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Papers by Robert Rudd

Proceedings of the 2007 Acm Ieee Conference on Supercomputing, 2007

We report the computational advances that have enabled the first micron-scale simulation of a Kel... more We report the computational advances that have enabled the first micron-scale simulation of a Kelvin-Helmholtz (KH) instability using molecular dynamics (MD). The advances are in three key areas for massively parallel computation such as on BlueGene/L (BG/L): fault tolerance, application kernel optimization, and highly efficient parallel I/O. In particular, we have developed novel capabilities for handling hardware parity errors and improving the speed of interatomic force calculations, while achieving near optimal I/O speeds on BG/L, allowing us to achieve excellent scalability and improve overall application performance. As a result we have successfully conducted a 2-billion atom KH simulation amounting to 2.8 CPUmillennia of run time, including a single, continuous simulation run in excess of 1.5 CPU-millennia. We have also conducted 9billion and 62.5-billion atom KH simulations. The current optimized ddcMD code is benchmarked at 115.1 TFlop/s in our scaling study and 103.9 TFlop/s in a sustained science run, with additional improvements ongoing. These improvements enabled us to run the first MD simulations of micron-scale systems developing the KH instability.

Incipient spallation damage was studied in Aluminum non-destructively using 3D X-ray tomography. ... more Incipient spallation damage was studied in Aluminum non-destructively using 3D X-ray tomography. Two samples were laser shocked and recovered using a weaker and stronger laser pulses. The X-ray tomography was performed at SSRL with a resolution of five microns. The 3D distribution of damage in both samples was clearly evident about a well defined spall plane. The results were analyzed

Phys Rev B, 2010

The pressure dependence of the quasistatic yield strength of vanadium in polycrystalline foils an... more The pressure dependence of the quasistatic yield strength of vanadium in polycrystalline foils and powders has been measured up to 90 GPa at room temperature using an implementation of a nonhydrostatic diamond-anvil cell technique, an extension of the technique of Meade and Jeanloz [C. Meade and R. Jeanloz, J. Geophys. Res. 93, 3261 (1988)]. A feature present in this work is the use of an in situ determination of the sample thickness and pressure profile using synchrotron x-ray techniques, allowing a determination of the strength at each point across the sample. Following an initial increase in the strength with pressure, a decrease in the strength of vanadium was observed starting at ˜40-50GPa . The softening of the yield strength of vanadium at high pressure is quite surprising and unusual, following the trend of an unusual softening of the shear modulus associated with a subtle phase transition from body-centered-cubic-to-rhombohedral structure in a recent x-ray diffraction experiment.

Experimental results showing significant reductions from classical in the Rayleigh-Taylor (RT) in... more Experimental results showing significant reductions from classical in the Rayleigh-Taylor (RT) instability growth rate due to high pressure material strength or effective lattice viscosity in metal foils are presented. On the Omega Laser in the Laboratory for Laser Energetics, University of Rochester, target samples of polycrystalline vanadium are compressed and accelerated quasi-isentropically at 1 Mbar pressures, while maintaining the samples in the solid-state. Comparison of the results with constitutive models for solid state strength under these conditions show that the measured RT growth is substantially lower than predictions using existing models that work well at low pressures and long time scales. High pressure, high strain rate data can be explained by the enhanced strength due to a phonon drag mechanism, creating a high effective lattice viscosity.

ABSTRACT We employ classical molecular dynamics (MD) to investigate species diffusivity in binary... more ABSTRACT We employ classical molecular dynamics (MD) to investigate species diffusivity in binary Yukawa mixtures. The Yukawa potential is used to describe the screened Coulomb interaction between the ions, providing the basis for models of dense stellar materials, inertial confined plasmas, and colloidal particles in electrolytes. We use Green-Kubo techniques to calculate self-diffusivities and the Maxwell-Stefan diffusivities, and evaluate the validity of the Darken relation over a range of thermodynamic conditions of the mixture. The inter-diffusivity (or mutual diffusivity) can then be related to the Maxwell-Stefan diffusivities through the thermodynamic factor. The latter requires knowledge of the equation of state of the mixture. To test these Green-Kubo approaches and to estimate the activity contribution we have also employed large-scale non-equilibrium MD. In these simulations we can extract the inter-diffusivity value by calculating the rate of broadening of the interface in a diffusion couple. We also explore thermodynamic conditions for possible non-Fickian diffusivity. The main motivation in this work is to build a model that describes the transport coefficients in binary Yukawa mixtures over a broad range of thermodynamic conditions up to 1keV.

