Fully resolved numerical simulation of particle laden flows

The collapse of granular columns in a viscous fluid is a common model case for submarine geophysical flows. In immersed granular collapses, dense packings result in slow dynamics and short runout distances, while loose packings are associated with fast dynamics and long runout distances. However, the underlying mechanisms of the collapse initiation and runout, particularly regarding the complex fluid-particle interactions at the pore scale, are yet to be fully understood. In this study, a three-dimensional approach coupling the lattice Boltzmann method and the discrete element method is adopted to investigate the influence of packing density on the collapsing dynamics. As a supplement to previous experimental measurements, the direct numerical simulation of fluid-particle interactions explicitly provides micromechanical evidence of the pore pressure feedback mechanism. In dense cases, a strong arborescent contact force network can form to prevent particles from sliding, resulting in a creeping failure behavior. In contrast, the granular phase is liquefied substantially in loose cases, leading to a rapid and catastrophic failure. This opposing dilative/contractive behavior linked to the initial packing is robust and does not depend on the column size. Furthermore, hydroplaning can take place in large enough loose cases due to the fast-moving surge front, which reduces the frictional resistance dramatically and thereby results in a long runout distance. More quantitatively, we are able to linearly correlate the normalized runout distance and the densimetric Froude number across a wide range of length scales, including small-scale numerical/experimental data and large-scale field data.