Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session H10: Granular Flows: Mixing, Segregation and Separation |
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Chair: Troy Shinbrot, Rutgers University Room: 110 |
Monday, November 23, 2015 10:35AM - 10:48AM |
H10.00001: Itokawa is not Brazil: granular segregation on asteroids Troy Shinbrot, Pinaki Chakraborty, Tapan Sabuwala Recent photographs of the asteroid Itokawa have revealed strong separation between regions populated almost entirely by sand and other regions consisting only of larger boulders. This size separation has been attributed to the Brazil Nut Effect (BNE), however we point out here that the BNE depends on conditions such as isotropic gravity, parallel sidewalls and periodic vertical shaking that are wholly absent on asteroids. On the other hand, surface areas of boulders and sand appear to be comparable on Itokawa, and in this situation it follows that the asteroid must have suffered many orders of magnitude more collisions with sand particles than with boulders. We observe that a sand particle will tend to bounce off of a boulder but will sink into a sea of similar sand particles, and so we predict that sand seas must grow on such asteroids. We carry out experiments and simulations to evaluate this and related predictions, and we demonstrate that this new mechanism of segregation based on simple counting of grains can produce the strong separation of sizes reported. [Preview Abstract] |
Monday, November 23, 2015 10:48AM - 11:01AM |
H10.00002: Underlying Asymmetry with Particle-Size Segregation Parmesh Gajjar, Kasper van der Vaart, Gael Epely-Chauvin, Nicolas Andreini, Nico Gray, Christophe Ancey Granular media have a natural tendency to self-organise when sheared, with different sized constituents counter-intuitively separating from each other. Not only does the segregation produce a rich diversity of beautiful patterns, but it can also have serious implications in both industrial and geophysical environments. Despite the universal importance, the individual particle dynamics during segregation are still poorly understand, with such an analysis proving to be difficult with conventional techniques such as binning and sidewall observation. This talk will present results of recent experiments that studied particle scale segregation dynamics during oscillatory shear. Refractive index matched scanning allowed examination of the interior of the flow, where it was observed that large and small particles have an underlying asymmetry that is dependant on the local particle concentration. Small particles were seen to segregate faster through regions of many large particles, whilst large particles rise slower through regions of many small particles. The asymmetry is quantified on both bulk and particle length scales, and is shown to have good agreement with a continuum model that uses a cubic segregation flux. [Preview Abstract] |
Monday, November 23, 2015 11:01AM - 11:14AM |
H10.00003: Size and density segregation in granular mixtures Deepak Tunuguntla, Thomas Weinhart, Anthony Thornton In recent years, quite a few mixture theory continuum models, e.g., Tunuguntla et al. (2014), have attempted to, qualitatively and quantitatively, predict particle segregation in bidisperse mixture flows over inclined channels. This ongoing continuum approach incorporates percolation-driven segregation phenomenon into a continuum transport equation given in terms of particle volume fraction of a particular species. The key feature behind these models lies upon the fact on how the total bulk pressure is distributed among the two particle species. Thereby, indicating the need for suitable pressure scalings which help us determine the proportion of the bulk pressure to be carried by each type of particle species. To investigate this in detail, fully three dimensional discrete particle simulations (DPMs) are used. Further, we project the discrete data onto a continuum field using the novel coarse graining technique, see Tunuguntla et al. (2015). With these constructed macroscopic fields, such as the partial and bulk stresses, at hand, we arrive at suitable pressure scalings taking into the effects of both particle particle size and density. Thence, providing us with DPMs validated pressure scalings required to predict particle segregation more accurately. [Preview Abstract] |
Monday, November 23, 2015 11:14AM - 11:27AM |
H10.00004: Modeling density segregation in granular flow Hongyi Xiao, Richard Lueptow, Paul Umbanhowar A recently developed continuum-based model accurately predicts segregation in flows of granular mixtures varying in particle size by considering the interplay of advection, diffusion and segregation. In this research, we extend the domain of the model to include density driven segregation. Discrete Element Method (DEM) simulations of density bidisperse flows of mono-sized particles in a quasi-2D bounded heap were performed to determine the dependence of the density driven segregation velocity on local shear rate, particle concentration, and a segregation length which scales with the particle size and the logarithm of the density ratio. With these inputs, the model yields theoretical predictions of density segregation patterns that quantitatively match the DEM simulations over a range of density ratios (1.11-3.33) and flow rates (19.2-113.6 cm$^{3}$/s). Matching experiments with various combinations of glass, steel and ceramic particles were also performed which reproduced the segregation patterns obtained in both the simulations and the theory. [Preview Abstract] |
