Granular materials are processed in many industries including agricultural, pharmaceutical, mining, and oil-sand and several geophysical processes such as landslides and avalanches. There are few models capable of predicting the flow of granular materials and particularly their collapse. Theoretical studies of the topic usually encounter difficulties in accurately predicting the collapse dynamics and final stable heaps. The two-dimensional gravitational collapse of cohesionless rectangular granular piles is numerically investigated in this paper. Piles surrounded by either air (dry case) or an oil-water mixture (wet case) undergo a dam-break collapse onto a horizontal base. The granular material is modeled as a perfectly plastic substance based on the Mohr-Coulomb law. The constitutive relations represent the granular material as a fluid, with a shear viscosity as a function of solids pressure, the second invariant of the deviatoric strain-rate tensor, and the internal angle of friction of the granular material. This two-phase flow problem (grains and liquid or air) is then formulated accordingly and solved by the mixture model method for the wet collapse and the level-set method for the dry collapse using COMSOL finite-element software. In both air and the liquid, stable heaps are achieved. The results are compared with experimental measurements of Balmforth & Kerswell [1] and Rondon et al. [2]. The model can closely predict the final shape of the collapsed dry pile. The final shape of the collapsed wet pile is also well-predicted when its initial packing concentration is relatively low. Further developments are needed to model the wet collapse of high-initial-concentration piles.

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