Two-dimensional Packing in Prolate Granular Materials

We investigate the two-dimensional packing of extremely prolate (aspect ratio $\alpha=L/D>10$) granular materials, comparing experiments with Monte-Carlo simulations. The average packing fraction of particles with aspect ratio $\alpha=12$ is $0.68\pm0.03$. We quantify the orientational correlation of particles and find a correlation length of two particle lengths. The functional form of the decay of orientational correlation is the same in both experiments and simulations spanning three orders of magnitude in aspect ratio. This function decays over a distance of two particle lengths. It is possible to identify voids in the pile with sizes ranging over two orders of magnitude. The experimental void distribution function is a power law with exponent $-\beta=-2.43\pm0.08$. Void distributions in simulated piles do not decay as a power law, but do show a broad tail. We extend the simulation to investigate the scaling at very large aspect ratios. A geometric argument predicts the pile number density to scale as $\alpha^{-2}$. Simulations do indeed scale this way, but particle alignment complicates the picture, and the actual number densities are quite a bit larger than predicted.Comment: 6 pages + 10 ps/eps figure

Manipulation of ultracold Bose gases in a time-averaged orbiting potential

De gemiddelde beweging van deeltjes in een oscillerende magnetische val, de zogeheten TOP-trap, kan worden gewijzigd door een verandering in de potentiële fase. De amplitude en energie van deze beweging kan worden gewijzigd door een geschikte fasesprong in het oscillerend (heen en weer slingeren van massa of energie) veld. Paul Cleary onderzocht toepassingen van een TOP-trap waarmee een wolk van ultrakoude atomen wordt gemanipuleerd. De TOP-trap bestaat uit een statische magneetval gecombineerd met een snel roterend magneetveld (het TOP-veld). In zijn onderzoek beschrijft Cleary de atomaire beweging in de TOP-val, in het bijzonder de fase van het roterende TOP-veld. Door numerieke en analytische methoden toont hij het belang aan van de fase van dit veld bij het inschakelen, wanneer het atomaire gas vanuit een statische in een TOP-val wordt overgebracht. De resulterende langzame ‘klotsende’ beweging van het centrum van de gaswolk is aanzienlijk vergeleken met de snelle microscopische beweging en is sterk afhankelijk van de initiële fase van het TOP-veld. Cleary laat zien dat deze klotsbeweging vervolgens kan worden onderdrukt door een goed-getimede sprong in de fase van het TOP-veld. Hij demonstreert hoe deze fasesprong optimaal wordt gekozen en toont vervolgens experimenteel zowel het inschakel-effect als de succesvolle demping van deze beweging aan

The role of advanced DEM based modelling tools in increasing comminution energy efficiency

Particle breakage is an essential part of mineral processing. The aim is to reduce run of mine mineral ore to an optimal size for liberating target minerals and for subsequent recovery by separation processes such as flotation. This size reduction is typically accomplished in a series of stages in a grinding circuit tailored to the properties of the particular mine ore. Commonly this involves two or more classes of equipment starting with crushers, followed by SAG mills and then sometimes ball mills. Occasionally, high pressure grinding rolls or other novel devices are substituted. Broadly, energy consumption increases and energy efficiency decreases with the fineness of the material produced by each piece of equipment

Using SPH one-way coupled to DEM to model wet industrial banana screens

Large banana screens with multiple decks are used extensively in the process separation of many valuable export commodities. They are high capacity vibrating screens with a curved profile. Discrete Element Method (DEM) modelling using non-spherical particles has previously provided significant insight into the operation of these dry industrial screens. Here we introduce the use of Smoothed Particle Hydrodynamics (SPH) to model the flow of slurry (water and fine material) through a double deck banana screen. This paper firstly reports on the underlying DEM model of the coarse particulate flow on a full-scale banana screen. We then use Smoothed Particle Hydrodynamics (SPH) to model the transport of fine particle slurry over and through the double deck banana screen. Finally, we combine the DEM with SPH models using a one-way coupling to simulate the effects of adding a slurry flow to coarse particulates on the banana screen. The key outcomes from this study are that; SPH is ideally suited for the high speeds and the high fragmented and filamentary nature of the fluid flow through the screen deck openings; the fluid only (SPH) model of slurry behaves similarly to the DEM approach in that more fluid is screened as the velocity slows, except near the earlier panels on the top deck; and, use of a porous media derived from DEM in one-way coupled approach with SPH produces clear and reasonable changes in fluid structure, separation and wetting of the screens consistent with slurry behaviour. Specifically, the fluid layer was much thicker in the coupled case, with slurry being trapped inside a coarse particle bed and which is sensitive to the fluid viscosity. © 2010 Elsevier Inc. All rights reserved