Modeling gravitational collapse of rectangular granular piles in air and water

• Uses CFD module of COMSOL to simulate gravitational collapse of granular piles.
• Level Set method with simple dry particle rheology predicts experimental results well.
• Dry model rheology modified to include pore pressure effects for submerged collapses.
• Mixture Model gives good predictions of initially loose submerged collapses.
• Simple empirical treatment presented for initially densely packed submerged piles.

The present paper is concerned with the two-dimensional collapse of piles of granular materials, a problem analogous to the classical dam break problem in hydraulics. This study is intended to aid in the development of constitutive equations and modeling procedures that can be applied to predict various flows involving high concentration liquid–particle mixtures. We consider the granular collapse as a test problem and attempt to validate our modeling by comparing our predictions with previously published granular collapse experiments. The time-dependent evolution of the collapsing granular piles is calculated by making use of COMSOL, a commercial finite element code that is designed to handle a wide variety of Multiphysics problems. We begin by considering the collapse of a rectangular block of dry granular material and calculate the temporal evolution of the free surface by making use of the Level Set method. Good agreement is found between these predictions and the laboratory experiments of Balmforth and Kerswell (2005). The collapse of granular material submerged in a water is then investigated using a Mixture Model approach. The experiments of Rondon et al. (2011) revealed drastically different collapse periods depending upon whether the initial pile was in a loose or a dense, compacted state. The simple Mixture Model approach gave reasonably good predictions of the Rondon et al. (2011) experiments for the case of initially loose piles that collapsed in about a second, but it was unsuccessful in simulating the collapse of the initially dense piles that were observed by Rondon et al. (2011) to take around 30–40 s. Some simple empirical modifications to the material constitutive behavior were able to roughly predict such long collapse times, but a more comprehensive and detailed investigation of the phenomenon is warranted.