Petaflop Opteron/Cell Roadrunner supercomputer is assembled in mid-2008. [Preview Abstract] |
Monday, June 25, 2007 4:15PM - 4:30PM |
D2.00002: Simulation of the Richtmyer-Meshkov and Rayleigh-Taylor Instability Using Atomistic Methods Kai Kadau, John L. Barber, Timothy C. Germann, Peter S. Lomdahl, Brad Lee Holian, Berni J. Alder We present large-scale atomistic simulations [molecular dynamics (MD) and direct simulation Monte-Carlo (DSMC)] of fluid instabilities that occur when a fluid interface is subjected to shock loading or gravitation [Richtmyer-Meshkov and Rayleigh-Taylor instability]. The atomistic methods reach the parameter range that is of importance for inertial confinement fusion (ICF) capsules subjected to high energy lasers. The results are compared to existing theoretical and experimental work from which we have strong evidence for the importance of fluctuations in such instabilities. References: 1.) Kai Kadau, Timothy C. Germann , Nicolas G. Hadjiconstantinou , Peter S. Lomdahl *, Guy Dimonte , Brad Lee Holian *, and Berni J. Alder, PNAS 101, 5851 (2004). 2.)K. Kadau et al. submitted (2007). [Preview Abstract] |
Monday, June 25, 2007 4:30PM - 4:45PM |
D2.00003: Molecular dynamics simulations of anomalous elastic response of covalent crystals to shock compression Keith McLaughlin, Ivan Oleynik, Sergey Zybin, Mark Elert, Carter White We have performed large-scale molecular-dynamics simulations of shock-wave propagation in single-crystal covalent solids such as diamond and silicon. An anomalous elastic response of these materials has been observed in the intermediate range of shock-wave intensities between the elastic-plastic split shock-wave regime and the shock-induced chemistry regime. The anomalous elastic response is characterized by the absence of plastic deformations in highly uniaxially compressed material. The unusual materials response in shock-compressed diamond is attributed to unique and complex constitutive relationships: both shear and longitudinal stresses are non-monotonic functions of compression. This example clearly demonstrates the necessity of generalization of the notion of the Hugoniot elastic limit (HEL) to include critical shear stresses in a criterion of materials yielding upon shock compression. [Preview Abstract] |
Monday, June 25, 2007 4:45PM - 5:00PM |
D2.00004: Shock-induced plasticity in bcc metals. E. Bringa, J. Hawreliak, P. Erhart, H. Lorenzana, J. Wark It is often assumed that a shocked material will evolve from a state of uniaxial (1D) strain towards a state of nearly hydrostatic (3D) strain due to shock-induced plasticity. We recently simulated the case of fcc copper, where dislocation nucleation and activity leads to a final state close to 3D strain after $\sim $0.1 ns, as confirmed by simulated X-range diffraction. Simulations with up to 350 million atoms, including defective crystals and ramp loading, were needed to reach such 3D state. This 1D to 3D transition has not been as studied for bcc metals. Although there are several studies of shocks in bcc Fe, a phase transition happens before shock-induced plastic activity appears. We have carried atomistic simulations of shocks in tantalum, using 0.5-50 million atoms, with samples nearly 1 micron long. Our samples include perfect single crystals, defective single crystals, and polycrystals. We find agreement with the experimental Hugoniot up to $\sim $15{\%} compression, but the complex elastic-plastic shock wave structure does not lead to full 3D relaxation within the 0.2 ns of simulated time. [Preview Abstract] |
Monday, June 25, 2007 5:00PM - 5:15PM |
D2.00005: Characteristic cluster size at coalescence following pressure-induced solidification. Fred Streitz, Jim Glosli, David Richards During the initial period of solidification, clusters of solid phase nucleate and grow rapidly as liquid is converted to solid. This rapid growth period continues until the clusters coalesce into a connected network and little liquid phase remains. Characterizing the nature of this network of clusters at coalescence is important to understanding the character of the solid at much later times. Using large scale MD simulations of liquid Ta under pressure the solidification processed is explore in detail from nucleation to coalescence. We extract growth and nucleation rates from our simulations, as well as cluster size distributions that can be compared against the predictions of simple models. We will show that the length scale for the distribution of cluster size at coalescence is set by the interplay of nucleation rate $j$ and growth rate $u$. In particular, we find that the characteristic cluster size at coalescence $l \sim (u/p) ^{1/4}$. [Preview Abstract] |
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