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

Towards better understanding of explosive welding by combination of numerical simulation and experimental study

Formation of cellular detonation in a stoichiometric mixture of aluminum particles in oxygen is studied by means of numerical simulation of shock-wave initiation of detonation in a flat and rather wide channel. By varying the channel width, the characteristic size of the cells of regular uniform structures for particle fractions of 1–10 µm is determined. The calculated cell size is in agreement with the estimates obtained by methods of an acoustic analysis. A relation is established between the cell size and the length of the characteristic zones of the detonation-wave structure (ignition delay, combustion, velocity and thermal relaxation).

Numerical Simulation of Formation of Cellular Heterogeneous Detonation of Aluminum Particles in Oxygen

The paper addresses the problem of searching for methods that can control, suppress, and attenuate explosive and detonation processes in homogeneous and heterogeneous media (mixtures of reactive gases and inert species). The analysis is performed by analytical and numerical methods. The problem of detonation suppression in a mixture of reactive gases and inert species (argon and sand particles) in a one-dimensional unsteady flow is formulated, and its solution is given. The effect of the particle diameter and concentration on the detonation velocity is determined; the parameters of the detonation wave in a stoichiometric hydrogen-oxygen mixture diluted by a chemically inert gas (argon) and particles is determined. The influence of the initial parameters of the mixture on the possibility of detonation suppression by inert particles is studied. It is shown that the detonation velocity substantially decreases with increasing volume fraction of particles. A decrease in the particle size with an unchanged volume fraction is also found to reduce the detonation velocity.

Mathematical modeling of detonation suppression in a hydrogen-oxygen mixture by inert particles

This work verifies that the W foil could be successfully welded on Cu through conventional explosive welding, without any cracks. The microstructure was observed through scanning electron microscopy (SEM), optical microscopy and energy-dispersive X-ray spectrometry (EDS). The W/Cu interface exhibited a wavy morphology, and no intermetallic or transition layer was observed. The wavy interface formation, as well as the distributions of temperature, pressure and plastic strain at the interface were studied through numerical simulation with Smoothed Particle Hydrodynamics (SPH). The welding mechanism of W/Cu was analyzed according to the numerical results and experimental observation, which was in accordance with the indentation mechanism proposed by Bahrani.

Numerical and Experimental Studies on the Explosive Welding of Tungsten Foil to Copper.

The present paper is devoted to experimental and theoretical investigation of shock-wave propagation in a mixture of a gas and solid particles with clearly defined boundaries of the two-phase region (cloud of particles). The effect of qualitative transformation of supersonic flow behind a shock wave in a cloud of particles is shown experimentally and substantiated theoretically for volume concentrations of the dispersed phase of 0.1–3%.

Interaction of a shock wave with a cloud of particles

Results of numerical and experimental modeling of jet formation during a high-velocity oblique impact of metal plates accelerated by an explosion are presented. Numerical simulations are performed by the method of molecular dynamics. The impact of metal plates is directly studied in experiments with flash radiography of phenomena ahead of and behind the impact point. A metallographic study of the samples is performed. It is demonstrated that numerical simulations by the method of molecular dynamics ensure a qualitatively and quantitatively adequate description of jet formation and evolution.

Numerical and experimental modeling of jet formation during a high-velocity oblique impact of metal plates

Results of an experimental and theoretical study of the problem of shock-wave compaction of a metallic powder enclosed into a metallic container with a transverse partition are presented. It is demonstrated that internal stresses in the partition generate disturbances, which evolve under explosive loading and compression of the partition together with the powder. In addition to the loss of stability in the powder, which occurs owing to filling of voids, wave formation on the partition is observed.

Effect of the metal structure on the loss of stability of a thin plate separating a powder compressed by a shock wave

Results of numerical simulations of fracture of titanium and aluminum nanocrystals by the molecular dynamics method are reported. The nanocrystals are subjected to uniaxial tension in a wide temperature range (300–1270 K). It is demonstrated that tension of titanium nanocrystals heated to temperatures above 0.7 of the melting temperature in a non-stressed nanocrystal first leads to a phase transition from the crystalline to liquid state, followed by fracture. This effect is not observed in the case of tension of the aluminum nanocrystal.

S. P. Kiselev

Papers

Interaction of a shock wave with a cloud of particles

Numerical and experimental modeling of jet formation during a high-velocity oblique impact of metal plates

Effect of the metal structure on the loss of stability of a thin plate separating a powder compressed by a shock wave

Numerical Simulation of Fracture of Titanium and Aluminum Nanocrystals by the Molecular Dynamics Method