  We discuss the use of the Discrete Element Method (DEM) to simulate the
dynamics of granular systems made up of elements with nontrivial geometries.
The DEM simulator is GPU accelerated and can handle elements whose shape is
defined as the union with overlap of diverse sets of spheres with
user-specified radii. The simulator can also handle complex materials since
each sphere in an element can have its own Young's modulus $E$, Poisson ratio
$\nu$, friction coefficient $\mu$, and coefficient of restitution CoR. To
demonstrate the simulator, we produce a "digital simulant" (DS), a replica of
the GRC-1 lunar simulant. The DS follows an element size distribution similar
but not identical to that of GRC-1. We validate the predictive attributes of
the simulator via several numerical experiments: repose angle, cone
penetration, drawbar pull, and rover incline-climbing tests. Subsequently, we
carry out a sensitivity analysis to gauge how the slope vs. slip curves change
when the element shape, element size, and friction coefficient change. The
paper concludes with a VIPER rover simulation that confirms a recently proposed
granular scaling law. The simulation involves more than 11 million elements
composed of more than 34 million spheres of different radii. The simulator
works in the Chrono framework and utilizes two GPUs concurrently. The GPU code
for the simulator and all numerical experiments discussed are open-source and
available on GitHub for reproducibility studies and unfettered use and
distribution.
