Abstract
Avalanches, debris flows, and landslides are geophysical hazards, which involve rapid mass movement of granular solids, water and air as a single-phase system. The dynamics of a granular flow involve at least three distinct scales: the micro-scale, meso-scale, and the macro-scale. This study aims to understand the ability of continuum models to capture the micro-mechanics of dry granular collapse. Material Point Method (MPM), a hybrid Lagrangian and Eulerian approach, with Mohr-Coulomb failure criterion is used to describe the continuum behaviour of granular column collapse, while the micromechanics is captured using Discrete Element Method (DEM) with tangential contact force model. The run-out profile predicted by the continuum simulations matches with DEM simulations for columns with small aspect ratios (‘h/r’ 2). Energy evolution studies in DEM simulations reveal higher collisional dissipation in the initial free-fall regime for tall columns. The lack of a collisional energy dissipation mechanism in MPM simulations results in larger run-out distances. Micro-structural effects, such as shear band formations, were observed both in DEM and MPM simulations. A sliding flow regime is observed above the distinct passive zone at the core of the column. Velocity profiles obtained from both the scales are compared to understand the reason for a slow flow run-out mobilization in MPM simulations.
Highlights
Geophysical hazards and industrial processes involves flow of dense granular material
This study aims to understand the ability of continuum models to capture the micro-mechanics of dry granular collapse
Material Point Method (MPM), a hybrid Lagrangian and Eulerian approach, with Mohr-Coulomb failure criterion is used to describe the continuum behaviour of granular column collapse, while the micromechanics is captured using Discrete Element Method (DEM) with tangential contact force model
Summary
Geophysical hazards and industrial processes involves flow of dense granular material. Different approaches have been adopted to model granular flows at different scales of description. Granular materials such as soils are modelled as a continuum. Granular materials exhibit many collective phenomena and the use of continuum mechanics to describe the macroscopic behaviour can be justified. Recent works on granular materials suggest that a continuum law may be incapable of revealing in-homogeneities at the grainscale level, such as orientation of force chains, which are purely due to micro-structural effects [2]. Granular flow is modelled as a frictional dissipation process in continuum mechanics but the lack of influence of inter-particle friction on the energy dissipation and spreading dynamics [5] is surprising. Multi-scale numerical modelling, i.e. discrete-element and continuum analyses, of the quasi-two dimensional collapse of granular columns are performed using Discrete Element (DEM) approach and Material Point Method (MPM) to understand the ability and limitations of continuum approaches in modelling dense granular flows
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