Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

A flux splitting scheme is proposed for the general nonequilibrium flow equations with an aim at removing numerical dissipation of Van-Leer-type flux-vector splittings on a contact discontinuity. The scheme obtained is also recognized as an improved Advection Upwind Splitting Method (AUSM) where a slight numerical overshoot immediately behind the shock is eliminated. The proposed scheme has favorable properties: high-resolution for contact discontinuities; conservation of enthalpy for steady flows; numerical efficiency; applicability to chemically reacting flows. In fact, for a single contact discontinuity, even if it is moving, this scheme gives the numerical flux of the exact solution of the Riemann problem. Various numerical experiments including that of a thermo-chemical nonequilibrium flow were performed, which indicate no oscillation and robustness of the scheme for shock/expansion waves. A cure for carbuncle phenomenon is discussed as well.

A Flux Splitting Scheme with High-Resolution and Robustness for Discontinuities(Proceedings of the 12th NAL Symposium on Aircraft Computational Aerodynamics)

X-ray photoelectron spectroscopy was applied to study the composition of the passive film formed on an extremely corrosion resistant amorphous Fe-10at.%Cr-13at.%P-7at.%C alloy in 1 N HCl. The passive film consists mainly of hydrated chromium oxyhydroxide which is a common major constituent of passive films on crystalline stainless steels. The extremely high corrosion resistance of the amorphous alloy can only in part be attributed to the formation of a protective hydrated chromium oxyhydroxide film.

ESCA Study of the Passive Film on an Extremely Corrosion-Resistant Amorphous Iron Alloy

Elements of Gas Dynamics

Supersonic mixing in airbreathing propulsion systems for hypersonic flights

With a growing demand for cheap access to space and high-speed flight, hypersonic research is experiencing an unceasing interest from worldwide parties. The vehicles designed for this purpose experience flight conditions where real gas effects caused by dissociation and ionization of the air molecules greatly influence the flow conditions and surface heat transfer. This makes the application of conventional perfect gas models ineffective and inaccurate. The study of this high-temperature flight regime and the effects on the flow is called aerothermodynamics. This presentation will introduce the research conducted at the Hypersonics lab of KAIST (South Korea) regarding the catalytic recombination of nitrogen on copper oxide (and copper). Catalytic recombination is a phenomenon where the dissociated air molecules recombine at the surface of the body. The extent to which this recombination occurs does not only depend on the flow condition but is also greatly influenced by the surface material. Copper coated heat flux probes are commonly used as a well-defined reference for experimental catalycity research. However, it has been found that impurities of the probes caused by oxidation (and thus the formation of copper oxide) may have a profound effect on the measured reference heat flux. In order to quantify this effect, heat transfer rates at the stagnation point of a blunt body are measured in a shock tube using thin-film gauges. This thesis presents experimental and theoretical results on the surface catalytic recombination phenomenon occurring in a test gas consisting of a mixture of nitrogen and krypton. The results found in this project will function as a building block towards a better understanding of catalycity which ideally leads to a more accurate design of hypersonic vehicles.

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Nitrogen Catalytic Recombination on Copper Oxide in Tertiary Gas Mixtures

Thermochemical nonequilibrium parameters of oxygen, O2, for a two-temperature model are proposed in the present work. The rovibrational nonequilibrium and chemical reactions of three low-lying electronic states of O2(X3Σg−, a1Δg, and b1Σg+) are investigated in one-dimensional post-shock flow environments, including the rovibrational state-to-state kinetics and electronic excitations due to heavy-particle collisions. Based on the results of rigorous state-resolved calculations and state-of-the-art shock-tube experimental data, the nonequilibrium parameters of O2 are proposed to improve the two-temperature model accuracy. For shock wave velocities above 3 km/s, the proposed nonequilibrium parameters of the two-temperature model better reproduce the results of the rigorous state-resolved calculations and shock-tube experimental data than do the previous parameters.

Gisu Park

Papers

Nitrogen Catalytic Recombination on Copper Oxide in Tertiary Gas Mixtures

Thermochemical nonequilibrium parameter modification of oxygen for a two-temperature model

Experimental and numerical study of oxygen catalytic recombination of SiC-coated material

Experimental study of surface roughness effect on oxygen catalytic recombination

Effect of titanium surface roughness on oxygen catalytic recombination in a shock tube