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

Turbulent dispersed multiphase flows are common in many engineering and environmental applications. The stochastic nature of both the carrier-phase turbulence and the dispersed-phase distribution makes the problem of turbulent dispersed multiphase flow far more complex than its single-phase counterpart. In this article we first review the current state-of-the-art experimental and computational techniques for turbulent dispersed multiphase flows, their strengths and limitations, and opportunities for the future. The review then focuses on three important aspects of turbulent dispersed multiphase flows: the preferential concentration of particles, droplets, and bubbles; the effect of turbulence on the coupling between the dispersed and carrier phases; and modulation of carrier-phase turbulence due to the presence of particles and bubbles.

Turbulent Dispersed Multiphase Flow

Discrete particle simulation of particulate systems: A review of major applications and findings

Thermophysical Properties Research Literature Retrieval Guide Edited by Y. S. Touloukian, J. K. Gerritsen and N. Y. Moore Second edition, revised and expanded. Book 1: Pp. xxi + 819. Book 2: Pp.621. Book 3: Pp. ix + 1315. (New York: Plenum Press, 1967.) n.p.

/pdf/heat-and-mass-transfer-1yaijfx8vp.pdf

Heat and Mass Transfer

To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.

Theory of cross-correlation analysis of PIV images : Image analysis as measuring technique in flows

https://sites.ualberta.ca/~jos/pbl/ces54p2055.pdf

An experimental and numerical study of turbulent swirling flow in gas cyclones

A comparison is made between experiments and simulations on a single sphere settling in silicon oil in a box. Cross-correlation particle imaging velocimetry measurements were carried out at particle Reynolds numbers ranging from 1.5 to 31.9. The particle Stokes number varied from 0.2 to 4 and at bottom impact no rebound was observed. Detailed data of the flow field induced by the settling sphere were obtained, along with time series of the sphere’s trajectory and velocity during acceleration, steady fall and deceleration at bottom approach. Lattice–Boltzmann simulations prove to capture the full transient behavior of both the sphere motion and the fluid motion. The experimental data were used to assess the effect of spatial resolution in the simulations over a range of 2–8 grid nodes per sphere radius. The quality of the flow field predictions depends on the Reynolds number. When the sphere is very close to the bottom of the container, lubrication theory has been applied to compensate for the lack of spatial resolution in the simulations.

/pdf/particle-imaging-velocimetry-experiments-and-lattice-47nk5kbs22.pdf

Particle imaging velocimetry experiments and lattice-Boltzmann simulations on a single sphere settling under gravity

Large eddy simulations were performed on the flow in a baffled stirred tank, drien by a Rushton turbine at Res 29,000. The simulation procedure consisted of a lattice ) Boltzmann scheme for discretizing the Naier ) Stokes equations, and a force- field technique for representing the action of the impeller on the fluid. The subgrid-scale model was a conentional Smagorinsky model with a Smagorinsky constant c s 0.12. s The uniform, cubic computational grid had a size of about 6 = 10 6 nodes. The com- puter code was implemented on a parallel, shared-memory computer platform. The results on the phase-resoled aerage flow, as well as on the turbulence characteristics, are compared with phase-resoled experimental data. The trailingortex structure in the ¤icinity of the impeller was well represented by the simulations.

/pdf/large-eddy-simulations-on-the-flow-driven-by-a-rushton-2qiptbtpuy.pdf

Large eddy simulations on the flow driven by a Rushton turbine

Fully resolved simulations of particles suspended in a sustained turbulent flow field are presented. To solve the Navier–Stokes equations a lattice-Boltzmann scheme was used. A spectral forcing scheme is applied to maintain turbulent conditions at a Taylor microscale Reynolds number of 61. The simulations contained between 2 and 10 vol % particles with a solid to fluid density ratio between 1.15 and 1.73. A lubrication force is used to account for subgrid hydrodynamic interaction between approaching particles. Results are presented on the influence of the particle phase on the turbulence spectrum and on particle collisions. Energy spectra of the simulations show that the particles generate fluid motion at length scales of the order of the particle size. This results in a strong increase in the rate of energy dissipation at these length scales and a decrease of kinetic energy at larger length scales.Collisions due to uncorrelated particle motion are observed (primary collisions), and collision frequencies are in agreement with theory on inertial particle collisions. In addition to this, a large number of collisions at high frequencies is encountered. These secondary collisions are due to the correlated motion of particles resulting from short-range hydrodynamic interactions and spatial correlation of the turbulent velocity field at short distances. This view is supported by the distribution of relative particle velocities, the particle velocity correlation functions and the particle radial distribution function.

/pdf/fully-resolved-simulations-of-colliding-monodisperse-spheres-3ywdaxnlrh.pdf

Fully resolved simulations of colliding monodisperse spheres in forced isotropic turbulence

s damping functions, was applied. The 3-D, aerage flow field was predicted with a high leel of accuracy. Furthermore, the simulations exhibitortex-core precession, that is, the core of the mainortex is obsered to moe about the geometrical axis of the cyclone in a quasi-periodic manner. The Strouhal number associated with the simulated ¤ortex-core precession was 0.53, whereas 0.49 was experimentally obsered in a similar geometry at approximately the same Reynolds number.

/pdf/simulation-of-vortex-core-precession-in-a-reverse-flow-11oi2gbbou.pdf

Jos Derksen

Papers

An experimental and numerical study of turbulent swirling flow in gas cyclones

Particle imaging velocimetry experiments and lattice-Boltzmann simulations on a single sphere settling under gravity

Large eddy simulations on the flow driven by a Rushton turbine

Fully resolved simulations of colliding monodisperse spheres in forced isotropic turbulence

Simulation of vortex core precession in a reverse-flow cyclone