Jens Honore Walther

PPM: a highly efficient parallel particle-mesh library for the simulation of continuum systems

This paper presents a highly efficient parallel particle-mesh (PPM) library, based on a unifying particle formulation for the simulation of continuous systems. In this formulation, the grid-free character of particle methods is relaxed by the introduction of a mesh for the reinitialization of the particles, the computation of the field equations, and the discretization of differential operators. The present utilization of the mesh does not detract from the adaptivity, the efficient handling of complex geometries, the minimal dissipation, and the good stability properties of particle methods.The coexistence of meshes and particles, allows for the development of a consistent and adaptive numerical method, but it presents a set of challenging parallelization issues that have hindered in the past the broader use of particle methods. The present library solves the key parallelization issues involving particle-mesh interpolations and the balancing of processor particle loading, using a novel adaptive tree for mixed domain decompositions along with a coloring scheme for the particle-mesh interpolation.The high parallel efficiency of the library is demonstrated in a series of benchmark tests on distributed memory and on a shared-memory vector architecture. The modularity of the method is shown by a range of simulations, from compressible vortex rings using a novel formulation of smooth particle hydrodynamics, to simulations of diffusion in real biological cell organelles.The present library enables large scale simulations of diverse physical problems using adaptive particle methods and provides a computational tool that is a viable alternative to mesh-based methods.

Barriers to superfast water transport in carbon nanotube membranes

Carbon nanotube (CNT) membranes hold the promise of extraordinary fast water transport for applications such as energy efficient filtration and molecular level drug delivery. However, experiments and computations have reported flow rate enhancements over continuum hydrodynamics that contradict each other by orders of magnitude. We perform large scale molecular dynamics simulations emulating for the first time the micrometer thick CNTs membranes used in experiments. We find transport enhancement rates that are length dependent due to entrance and exit losses but asymptote to 2 orders of magnitude over the continuum predictions. These rates are far below those reported experimentally. The results suggest that the reported superfast water transport rates cannot be attributed to interactions of water with pristine CNTs alone.

Aeroelastic analysis of bridge girder sections based on discrete vortex simulations

Two-dimensional viscous incompressible flow past bridge girder cross-sections are simulated using the discrete vortex method. The flow around stationary cross-sections as well as cross-sections undergoing cross-wind vertical (bending) and rotary (torsional) motions are investigated for assessment of drag coefficient, Strouhal number and aerodynamic derivatives for application in aeroelastic analyses. Good to excellent agreement with wind tunnel test results is demonstrated for analyses of forced wind loading, flutter wind speed and vertical vortex-induced response of four practical girder cross-sections. The success of the simulations is attributed to the bluff nature of the cross-sections and to the two-dimensional (2-D) nature of flow around bridge girders.

Discrete vortex simulation of flow around five generic bridge deck sections

Abstract Two-dimensional viscous incompressible flow past five generic bridge deck cross sections are investigated by means of the discrete vortex method. The analyses yields root mean square lift coefficients and Strouhal numbers for fixed cross sections and aerodynamic derivatives for the cross sections undergoing forced oscillatory cross wind and twisting motion. Fair agreement is established between the present simulations and wind tunnel test results reported in the literature.

Thermophoretic Motion of Water Nanodroplets Confined inside Carbon Nanotubes

We study the thermophoretic motion of water nanodroplets confined inside carbon nanotubes using molecular dynamics simulations. We find that the nanodroplets move in the direction opposite the imposed thermal gradient with a terminal velocity that is linearly proportional to the gradient. The translational motion is associated with a solid body rotation of the water nanodroplet coinciding with the helical symmetry of the carbon nanotube. The thermal diffusion displays a weak dependence on the wetting of the water-carbon nanotube interface. We introduce the use of the moment scaling spectrum (MSS) in order to determine the characteristics of the motion of the nanoparticles inside the carbon nanotube. The MSS indicates that affinity of the nanodroplet with the walls of the carbon nanotubes is important for the isothermal diffusion and hence for the Soret coefficient of the system.

