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.

1st International Conference on the Material Point Method MPM 2017Transient granular flows, such as rock falls, debris flows, and aerial and submarine avalanches, occur very often in nature. In the geotechnical context, transient movements of large granular slopes are a substantial factor of risk due to their destructive force and the transformations they may produce in the landscape. This paper investigates the ability of MPM, a continuum approach, to reproduce the evolution of a granular slope destabilised by an external energy source. In particular, a central issue is whether the power-law dependence of run-out distance and time observed with respect to the initial geometry or energy can be reproduced by a simple Mohr-Coulomb plastic behaviour. The effect of base friction on the run-out kinematics is studied by comparing the data obtained from the DEM and MPM simulations. The mechanism of energy dissipation is primarily through friction and the MPM is able to predict the run-out response in good agreement with the DEM simulations. At very low excitation energies, the DEM simulations show longer run-out in comparison to the MPM due to local destabilization at the flow front. At low input energies, a larger fraction of the energy is consumed in the destabilisation process, hence the amount energy available for flow is less. However, at higher input energy, where most of the energy is dissipated during the spreading phase, the run-out distance has a weak dependence on the distribution of velocity in the granular mas

The Material Point Method (MPM) is a hybrid Eulerian-Lagrangian approach capable of simulating large deformation problems of history-dependent materials. While the MPM can represent complex and evolving material domains by using Lagrangian points, boundary conditions are often applied to the Eulerian nodes of the background mesh nodes. Hence, the use of a structured mesh may become prohibitively restrictive for modeling complex boundaries such as a landslide topography. We study the suitability of unstructured background mesh with isoparametric elements to model irregular boundaries in the MPM. An inverse mapping algorithm is used to transform the material points from the global coordinates to the local natural coordinates. Dirichlet velocity and frictional boundary conditions are applied in the local coordinate system at each boundary node. This approach of modeling complex boundary conditions is validated by modeling the dynamics of a gravity-driven rigid block sliding on an inclined plane. This method is later applied to a flume test of controlled debris flow on an inclined plane conducted by the United States Geological Survey (USGS).

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