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

https://bura.brunel.ac.uk/bitstream/2438/13101/1/FullText.pdf

Cold spraying - A materials perspective

In conventional thermal spraying, the spray particles are partially or fully molten when they deposit on the substrate. Cold spraying, in contrast, uses less thermal and more kinetic energy. In this process, solid particles impact on the substrate at high velocities and form excellent coatings. Due to comparatively low temperatures and typically inert process gases, cold spraying is particularly suitable for heat and oxidation sensitive materials. In recent years, modeling and computational methods have been widely used to study this relatively new spraying process, particularly to describe impact conditions of particles, to improve nozzle design, and to provide a better understanding of the thermo-mechanical processes that lead to particle bonding and deposition. This paper summarizes the state of the art in these theoretical studies, alongside a comprehensive description of the process. The paper also discusses the prediction of coating properties in the light of modeling combined with experimental investigations.

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From Particle Acceleration to Impact and Bonding in Cold Spraying

Cold spraying has attracted serious attention since unique coating properties can be obtained by the process that are not achievable by conventional thermal spraying This uniqueness is due to the fact that coating deposition takes place without exposing the spray or subtrate material to high temperatures and, in particular, without melting the sprayed particles Thus, oxidation and other undesired reactions can be avoided Spryy particles adhere to the substrate only because of their high kinetic energy on impact For successful bonding, powder particles have to exceed a critical velocity on impact, which is dependent on the properties of the particular spray material This requires new concepts for the description of coating formation but also indicates applications beyond the market for typical thermal spray coatings The present contribution summarizes the current “state of the art” in cold spraying and demonstrates concepts for process optimization

The cold spray process and its potential for industrial applications

For cold spraying, a method for the construction of the window of deposition and the selection of optimum process parameters is presented. Initially, particle impact velocity and the critical particle velocity for bonding are worked out and expressed explicitly in terms of key process and material parameters. Subsequently, the influence of particle velocity on coating characteristics is examined in view of the results of experiments and simulations. It has been found that main coating characteristics can be described as a unique function of the ratio of particle velocity to critical velocity, here referred to as η. Finally, coating properties are linked directly to primary process parameters via parameter selection maps, where contours of constant η are plotted on a plane of gas temperature versus gas pressure. Inferences of the presented method and the resulting parameter selection maps are discussed for the example of copper as feedstock material.

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On Parameter Selection in Cold Spraying

This article presents the peculiarities of the supersonic nozzle design for the cold gas-dynamic spraying. The procedure to produce the high particle velocity by correct choice of the geometrical dimensions of the accelerating nozzles is described. Numerical and experimental research of wedge-shaped nozzles shows that there is a nozzle with its particular dimensions for a given type of particles that produces the maximum possible particle velocity at the moment of impact on a target surface.

The features of cold spray nozzle design

This paper presents an overview of results of recent studies conducted at the Institute of Theoretical and Applied Mechanics of the Siberian Division of the Russian Academy of Science in the field of gas dynamics and heat transfer of the supersonic air jet under conditions typically used in the cold spray process. These studies are related to various aspects of the problem including a flow in the nozzle and the outflow of the jet, as well as effects of the interaction of the jet with a flat obstacle. They are conducted with a supersonic nozzle with a rectangular section at the exit with a Mach number M
 0 between 2 and 3.5. The gas flow in the nozzle is theoretically and experimentally studied. It is shown that the boundary layer on the walls of the nozzle affects significantly the flow parameters (for example, Mach number M, pressure p, temperature T, and density ρ of the gas). A method of calculation of the gas parameters in the flow core of the nozzle is suggested, and it is shown that they depend mainly on the ratio of the nozzle width to its length. The results of the investigation of the supersonic air jets with stagnation temperature ranging from 300–600 K flowing in the atmosphere are presented. The corresponding dimensions of the jets, profiles, and axial distributions of the gas parameters are obtained. The interactions of the supersonic jet with the flat obstacle are studied. Self-similarity of the distribution of the pressure and of the Mach number on the obstacle surface is shown for the jets with various values of the Mach number and the angle of impingement. The oscillation regimen of the jet impingement, as well as a compressed layer structure is observed with the aid of a Schliren visualization technique. Some problems of heat exchange of the jets with the obstacle are considered. Distributions of stagnation temperature and heat exchange coefficient in the near-wall jet are obtained. The temperature of the obstacle for the stationary case is calculated, and it is shown that for heat conductive materials the surface temperature is lower than the stagnation temperature due to the redistribution of heat inside of the substrate.

On some aspects of gas dynamics of the cold spray process

Cold spraying of in situ produced TiB2–Cu nanocomposite powders

An analytical model of the kinetics of coating formation during cold spray is presented. The model is used to correct experimental data on deposition efficiency. The experimentally observed values are shown to be affected by experimental conditions, such as the velocity of substrate motion, the number of passes, the mass of a single portion of powder, and the exposure time of a given surface section. It is noted that experimental conditions can exert a significant effect on the consequences of the high-speed interaction of particles with a substrate. Relations are suggested that allow one to correct the results of deposition efficiency determined experimentally and to avoid mistakes in interpreting the data obtained.

Measurements of cold spray deposition efficiency

We present results of an experimental study of the specific features of the gas-dynamic formation of coatings from metallic powders (dp<50 μm) on substrates of various materials depending on the particle velocity (200–1200m/sec), the jet temperature (300–700K), and other parameters. Results of a prospecting study of the implementation of the methods of particle acceleration in supersonic (M=2.0–3.0) rectangular nozzles are described. The rate of bond formation in a cold particle-cold substrate contact occurring in gas-dynamic spraying is estimated within the framework of the concepts applied in analysis of gas-dynamic spraying.

V. F. Kosarev

Papers

The features of cold spray nozzle design

On some aspects of gas dynamics of the cold spray process

Cold spraying of in situ produced TiB2–Cu nanocomposite powders

Measurements of cold spray deposition efficiency

Gas-dynamic spraying. An experimental study of the spraying process