Two dimensional discrete vortex method for application to bluff body aerodynamics

Two-dimensional viscous incompressible flow past a flat plate of finite thickness and length is simulated using the discrete vortex method. Both a fixed plate and a plate undergoing a harmonic heave and pitch motion are studied. The Reynolds number is 104 and the record onset flow speed, Ufc is in the range 2–14. The fundamental kinematic relation between the velocity and the vorticity is used in a novel approach to determine the surface vorticity. An efficient influence matrix technique is used in a fast adaptive multipole algorithm context to obtain a mesh-free method. The numerical results are compared with the steady-state Blasius solution, and with the inviscid solution for the flow past an oscillating plate by Theodorsen.

Water–Carbon Interactions 2: Calibration of Potentials using Contact Angle Data for Different Interaction Models

Molecular dynamics simulations of water droplets on graphite are carried out to determine the contact angle for different water–carbon potential functions. Following the procedure of Werder et al. [J. Phys. Chem. B, 107 (2003) 1345], the C–O Lennard–Jones well depth is varied to recover the experimental value for the contact angle using a 2000-molecule water droplet and compensating for the line tension effect that lowers the contact angle for increasing droplet size. For the discrete graphite surface model studied by Werder et al., the effects of adding C–H Lennard–Jones interactions and changing the long-range cut-off distance are considered. In addition, a continuum graphite surface model is studied for which the water–graphite interaction energy depends only on the normal distance (z) from the water oxygen to the surface. This new model, with z -10 repulsion and z -4 attraction, is formulated in terms of the standard Lennard–Jones parameters, for which the recommended values are sgr CO=3.19 Å and ε CO=0.3651 kJ/mol.

Strain engineering of Kapitza resistance in few-layer graphene.

We demonstrate through molecular dynamics simulations that the Kapitza resistance in few-layer graphene (FLG) can be controlled by applying mechanical strain. For unstrained FLG, the Kapitza resistance decreases with the increase of thickness and reaches an asymptotic value of 6 × 10(-10) m(2)K/W at a thickness about 16 nm. Uniaxial cross-plane strain is found to increase the Kapitza resistance in FLG monotonically, when the applied strain varies from compressive to tensile. Moreover, uniaxial strain couples the in-plane and out-of-plane strain/stress when the surface of FLG is buckled. We find that with a compressive cross-plane stress of 2 GPa, the Kapitza resistance is reduced by about 50%. On the other hand it is almost tripled with a tensile cross-plane stress of 1 GPa. Remarkably, compressive in-plane strain can either increase or reduce the Kapitza resistance, depending on the specific way it is applied. Our study suggests that graphene can be exploited for both heat dissipation and insulation through strain engineering.

Kapitza Resistance between Few-Layer Graphene and Water: Liquid Layering Effects

The Kapitza resistance (RK) between few-layer graphene (FLG) and water was studied using molecular dynamics simulations. The RK was found to depend on the number of the layers in the FLG though, surprisingly, not on the water block thickness. This distinct size dependence is attributed to the large difference in the phonon mean free path between the FLG and water. Remarkably, RK is strongly dependent on the layering of water adjacent to the FLG, exhibiting an inverse proportionality relationship to the peak density of the first water layer, which is consistent with better acoustic phonon matching between FLG and water. These findings suggest novel ways to engineer the thermal transport properties of solid-liquid interfaces by controlling and regulating the liquid layering at the interface.

Large‐scale parallel discrete element simulations of granular flow

Purpose – The purpose of this paper is to present large‐scale parallel direct numerical simulations of granular flow, using a novel, portable software program for discrete element method (DEM) simulations.Design/methodology/approach – Since particle methods provide a unifying framework for both discrete and continuous systems, the program is based on the parallel particle mesh (PPM) library, which has already been demonstrated to provide transparent parallelization and state‐of‐the‐art parallel efficiency using particle methods for continuous systems.Findings – By adapting PPM to discrete systems, results are reported from three‐dimensional simulations of a sand avalanche down an inclined plane.Originality/value – The paper demonstrates the parallel performance and scalability of the new simulation program using up to 122 million particles on 192 processors, employing adaptive domain decomposition and load balancing techniques.