The kinetics of micro-structural evolution (i.e. phase transitions and twinning) is not well unde... more The kinetics of micro-structural evolution (i.e. phase transitions and twinning) is not well understood. For small strain rates the effect of kinetics is negligible due to the rapid rate at which these evolutions occur; however, for the large strain rates that can occur under dynamic loading conditions the micro-structure, and hence the strength of the material, may depend on the details of the kinetics. Therefore, in this work we investigated kinetics of one particular micro-structural evolution process--- twinning in bcc tantalum. To perform this investigation we performed a series of Molecular Dynamics simulations over a range of temperatures and pressures to determine the twinning threshold and understand the kinetics. We present here the results of these simulations, where one notable result was that the twinning threshold had an unexpected dependence on temperature.

The modeling of hydrodynamic phenomena has almost exclusively been the purview of continuum mecha... more The modeling of hydrodynamic phenomena has almost exclusively been the purview of continuum mechanics, specifically, through the use of the Navier-Stokes equation and closely related variants. Nevertheless, at the smallest length scales, where atomistic effects become important, it is not clear that this continuum approach provides a complete description of fluid behavior. To understand the effects of atomistics, we have performed a 62.5-billion-atom, fully 3-D molecular dynamics simulation of a cubic micron of molten copper and aluminum. The shear flow at 2 km/s exhibits complex phenomena associated with a Kelvin-Helmholtz (KH) instability. In this presentation we will discuss the initiation and early evolution of the KH instability, focusing specifically on the effects of full atomistic resolution.

The embedded atom method (EAM) potentials have been used extensively since introduced by Daw and ... more The embedded atom method (EAM) potentials have been used extensively since introduced by Daw and Baskes in the mid 1980's due to their simple incorporation of many-body effects that are missed by simple pair potentials. The computational cost of the inclusion of this additional physics has traditionally been a second pass over the pair data. We will report on an implementation of the EAM model within a molecular dynamics algorithm (MD) that does not require this second pass, substantially reducing the computer time and memory required for evaluation of the potential. The second pass is avoided by using a forward extrapolation in time of the density derivative of the embedding function dF(rho(t))/drho. The error in this approximation is controllable and consistent with the error introduced by the finite time step numerical integrators used in the MD.

Phys Rev B, 2004

We study the nucleation of GaN islands grown by plasma-assisted molecular-beam epitaxy on AlN(000... more We study the nucleation of GaN islands grown by plasma-assisted molecular-beam epitaxy on AlN(0001) in a Stranski-Krastanov mode. In particular, we assess the variation of their height and density as a function of GaN coverage. We show that the GaN growth passes four stages: (i) initially, the growth is layer by layer, (ii) subsequently, two-dimensional precursor islands form, which (iii) transform into genuine three-dimensional islands. Island height and density increase with GaN coverage until the density saturates. (iv) During further GaN growth, the density remains constant and a bimodal height distribution appears. A fit of an equilibrium model for Stranski-Krastanov growth to the variation of island distributions as a function of coverage is discussed.

Mrs Bulletin, Nov 30, 2010

... Plasmas 17, 056314 (2010). 18. JR Asay, T. Ao, TJ Vogler, J.-P. Davis, GT Gray III, J. Appl. ... more ... Plasmas 17, 056314 (2010). 18. JR Asay, T. Ao, TJ Vogler, J.-P. Davis, GT Gray III, J. Appl. Phys. 106, 073515 (2009). 19. DK Bradley, JH Eggert, RF Smith, ST Prisbrey, DG Hicks, DG Braun, J. Biener, AV Hamza, RE Rudd, GW Collins, Phys. Rev. Lett. 102, 075503 (2009). 20. ...

We use equilibrium classical molecular dynamics and Green-Kubo techniques to calculate the diffus... more We use equilibrium classical molecular dynamics and Green-Kubo techniques to calculate the diffusivity and viscosity of Al-Cu molten mixtures. We calculate both the self-diffusivities and the Maxwell-Stefan diffusivities, and evaluate the validity of the Darken relation for this system. We compare the results with those from experiments reported in the literature. We have constructed an analytic model that is fit to the MD results. This transport model has been implemented in a continuum hydrodynamics code. Both the continuum code and extremely large-scale molecular dynamics have been used to simulate the development of vortices due to the Kelvin-Helmholtz instability in a shear layer, and we discuss the results of that comparison.

