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Phase Separation in Amphiphilic Fluids

Bruce M. Boghosian, Center for Computational Science, Boston University
Peter V. Coveney, Department of Chemistry, Queen Mary and Westfield College, University of London

Lattice models have been used to study the equilibrium properties of amphiphilic fluids for over a decade now. many interesting questions about these fluids, however, center on their behavior away from equilibrium. Droplet growth rates, fluctuations of interfaces loaded with surfactang, vesicle formation, and rheology in the presence of shearing flow, for example, all involve hydrodynamics, and therefore require a model with a conserved momentum. In our research, we present recent simulational results from a three-dimensional lattice-gass hydrodynamic model of amphiphilic fluids. These results include spherical and "wormlike" micelle formation, sponge sphases and lamellae.

Visualization of Magnetically Confined Toroidal Plasmas

Donald A. Spong, Steve Hirshman, Jim Lyon, Don Batchelor, Lee Berry, Raul Sanchez, Andrew Ware, and John Whitson, Oak Ridge National Laboratory

The configurational design optimization and physics analysis of magnetically confined plasmas in magnetic fusion research rely heavily on high performance computing and simulation. Scientific visualization techniques are an important tool in this effort as they provide a necessary ergonomic link between the numerical results and the intuition of the human researcher. We have developed a variety of visualization methods for displaying three-dimensional optimized stellarator configurations and for visualizing the complex structures which are generated in plasma instability simulations.

Oak Ridge National Labortatory

Visualization of Magnetic Colloid Dynamics

Pieter B. Visscher, Dept. of Physcs and Astronomy and MINT Center, University of Alabama
Mourad Benakli, Dept. of Physcs and Astronomy and MINT Center, University of Alabama

We present three-dimensional animated visualizations of Brownian dynamics simulations of a magnetic colloid. The system is chosen to model a material of great technological interest, the magnetic ink (a colloidal suspension of cigar-shaped iron or iron-oxide particles) that is painted on a substrate in the manufacture of magnetic tapes and disks. We have modeled the response of the colloid to an AC field in a direction perpendicular to an applied DC field, i.e. the transverse susceptibility.

The simulation shows a system of cigar-shaped particles, initially randomly oriented in a gel-like structure in the absence of a magnetic field. When the DC magnetic field is turned on, the particles align along the field. The AC susceptibility is measured by superposing a small transverse AC field and recording the resulting AC magnetization, and is plotted as a function of time. This plot has also been obtained experimentally, and we see good semiquantitative agreement. This probe (transverse susceptibility) is quite sensitive to how easily the structure is broken up by the orienting DC field, hence to the detailed structure and has been used industrially as a gauge of dispersion quality (lack of clumping). The user can freeze the system in a particular configuration and rotate or zoom it with the mouse to examine the orientation and local structure.

Supported by NSF-MRSEC, grant # DMR-9809423.

University of Alabama

Visiometrics: Vortex dominated Flows

Norman J. Zabusky and Deborah Silver, Laboratory for Visiometrics and Modeling, Mechanical & Aerospace Engineering & CAIP Center, Rutgers, The State University of New Jersey

The goal of large-scale simulations and experiments in computational science and technology is a quantitative and mathematical understanding for the model being investigated. We are employing the visiometrics and modeling paradigm. Visiometrics involves two categories of related processes:

  • Visualization (including data projection, image rendering, feature extraction and classification), and
  • Quantification (or feature measurement, space-time feature identification, and feature tracking).

Visualization involves projecting the data into alternate spaces (e.g. wave number space for spectra) or lower dimensional spaces (e.g. space-time diagrams), rendering the features, and isolating observed coherent" regions. Examples of coherent structures from fluid dynamics include: wave packets; shock waves; solitons; eddies; holes; bubbles; rings; tubes; spikes; etc.

Quantification involves measuring properties of coherent features (or objects) that are defined according to a physically based rule or that lie above some threshold. This includes: content (volume, mass, charge. circulation, etc. ); moments about extrema (e.g. quadratic forms of thresholded objects); gradients, curvatures, torsion's of skeletons of tubular or layered domains; critical points of velocity fields; eigenvalues and eigenvectors of tensor fields, etc.; and properties of space-time trajectories of these objects in the original or lower-dimensional (projected) spaces.

We are developing tools to easily access, manipulate, extract, track and archive essential features of one or more computer simulations, or laboratory or field experiments. The cogent assimilation and juxtaposition of measured and simulated data sets and their thresholded projections into lower dimensions will provide insights into the causes of observed phenomena. These insights will show the way toward formulating hierarchies of mathematical models and will result in increased reliability of predictions.

Areas of interest include novel algorithms to manage, visualize and quantify massive data sets using numerical, geometrical and graphical techniques. Quantification encompasses identifying coherent regions and structures, extracting these regions, simplifying them and fitting terse mathematical descriptions to them. Parallel algorithms for visualizing, quantifying, and managing hierarchical datasets are being developed.

