Bulletin of the American Physical Society
18th Biennial Intl. Conference of the APS Topical Group on Shock Compression of Condensed Matter held in conjunction with the 24th Biennial Intl. Conference of the Intl. Association for the Advancement of High Pressure Science and Technology (AIRAPT)
Volume 58, Number 7
Sunday–Friday, July 7–12, 2013; Seattle, Washington
Session P4: TM Molecular Dynamics II |
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Chair: Tommy Sewell, University of Missouri-Columbia Room: Vashon |
Wednesday, July 10, 2013 11:00AM - 11:15AM |
P4.00001: DPDE Based Mesoscale Simulations of Shock Response of HE Composites Parveen Sood, Sunil Dwivedi, Naresh Thadhani, John Brennan, Yasuyuki Horie The dissipative particle dynamics with energy conservation (DPDE) method is extended to simulate the shock response of high explosive (HE) composites at the micron length scale. Originally developed for soft matter, DPDE has been successfully used to simulate polymeric and soft bio materials. However, the method has yet to be shown applicable to various mesoscale responses of HE composites, including intra-grain fracture, sublimation during reaction initiation, and coupling of the gas and solid phases. The long-term objectives of the present work are to develop the DPDE code and simulate the shock response of RDX at the micron length scale and later couple it with the finite element method for a generic computational approach. The statistical volume element (SVE) is composed of micron-sized RDX particles stacked in a simple cubic configuration whose interactions are simulated using the Lennard-Jones potential. The dissipative and random force contributions of the DPDE method are used to account for the heat transport phenomena. The artificial viscosity terms are added for the first time in the DPDE formulation to successfully damp out numerical oscillations. The results show that the shock propagation in the chosen SVE with wall-boundary conditions predicts the shock response of RDX in reasonable agreement with data available in the literature. [Preview Abstract] |
Wednesday, July 10, 2013 11:15AM - 11:30AM |
P4.00002: Modeling the Shock Compression of Coarse-Grained RDX using Constant Energy Dissipative Particle Dynamics Joshua D. Moore, Sergei Izvekov, John K. Brennan, Martin Lisal Mechanical stimulation of energetic materials often incites responses over a wide range of spatial and temporal scales, with a strong dependence upon micron-scale defects. Modeling these materials atomistically remains a challenge due to the length and time scales required, as billions of molecules would be necessary to model micron-size defects. To overcome these challenges, we have implemented multiscale techniques to bridge the atomistic and mesoscale descriptions by coarse-graining RDX through a multiscale coarse-grain force-matching technique, resulting in a density-dependent potential. The resulting model reproduces several atomistic properties, but only those properties which depend on intermolecular interactions. Properties that depend on the coarse-grained intramolecular degrees-of-freedom (d.o.f.) are underestimated. Implementing traditional molecular dynamics to simulate the mechanical response of such models inevitably results in inaccurate energy and momentum exchange due to these unaccounted d.o.f. To correct this, we utilize the constant-energy Dissipative Particle Dynamics method (DPD-E), which provides a mechanism to account for all coarsened d.o.f. through the inclusion of a coarse-grain particle internal energy. This work presents results for shock simulations of RDX using DPD-E with results assessed by direct comparison to fully atomistic simulations. [Preview Abstract] |
Wednesday, July 10, 2013 11:30AM - 11:45AM |
P4.00003: Laminar, cellular, transverse, and turbulent detonations in condensed phase energetic materials from molecular dynamics simulations Aaron Landerville, Vasily Zhakhovsky, Mikalai Budzevich, Carter White, Ivan Oleynik The development of instabilities in condensed phase detonation is simulated using moving window molecular dynamics and a generic AB model of a high explosive. An initially planar detonation front with one-dimensional flow becomes unstable through the development of transverse perturbations. Highly inhomogeneous and complex two-dimensional cellular and transverse, and three-dimensional turbulent detonation structures were observed depending on the physico-chemical properties of the AB energetic material, sample geometry, and boundary conditions. The different regimes of condensed-phase detonation simulated by a moving window molecular dynamics technique exhibit structures, although at a much smaller scale, similar to those observed in gases and diluted liquids. [Preview Abstract] |
Wednesday, July 10, 2013 11:45AM - 12:00PM |
P4.00004: Nano-scale spinning detonation in condensed phase energetic materials Vasily Zhakhovsky, Mikalai Budzevich, Aaron Landerville, Carter White, Ivan Oleynik Single- and multi-headed spinning detonation waves are observed in molecular dynamics simulations of a condensed phase detonation of an energetic material (EM) confined in round tubes of different radii. The EM is modeled using a modified AB Reactive Empirical Bond Order potential. The thermochemistry and reactive equation of state are varied by adjusting the barrier height for the exothermic reaction AB$+$B $\rightarrow$ A$+$BB. This allows us to study the evolution of the detonation-wave structure as a function of physico-chemical properties of the AB explosive. The detonation wave is found to exhibit a pulsating planar front in a tube of 8 nm radius, which later collapses due to the development of longitudinal perturbations. Upon increase of the tube's radius to 16 nm, the detonation wave structure is stabilized through the development of a single-headed spinning detonation. The spinning detonation displays a \textit{four-wave} configuration, including incident, oblique, transverse, and contact shock waves. The contact shock generated by a contact discontinuity is observed for the first time in our MD simulations. A multi-headed turbulent-like detonation structure develops within tubes of larger radii, and exhibit features similar to those observed in gases. [Preview Abstract] |
Wednesday, July 10, 2013 12:00PM - 12:15PM |
P4.00005: Direct first-principles simulation of shock waves in silicon Oliver Strickson, Emilio Artacho Density functional theory calculations of thousands of atoms are performed for the direct, non-equilibrium simulation of shock waves, using the SIESTA method and implementation of DFT. We perform a simulation of an elastic shock wave in silicon. We compare simulations using the direct method with equilibrium simulations of post-shock states found such that they lie on the Hugoniot locus, and simulations performed using existing empirical potentials for silicon. System size effects are addressed using conventional empirical interatomic potentials. [Preview Abstract] |
Wednesday, July 10, 2013 12:15PM - 12:30PM |
P4.00006: The dissociation constant of water at extreme conditions Otto E. Gonzalez-Vazquez, Luigi Giacomazzi, C. Pinilla, Sandro Scandolo Only one out of 10$^7$ water molecules is dissociated in liquid water at ambient conditions, but the concentration of dissociated molecules increases with pressure ad temperature, and water eventually reaches a fully dissociated state when pressure exceeds 50-100 GPa and temperature reaches a few thousand Kelvin. The behavior of the dissociation constant of water (pKa) at conditions intermediate between ambient and the fully dissociated state is poorly known. Yet, the water pKa is a parameter of primary importance in the aqueous geochemistry as it controls the solubility of ions in geological fluids. We present results of molecular dynamics calculations of the pKa water at extreme conditions. Free-energy differences between the undissociated and the dissociated state are calculated by thermodynamic integration along the dissociation path. The calculations are based on a recently developed all-atom polarizable force-field for water, parametrized on density-functional theory calculations. [Preview Abstract] |
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