Monday, November 23, 2015 11:27AM - 11:40AM |
H10.00005: Particles size segregation and roll waves in dense granular flows Sylvain Viroulet, James Baker, Peter Kokelaar, Nico Gray Geophysical granular flows, such as landslides, snow avalanches and pyroclastic flows commonly involve particles with different sizes which are prone to segregate during the flow. This particle-size segregation may lead to the formation of regions with different frictional properties which can have a feedback on the flow. This study aims to understand this effect in the context of bi-disperse roll waves in shallow granular free-surface flows. Experiments have been performed in a 3 meter long chute using several mixtures of spherical glass beads of diameter 75-150 and 400-600 microns flowing on a rough bed. These show that the waves propagate at constant speed that depends on the initial mixture composition. In addition, during their propagation, a higher concentration of large particles is localized at the front of the waves. A theoretical and numerical approach is presented using depth-averaged equations for the conservation of mass, momentum and depth-averaged small particle concentration. Results without frictional feedback are investigated and compared to those that include the enhanced frictional resistance to motion of the large grains. [Preview Abstract] |
Monday, November 23, 2015 11:40AM - 11:53AM |
H10.00006: Continuum modeling of segregation for tridisperse granular materials in developing chute flow Zhekai Deng, Paul Umbanhowar, Richard Lueptow Predicting segregation and mixing of size polydisperse granular material is a challenging problem and is relevant to many industrial applications. We develop and implement a continuum-based theoretical model that captures the effects of segregation, diffusion and advection on size tridisperse granular flow in developing quasi-two-dimensional chute flow. Unlike segregation models that rely on arbitrary fitting parameters, our model uses parameters based on kinematics measured using discrete element method (DEM) simulations. The model depends on both the P\'eclet number, Pe, which we defined as the ratio of the segregation rate to the diffusion rate, and the relative segregation strength between particle species. At large Pe, segregation dominates and chute flow consists of distinct stratified regions of small(bottom), medium(center) and large (top) particles, whereas at small Pe, diffusion dominates, which results in a well mixed flow. As relative segregation strength between any two particle species is increased, the segregation between them becomes quicker. However, as relative segregation strength between them is decreased, they remain mixed with each other. Preliminary results from DEM simulations support our theoretical model. [Preview Abstract] |
Monday, November 23, 2015 11:53AM - 12:06PM |
H10.00007: Self Propelled particle systems: A study of the onset of organized motion Ajinkya Kulkarni, Srikanth Vedantam, Mahesh Panchagnula Swimming micro-organisms in liquid suspensions are known to produce interesting patterns. The objective of this study is to investigate the onset of organized motion in Self Propelled particle systems in confined two-dimensional spaces. A dynamic Vicsek model including particle inertia has been proposed. In this approach, the particles are modeledas soft disks with finite mass. The particles are required to align their local motion to their immediate neighborhood, similar to standard Vicsek model. We study the dynamics of organized motion and diffusion properties of the particles as a function of the local co-ordination coefficient (LCC) and thrust generation ability. Firstly, we observe a hysteretic phase transition from disorganized thermal motion to organized vortical motion as LCC is increased. In addition, we observe a sensitive dependence of the realized state of the system to the initial conditions for the particles near to the critical LCC. Finally the energy budget of the system -- including potential and kinetic energies as well as dissipation with time -- is used to understand the motivation for the phase transition. [Preview Abstract] |
Monday, November 23, 2015 12:06PM - 12:19PM |
H10.00008: ABSTRACT WITHDRAWN |
Monday, November 23, 2015 12:19PM - 12:32PM |
H10.00009: Pattern formation in triboelectrically charged binary packings Andre Schella, Thomas Vincent, Stephan Herminghaus, Matthias Schr\"oter Electrostatic self-assembly is an interesting route to aim at creating well-defined microstructures [1]. In this spirit, we study the process of self-assembling for vertically shaken granular materials. Our system consists from 1 to 400 plastic beads of 3mm size made from Teflon and Nylon in 2D and 3D geometries. We find self-organization in four, five and sixfold order which is due to charging of the system via triboelectric effects between the grains. We observe that the binary system solidifies on a time scale of a few minutes. Image processing is used to extract the structural and dynamical properties of the assemblies. The mixture ratio is tuned from 1:5 to 5:1 and the humidity level is varied between 10\% and 90\% leading to various transitions between the morphologies. \\[4pt] [1] B. Grzybowksi et al. , Nat. Materials 2, 241-245 (2003) [Preview Abstract] |
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