TASK Quarterly   Scientific Bulletin of the Centre of Informatics - Tricity Academic Supercomputer & networK   ISSN 1428-6394

    3/98 "Granular Dynamics Simulations"

    Guest editor: Jacek Tejchman, tejchmk@pg.gda.pl
    Technical University of Gdansk, Poland

    Contents:

    • Paul Cleary, Discrete Element Modelling of Industrial Granular Flow Applications - abstract,
    • S.Luding, Clustering Instabilities, Arching and Anomalous Interaction Probablities as Examples for Cooperative Phenomena in Dry Granular Media - abstract,
    • Kazuyoshi Iwashita and Masanobu Oda, Shear Band Development in Modified DEM: Importance of Coupe Stress - abstract,
    • Colin Thornton, DEM Simulations of Quasi-Static Shear Deformation of Granular Media - abstract,
    • Jacek Tejchman, FE-Simulations of Rapid Silo Flow with a Polar Elasto-Plastic Constitutive Model - abstract,

    From the History of Science and Technology in Ancient Gdansk


    Abstracts:

  • Paul Cleary, Discrete Element Modelling of Industrial Granular Flow Applications,
  • Discrete element methods for modelling granular flows have now developed sufficiently for them to be applied to complex industrial and mining applications with an expectation that they can predict these flows reasonably well. Furthermore detailed quantitative predictions can be made using these models, allowing them to be validated against carefully designed experiments and then iteratively improved. The models allow existing equipment and processes to be carefully analysed. The resulting enhanced understanding can then be used to help improve them or to create new ones. DEM modelling of a wide range of industrial applications are described in this paper, including ball mill operation, dragline bucket filling, vibrational segregation by size and density, flow in centrifugal mills, flows from slot hoppers, idler induced segregation, cutter bias for commodity samplers and mixing in tumblers.

  • S.Luding, Clustering Instabilities, Arching and Anomalous Interaction Probablities as Examples for Cooperative Phenomena in Dry Granular Media,
  • In a freely cooling granular material fluctuations in density and temperature cause position dependent energy loss. Due to strong local dissipation, pressure and energy drop rapidly and material moves from `hot' to `cold' regions, leading to even stronger dissipation and thus causing the density instability. The assumption of `molecular chaos' is valid only in the homogeneous cooling regime. As soon as the density instability occurs, the impact parameter is not longer uniformly distributed. The pair-correlation and the  structure functions show that the molecular chaos assumption --- together with reasonable excluded volume modeling --- is important for short distances and irrelevant on large length scales.
    In this study, the probability distribution of the collision frequency is examined for pipe flow and for freely cooling granular materials as well. Uncorrelated events lead to a Poisson distribution for the collision frequencies. In contrast, the fingerprint of the cooperative phenomena discussed here is a power-law decay of the probability for many collisions per unit time.

  • Kazuyoshi Iwashita and Masanobu Oda, Shear Band Development in Modified DEM: Importance of Coupe Stress,
  • Numerical simulation tests were carried out using the distinct element method (DEM) with great interest in how large couple stress is actually generated in a shear band. To do this, the conventional DEM was modified slightly such that the effect of rolling resistance at contact points can be taken into account (called MDEM). It is found that MDEM provides a powerful tool in simulating not only the generation of large voids inside a shear band but also the high gradient of particle rotation along the shear band boundaries, in a quite similar manner to those of natural granular soils, and that couple stress, although it is very small, is certainly developed in a consistent manner with the particle rotation gradient developed in a shear band.

  • Colin Thornton, DEM Simulations of Quasi-Static Shear Deformation of Granular Media
  • ,
    Results are presented of numerically simulated three dimensional quasi-static shear deformation of polydisperse systems of spheres.  Results of axisymmetric compression test simulations are reported for both dense and loose systems.  The versatility of numerical simulation is demonstrated by presenting results obtained from simulations of general radial deviatoric loading, a constant deviatoric strain test and a multi-axial plane strain test.  In all cases, the simulated stress-strain dilation responses are in axcellent qualitative agreement with the behaviour of granular materials observed in physical experiments.

  • Jacek Tejchman, FE-Simulations of Rapid Silo Flow with a Polar Elasto-Plastic Constitutive Model,
  • The paper presents results on numerical modelling of rapid flow of granular materials in a model silo with convergent smooth walls. The calculations were performed with a finite element method based on a polar elasto-plastic constitutive relation by Mühlhaus. It differs from the conventional theory of plasticity by the presence of Cosserat rotations and couple stresses using a mean grain diameter as a characteristic length. The characteristic length causes that numerical results do not depend upon the mesh discretisation. The model tests on rapid silo flow of glass beads performed by Renner in a glass hopper with a large wall inclination from the bottom were numerically simulated. The plane strain FE-calculations were performed by taking into account inertial forces and linear viscous damping. A satisfactory agreement between numerical and experimental results was obtained. Advantages and limitations of a continuum approach for simulations of rapid silo flow were outlined.