Multi-scale modelling of granular avalanches

Abstract

A study of the rheology of planar granular flow of dumbbells using discrete element method simulations Physics of Fluids 28, 103301 (2016) 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), however MPM predicts larger run-out distances for columns with higher 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.

The role of constitutive models in MPM simulations of granular column collapses

“…It will be shown later that the convex shape is related to a process called avalanching which is mesh-dependent. Increasing the number of material points in a coarse mesh did not significantly improve the results as was also observed by Kumar [16]. However, a refinement of the mesh, which indirectly initialised more material points, improved the results significantly.…”

“…Hence, the run-out distances were largely overestimated and numerical damping had to be applied in order to match the experimental results. Kumar [16] carried out simulations of the column collapse with both MPM and DEM and showed that MPM with a Mohr-Coulomb model suffered from insufficient dissipation of energy in comparison with DEM. It was attributed to the absence of inter-particle collisions which dissipates some energy.…”

“…Many MPM simulations presented in the literature used simple failure criteria such as Mohr-Coulomb (e.g. [1,3,16,34,35]). The model parameters were chosen as being close to the critical state values (u 0 % u 0 cs and w % 0) favouring the large strained areas and neglecting their mechanical behaviour at small strains.…”

“…They commented on the influence of the shape of the grains on the angle repose and discussed the dilatancy characteristics of the granular material but did not consider it in the simulations. Kumar [16] carried out DEM simulations to investigate the role of the initial grain properties and showed that it had had a significant influence on the flow kinematics and the internal flow structure.…”

Numerical simulation of granular mixing in a rotary drum using a generalized interpolation material point method

“…Similar results were presented by Kumar in the simulation of slope subjecting horizontal excitation. 44…”