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

An efficient ghost-cell immersed boundary method (GCIBM) for simulating turbulent flows in complex geometries is presented. A boundary condition is enforced through a ghost cell method. The reconstruction procedure allows systematic development of numerical schemes for treating the immersed boundary while preserving the overall second-order accuracy of the base solver. Both Dirichlet and Neumann boundary conditions can be treated. The current ghost cell treatment is both suitable for staggered and non-staggered Cartesian grids. The accuracy of the current method is validated using flow past a circular cylinder and large eddy simulation of turbulent flow over a wavy surface. Numerical results are compared with experimental data and boundary-fitted grid results. The method is further extended to an existing ocean model (MITGCM) to simulate geophysical flow over a three-dimensional bump. The method is easily implemented as evidenced by our use of several existing codes.

A Ghost-Cell Immersed Boundary Method for Flow in Complex Geometry

Polymeric materials display distinguished characteristics which stem from the interplay of phenomena at various length and time scales. Further development of polymer systems critically relies on a comprehensive understanding of the fundamentals of their hierarchical structure and behaviors. As such, the inherent multiscale nature of polymer systems is only reflected by a multiscale analysis which accounts for all important mechanisms. Since multiscale modelling is a rapidly growing multidisciplinary field, the emerging possibilities and challenges can be of a truly diverse nature. The present review attempts to provide a rather comprehensive overview of the recent developments in the field of multiscale modelling and simulation of polymeric materials. In order to understand the characteristics of the building blocks of multiscale methods, first a brief review of some significant computational methods at individual length and time scales is provided. These methods cover quantum mechanical scale, atomistic domain (Monte Carlo and molecular dynamics), mesoscopic scale (Brownian dynamics, dissipative particle dynamics, and lattice Boltzmann method), and finally macroscopic realm (finite element and volume methods). Afterwards, different prescriptions to envelope these methods in a multiscale strategy are discussed in details. Sequential, concurrent, and adaptive resolution schemes are presented along with the latest updates and ongoing challenges in research. In sequential methods, various systematic coarse-graining and backmapping approaches are addressed. For the concurrent strategy, we aimed to introduce the fundamentals and significant methods including the handshaking concept, energy-based, and force-based coupling approaches. Although such methods are very popular in metals and carbon nanomaterials, their use in polymeric materials is still limited. We have illustrated their applications in polymer science by several examples hoping for raising attention towards the existing possibilities. The relatively new adaptive resolution schemes are then covered including their advantages and shortcomings. Finally, some novel ideas in order to extend the reaches of atomistic techniques are reviewed. We conclude the review by outlining the existing challenges and possibilities for future research.

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A Review of Multiscale Computational Methods in Polymeric Materials

CFD–DEM model for coupled heat and mass transfer in a spout fluidized bed with liquid injection

UV degradation model for polymers and polymer matrix composites

A novel nonintrusive technique is presented to investigate hydrodynamic and thermal behavior of gas–solid spout-fluidized beds with liquid injection, by simultaneously capturing visual and infrared images. Experiments were performed in a pseudo-2D bed with draft plates filled with glass or γ-alumina particles to investigate the effect of liquid injection and particle properties on the flow characteristics. For the glass particles under dry and wet conditions, time-averaged particle velocities show similar quasi-steady-state behavior. However, under wet conditions, lower particle velocities were observed in both spout and annulus as compared with the dry system. Whereas, γ-alumina particles do not show considerable variation in the particle velocities under dry and wet conditions and fluidize well at higher liquid injection rates. Additionally, for the glass particles, the particle temperature significantly decreases as compared to the γ-alumina particles. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1146–1159, 2015

Experimental study of hydrodynamics and thermal behavior of a pseudo‐2D spout‐fluidized bed with liquid injection

Viscoelastic fluids exhibit elastic instabilities in simple shear flow and flow through curved streamlines. Surprisingly, we found in a porous medium such fluids show strikingly different hydrodynamic instabilities depicted by very large sideways excursions and presence of fast and slow moving lanes which have not been reported before. Particle image velocimetry (PIV) measurements through a pillared microchannel, provide experimental evidence of such instabilities at very low Reynolds number (< 0.01). We observe a transition from a symmetric laminar to an asymmetric flow, which finally transforms to a nonlinear aperiodic flow with strong lateral movements. The instability is characterized by a rapid increase in spatial and temporal fluctuations of velocity components and pressure at a critical Deborah number (De). Our experiments reveal the presence of a fascinating interplay between pore space and fluid rheology.

Elastic Instabilities in Flows through Pillared Micro channels

A simulation approach to study photo-degradation processes of polymeric coatings

In this paper, we report the extension of an earlier developed Direct Numerical Simulation (DNS) model to study coupled heat and mass transfer problems in particulate flows. The DNS model builds on an efficient ghost-cell based Immersed Boundary Method (IBM) which implicitly incorporates the boundary conditions into the discretized momentum, thermal and species conservation equations of the fluid phase. On the particles an exothermic surface reaction takes place. The heat and mass transport is coupled through the particle temperature, which offers a dynamic boundary condition for the fluid phase thermal energy equation. The present simulations are performed for four fluid-solid systems. Following the case of the unsteady mass and heat diffusion in a large pool of quiescent fluid, we consider a stationary sphere under forced convection. In both cases variable reaction rates are imposed at the particle surface, and the particle temperatures obtained from DNS show a good agreement with analytical/empirical solutions. After that, we apply our DNS model to the three-bead reactor and finally, a dense particle array consisting of hundreds of particles distributed in a random fashion is studied. The concentration and temperature profiles are compared with a ID heterogeneous reactor model and the heterogeneity inside the array is discussed. .

Elias A.J.F. Peters

Papers

Experimental study of hydrodynamics and thermal behavior of a pseudo‐2D spout‐fluidized bed with liquid injection

Elastic Instabilities in Flows through Pillared Micro channels

A simulation approach to study photo-degradation processes of polymeric coatings

Immersed boundary method

Direct numerical simulation of fluid flow and dependently coupled heat and mass transfer in fluid-particle systems