Direct numerical simulations in computational fluid dynamics provide the data sets for these studies, including simulations done on a CRAY Y/MP and C-90 at PSC and the Connection Machine CM5 at NCSA. We are presently studying: 2D and 3D shock-interface and shock-bubble interactions and compressible coherent structures and turbulence; and 2D vortex dynamics and turbulence via pseudospectral and contour dynamics/surgery codes; 3D vortex tube interactions, reconnection, intermittency and turbulence via pseudospectral and Biot-Savart codes.

The Geodynamo

Gary A Glatzmaier, University of California, Santa Cruz

We illustrate the three-dimensional structure of the simulated geomagnetic field generated in the Earth's convecting fluid core. Our geodynamo simulations now span more than a million years, using an average computational time step of about 15 days. At the surface of our model Earth, the simulated magnetic field has an intensity, an axial dipole dominated structure, and a westward drift of the non-dipolar structure that are all similar to the Earth's. In addition, several spontaneous reversals of the magnetic dipole polarity have occurred during our simulations, similar to those seen in the Earth's paleomagnetic record.

Earth & Planetary Sciences

Z-Pinch X-ray Source Simulations

Tom Mehlhorn, Sandia National Laboratories*, Albuquerque, NM

Three dimensional radiation magneto-hydrodynamic (rad-MHD) simulations are required to understand the radiation production scaling of wire array z-pinches imploded by pulsed power drivers. Sandia is developing a rad-MHD version of ALEGRA, an arbitrary Lagrangian-Eulerian material dynamics code, to perform such simulations on various platforms, including the ASCI Red Machine. Experimentally, Sandia has generated z-pinch powers up to 280 TW utilizing tungsten wire arrays on the Z Machine. Increasing the number of wires in the array appears to decrease the perturbation seed to the magnetic Rayleigh-Taylor (MRT) instability and decrease the FWHM of the radiation pulse. The scaling of radiation pulse width with initial perturbation amplitude has been studied primarily with 2-D simulations. We report on the first massively parallel simulations of the evolution of the MRT instability in three dimensions.

* Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.

Wavepacket-Wavepacket Scattering

Jon J.V. Maestri, Rubin H. Landau, Physics Department, Oregon State University
Manuel J. Paez, Physics Department, University of Antioquia, Medellin, Colombia

A simple and explicit technique for the numerical solution of the two-particle, time-dependent Schrvdinger equation is assembled and tested. The technique can handle interparticle potentials that are arbitrary functions of the coordinates of each particle, arbitrary initial and boundary conditions, and multi-dimensional equations. Plots and animations are given here and on the World Wide Web of the scattering of two wavepackets in one dimension.

Smoothed Particle Hydrodynamics

Brian Schlatter, Rubin H. Landau, Department of Physics
Keith Vertanen, Department of Computer Science
Oregon State University, Corvallis, OR 97331

We have implemented a fluid flow simulation using the smoothed particle hydrodynamics method. The implementation runs in parallel on distributed memory machines using the MPI message passing libraries. Shown in the image is a color contour plot of the velocity field for flow in a channel around an object (the Beaver).

Materials Science Collaborative Activities

M. Ishida and Y. Zempo, (Sumitomo Chemical Co. Ltd.)
S. Ohnishi (NEC Corp.)
A. Fukumoto, K. Miwa (TOYOTA Central R&D Labs.)
S. Itoh (TOSHIBA Corp.)
H. Odaka (Asahi Glass Co. Ltd.)
M. Iwasawa, T. Saito (Sony Corp.)
A. Kato, H. Rikukawa (FDK Corp.)
M. Nishitsuka (PIONEER Electronic Corp.)
M. Takimoto, H. Shibuki, Y. Ohtani (Fuji Research Institute Corp.)
Y. Inoue, M. Tanoura (Mitsubishi Heavy Industries, Ltd.)

As a research community for computational materials science and related fields, the CAMP project was established in 1990 by researchers and engineers in Japanese industries to share knowledge and techniques of computational methods, and to make a base code for anyone to handle easily. Our first code named CAMP-Atami, a Car-Parrinello type of program, was available for public use in 1994. Several new programs are being developed; one in a plane wave basis, one in a mixed basis, one by an adaptive coordinate method, one based on parallel computing, one for pseudopotential generation, etc. Recently published computational techniques are also involved in the programs.

Simulating Materials Properties

J. Bernholc, E. L. Briggs, M. Buongiorno Nardelli, K. Rapcewicz, C. Roland and M. Wensell, North Carolina State University

Large-scale ab initio dynamics can reliably predict the atomic pathways of phase transformations, failure mechanisms under mechanical load and even chemical reactions. We use interactive 3D visualizations, videos and images to illustrate the impact of modern visualization techniques on both the initial analysis and presentations towide audiences. The specific examples include C60 dynamics, surface melting of Si, ductile and brittle behavior of carbon nanotubes, and surfaces of nitride semiconductors.

