Investigating the inflow into a granular bed using a locally resolved method

Discrete Element Method – Computational Fluid Dynamics (DEM/CFD) simulations of industrial-scale granular systems employ spatial averaging (porous media approach) for the fluid-particle interaction in the whole domain, which can lead to poor accuracy, for instance at flow inlets , as local particle bulk morphology is not resolved. This paper presents an approach where the interstitial flow in crucial areas with large gradients can be resolved locally in an otherwise unresolved domain, so that a mixed resolved-unresolved method is realized. As a generic example to show the feasibility and performance of the new approach, the inflow of ambient air into a flat-bottom hopper through a narrow orifice is investigated. In an experimental setup, the vertical profile of the pressure decay through the inlet and across the packing is chosen for comparison with respective simulations. Results obtained with the conventional porous media method and the locally resolved approach are compared to these experiments for varying volume flow rates and for two different particle shapes. Spheres of different size as well as dodecahedrons are examined. It is found that although averaging methods already provide good approximations, the locally resolved method can improve the result especially when conventional drag laws are not applicable due to wall effects or if large velocity gradients exist.