Typically hydrodynamic phenomena are modeled with continuum mechanics via integration of the Navi... more Typically hydrodynamic phenomena are modeled with continuum mechanics via integration of the Navier-Stokes (NS) equation or a closely related variant. However, as fluids are studied at smaller and smaller length scales atomistic effects can, and will, ultimately dominate; furthermore, even at micron scales it is not clear that the NS equation provides a complete description of the fluid, e.g. due to the initiation of instabilities at the molecular scale in initially quiescent fluids. To assess the effect of atomistic behavior on one particular hydrodynamic phenomenon--the Kelvin-Helmholtz instability--we have performed a very large molecular dynamics simulation of molten metals undergoing shear flow. Nine billion copper and aluminum atoms were sheared at a speed of 2000 m/sec for a total simulated time of more than a nanosecond. We present here results showing the initiation of the instabilities, the crossover to hydrodynamics, and the evolution and scaling behavior of the KH instab...

The pressure-dependences of the quasi-static yield strength of polycrystalline forms of V and Ta-... more The pressure-dependences of the quasi-static yield strength of polycrystalline forms of V and Ta-W alloys have been measured at room temperature using an implementation of a non-hydrostatic diamond anvil cell technique. A new feature of this work is the use of in situ determination of the sample thickness and pressure with synchrotron X-ray. We adopt the conventional technique [C. Meade and R. Jeanloz, J. Geophys. Res. 93, 3261 (1988)] and the pointwise approach to determine the strength under Tresca yield criterion. The result shows the trend of the pointwise analysis is similar to the one of conventional approach.

DYMAT 2009 - 9th International Conferences on the Mechanical and Physical Behaviour of Materials under Dynamic Loading, 2009

... Barton, T. Arsenlis, SG Glendinning, A. Hamza, D. Swift, A. ... BA Remington,a H.-S. Park,a S... more ... Barton, T. Arsenlis, SG Glendinning, A. Hamza, D. Swift, A. ... BA Remington,a H.-S. Park,a ST Prisbrey,a SM Pollaine,a RM Cavallo,a RE Rudd,a KT Lorenz,a R. Becker,a J. Bernier,a N.Barton,a T. Arsenlis,a SG Glendinning, A. Hamza,a D. Swift, A. Jankowski,b MA Meyersc ...

Journal of Physics: Conference Series, 2014

Multiscale strength (MS) models are constructed to capture a natural hierarchy in the deformation... more Multiscale strength (MS) models are constructed to capture a natural hierarchy in the deformation of metals such as V and Ta starting with atomic bonding and extending up through the mobility of individual dislocations, the evolution of dislocation networks and so on until the ultimate material response at the scale of an experiment. In practice, the hierarchy is described by quantum mechanics, molecular dynamics, dislocation dynamics, and so on, ultimately parameterizing a continuum constitutive model. We review the basic models and describe how they operate at extremely high pressures and strain rates, such as in Rayleigh-Taylor plastic flow experiments. The models use dislocation density as a state variable, and describe time-dependent, as well as rate-dependent, plasticity. They make interesting and testable predictions about transients in plastic flow. There are also clear challenges, however. The current MS models do not include a variety of mechanisms known to be important at low rates. Still, MS models provide compelling insight into plastic deformation of metals under extreme pressures and strain rates. well below the melt curve, by minimizing shock heating. Techniques based on the Rayleigh-Taylor instability have been developed over the past few decades to infer the flow stress in ramp-compressed materials, and to test constitutive models against the observed materials behavior . We also consider the response of dynamically loaded material at the level of unit cells in the lattice, as probed by in-situ x-ray diffraction [8-10].

Proceedings of the 2007 ACM/IEEE conference on Supercomputing - SC '07, 2007

ABSTRACT We present theoretical and computational analysis of the deformation regimes accessed by... more ABSTRACT We present theoretical and computational analysis of the deformation regimes accessed by recent Rayleigh-Taylor (RT) material strength experiments in vanadium (V) and tantalum (Ta) done at the Omega laser at high pressures (>1 Mbar) and high strain rates (106 - 108 sec-1). Within the context of the LLNL multiscale models, the V-RT experiment appears to be dominated by deformation in the drag regime, whereas the Ta-RT experiment resides largely within the thermal activation regime.