69 research outputs found
DEM simulations of polydisperse media: efficient contact detection applied to investigate the quasi-static limit
Discrete element modeling (DEM) of polydisperse granular materials is significantly more computationally expensive than modeling of monodisperse materials as a larger number of particles are required to obtain a representative elementary volume, and standard contact detection algorithms become progressively less efficient with polydispersity. This paper presents modified contact detection and inter-processor communication schemes implemented in LAMMPS which account for particles of different sizes separately, greatly improving efficiency. This new scheme is applied to the inertial number (I), which quantifies the ratio of inertial to confining forces. This has been used to identify the quasi-static limit for shearing of granular materials, which is often taken to be I=10−3. However, the expression for the inertial number contains a particle diameter term and therefore it is unclear how to apply this for polydisperse media. Results of DEM shearing tests on polydisperse granular media are presented in order to determine whether I provides a unique quasi-static limit regardless of polydispersity and which particle diameter term should be used to calculate I. The results show that the commonly used value of I=10−3 can successfully locate the quasi-static limit for monodisperse media but not for polydisperse media, for which significant variations of macroscopic stress ratio and microscopic force and contact networks are apparent down to at least I=10−6. The quasi-static limit could not be conclusively determined for the polydisperse samples. Based on these results, the quasi-staticity of polydisperse samples should not be inferred from a low inertial number as currently formulated, irrespective of the particle diameter used in its calculation
Hydrodynamic oscillations and variable swimming speed in squirmers close to repulsive walls
We present a lattice Boltzmann study of the hydrodynamics of a fully resolved
squirmer, radius R, confined in a slab of fluid between two no-slip walls. We
show that the coupling between hydrodynamics and short-range repulsive
interactions between the swimmer and the surface can lead to hydrodynamic
trapping of both pushers and pullers at the wall, and to hydrodynamic
oscillations in the case of a pusher. We further show that a pusher moves
significantly faster when close to a surface than in the bulk, whereas a puller
undergoes a transition between fast motion and a dynamical standstill according
to the range of the repulsive interaction. Our results critically require
near-field hydrodynamics; they further suggest that it should be possible to
control density and speed of squirmers at a surface by tuning the range of
steric and electrostatic swimmer-wall interactions.Comment: 5 + 8 pages, 4 + 4 Figure
Field-Induced Breakup of Emulsion Droplets Stabilized by Colloidal Particles
We simulate the response of a particle-stabilized emulsion droplet in an
external force field, such as gravity, acting equally on all particles. We
show that the field strength required for breakup (at fixed initial area
fraction) decreases markedly with droplet size, because the forces act
cumulatively, not individually, to detach the interfacial particles. The
breakup mode involves the collective destabilization of a solidified particle
raft occupying the lower part of the droplet, leading to a critical force per
particle that scales approximately as .Comment: 4 pages, plus 3 pages of supplementary materia
Improved hydrogen gas production in microbial electrolysis cells using inexpensive recycled carbon fibre fabrics
Growing energy demands of wastewater treatment have made it vital for water companies to develop less energy intensive processes for treating wastewater if net zero emissions are to be achieved by 2050. Microbial electrolysis cells (MECs) have the potential to do this by treating water and producing renewable hydrogen gas as a product, but capital and operational costs have slowed their deployment. By using recycled carbon fibre mats, commercially viable MECs can brought closer to reality, where recycled carbon fibre anode MECs treating real wastewater (normalised ~3100 L d−1) were producing 66.77 L H2 d−1 while graphite felt anode MECs produced 3.65 L H2 d−1 per 1 m3 reactor, anodes costing £5.53 m−2 and £88.36 m−2 respectively, resulting in a total anode cost saving of 93%. This could incentivise the development of larger pilot systems, opening the door for generating greater value and a more sustainable wastewater treatment industry
Dynamics of a spherical colloid at a liquid interface:A lattice Boltzmann study
We study the dynamics of a spherical colloidal particle pulled along the fluid-fluid interface using lattice Boltzmann (LB) simulations. We consider an interface with a finite width and include both the effects of the thermodynamics of the interface and the particle wetting, characterized by the contact angle h between the particle surface and the interface, in addition to the viscosity ratio k between the two fluids. We characterize the particle dynamics by applying a constant pulling force along the interface and measure both the translational and the rotational dynamics as a function of the contact angle and the viscosity ratio. We observe that the hydrodynamic drag is reduced and the particle rotation is increased when the particle resides more in the low viscosity fluid, in agreement with previous hydrodynamic theories. We also study the case where the particle rotation is suppressed, and find an overall increase of the drag coefficient
targetDP: an Abstraction of Lattice Based Parallelism with Portable Performance
To achieve high performance on modern computers, it is vital to map
algorithmic parallelism to that inherent in the hardware. From an application
developer's perspective, it is also important that code can be maintained in a
portable manner across a range of hardware. Here we present targetDP (target
Data Parallel), a lightweight programming layer that allows the abstraction of
data parallelism for applications that employ structured grids. A single source
code may be used to target both thread level parallelism (TLP) and instruction
level parallelism (ILP) on either SIMD multi-core CPUs or GPU-accelerated
platforms. targetDP is implemented via standard C preprocessor macros and
library functions, can be added to existing applications incrementally, and can
be combined with higher-level paradigms such as MPI. We present CPU and GPU
performance results for a benchmark taken from the lattice Boltzmann
application that motivated this work. These demonstrate not only performance
portability, but also the optimisation resulting from the intelligent exposure
of ILP.Comment: 4 pages, 1 figure, to appear in proceedings of HPCC 2014 conferenc
Dynamic clustering and re-dispersion in concentrated colloid-active gel composites
International audienceWe study the dynamics of quasi-two-dimensional concentrated suspensions of colloidal particles in active gels by computer simulations. Remarkably, we find that activity induces a dynamic clustering of colloids even in the absence of any preferential anchoring of the active nematic director at the particle surface. When such an anchoring is present, active stresses instead compete with elastic forces and re-disperse the aggregates observed in passive colloid-liquid crystal composites. Our quasi-two-dimensional "inverse" dispersions of passive particles in active fluids (as opposed to the more common "direct" suspensions of active particles in passive fluids) provide a promising route towards the self-assembly of new soft materials
CP2K – Scalable Atomistic Simulation for the PRACE Community
This report describes the work undertaken under PRACE-1IP to support the European scientific communities who make use of CP2K in their research. This was done in two ways – firstly, by improving the performance of the code for a wide range of usage scenarios. The updated code was then tested and installed on the PRACE CURIE supercomputer. We believe this approach both supports existing user communities by delivering better application performance, and demonstrates to potential users the benefits of using optimized and scalable software like CP2K on the PRACE infrastructure
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