  This article presents the development and validation of a hybrid multi-sphere
discrete element framework - Rigid3D, for the simulation of granular systems
with arbitrarily shaped particles in 3D space. In this DEM framework, a
non-spherical particle is approximated by three different geometric models: (1)
multi-sphere model with overlapping spheres (MS model), (2) particle surface
with triangle mesh (surface model), and (3) discretized particle body with
polyhedral cells (cell model). The multi-sphere approach will be the "engine"
for efficient DEM simulations, while the particle's mesh and cell models will
be updated simultaneously according to the position and orientation of their
associated MS model, for use in particle-related inter-phase couplings in a
multi-phase flow. In this sense, Rigid3D tries to combine the best of both
worlds in multi-sphere and polyhedral DEMs: multi-sphere method for the
efficiency and acceptable accuracy in the DEM simulation of granular flows,
while the surface and cell models for the couplings between particles and other
phases (continuous or dispersed phases) without affecting the performance of
DEM simulations.