Simulating Liquid Semiconductors

V.V. Godlevsky, M. Jain, J.J. Derby and J.R. Chelikowsky, Department of Chemical Engineering and Materials Science, Minnesota Supercomputing Institute, University of Minnesota

One of the most exciting developments in computational material science has been the development of simulation methods using quantum forces. It is no longer necessary to employ empirical interatomic potentials in molecular dynamics simulations. We illustrate these techniques by examining liquid ZnTe. This is part of a larger project encompassing II-VI liquid semiconductors, e.g., to date we have examined ZnTe, CdTe and HgTe in the liquid and solid state. One of the most interesting issues in these materials concerns the conductivity of the liquid state. Most Group IV and III-VI semiconductors are metallic in the liquid state; however, some II-VI semiconductors remain semiconducting in the melt. In order to examine this problem in more detail, we have calculated the conductivity of a prototypical III-V semiconductor (GaAs) and compared its properties to II-VI semiconductors. Our simulations use the pseudopotential density functional method (PDFM) to calculate quantum forces in the melt. We prepare a liquid state ensemble using supercells and Langevin dynamics to thermalize the liquid. Our demonstration illustrates the microstructure of the liquid state and the self-diffusion of Zn and Te within the melt. We summarize our studies of II-VI semiconductors and illustrate how the microstructure of the liquid can affect the conductivity of the liquid state. We will also discuss why it may be difficult to grow Te II-VI's from the melt.

Reference: V. Godlevsky M. Jain, J.J. Derby and J.R. Chelikowsky, Phys. Rev. Lett. 81, 4959 (1998)

Neutron Star Coalescences

Mark Miller, Wai-Mo Suen and Malcolm Tobias, NCSA/Potsdam/Washington U Collaboration

General Relativistic Astrophysics---astrophysics that involves strong and dynamical gravitational fields requiring the full Einstein equations for its understanding---is becoming a promising research area, due to the large amount of data in high energy astronomy, and to the great promise of gravitational wave astronomy made possible by the Laser Interferometer Gravitational Wave Observatories, LIGO/VIRGO and LISA. In particular, Computational General Relativistic Astrophysics, with the recent stunning increases in computer power, may hold the key to the understanding of many observations in high energy astronomy and gravitational wave astronomy.

We show a simulation of two neutron stars in a grazing collision based on the full Einstein equations coupled to general relativistic hydrodynamic equations. The coalescences of neutron star binaries have attracted much attention as sources of gravitational radiation and candidates of gamma-ray burst.

The simulation is carried out with a 3D code called "Cactus" that is currently being developed by our NCSA/Potsdam/Wash U Numerical Relativity Collaboration, together with other research groups in the community. The study of neutron star coalescences is performed as part of the NASA Neutron Star Grand Challenge Project (NASA HPCC NCCS5-153). The computation is carried out at the NSF SuperComputing Centers under National Resource Allocation MCA 93S025.

Adatom diffusion on Al(100)

Mark Miller, Wai-Mo Suen and Malcolm Tobias, NCSA/Potsdam/Washington U Collaboration

General Relativistic Astrophysics---astrophysics that involves strong and dynamical gravitational fields requiring the full Einstein equations for its understanding---is becoming a promising research area, due to the large amount of data in high energy astronomy, and to the great promise of gravitational wave astronomy made possible by the Laser Interferometer Gravitational Wave Observatories, LIGO/VIRGO and LISA. In particular, Computational General Relativistic Astrophysics, with the recent stunning increases in computer power, may hold the key to the understanding of many observations in high energy astronomy and gravitational wave astronomy.

We show a simulation of two neutron stars in a grazing collision based on the full Einstein equations coupled to general relativistic hydrodynamic equations. The coalescences of neutron star binaries have attracted much attention as sources of gravitational radiation and candidates of gamma-ray burst.

The simulation is carried out with a 3D code called "Cactus" that is currently being developed by our NCSA/Potsdam/Wash U Numerical Relativity Collaboration, together with other research groups in the community. The study of neutron star coalescences is performed as part of the NASA Neutron Star Grand Challenge Project (NASA HPCC NCCS5-153). The computation is carried out at the NSF SuperComputing Centers under National Resource Allocation MCA 93S025.

Materials Science at MSI

Keith M. Glassford, Molecular Simulations Inc.

The advances in theoretical and computational methods combined with the rapid progress in computer hardware has now made it possible to investigate the material properties from the sub-atomic scale, through the atomic and molecular scale, up to the mesoscale level. MSI will discuss how computer modeling can help scientists and engineers predict material properties, design and characterize new materials, and optimize existing processes using molecular modeling software. The focus will be towards the chemical and materials industry using MSI's software to understand the chemistry and physics of molecules, polymers and bulk materials using various simulation strategies ranging from force fields to first principles quantum mechanics.