An implementation of the soft-sphere discrete element method in a high-performance parallel gravity tree-code

We present our implementation of the soft-sphere discrete element method (SSDEM) in the parallel gravitational -body code , a well-tested simulation package that has been used to provide many successful results in the field of planetary science. The implementation of SSDEM allows for the modeling of the different contact forces between particles in granular material, such as various kinds of friction, including rolling and twisting friction, and the normal and tangential deformation of colliding particles. Such modeling is particularly important in regimes for which collisions cannot be treated as instantaneous or as occurring at a single point of contact on the particles' surfaces, as is done in the hard-sphere discrete element method already implemented in the code. We check the validity of our soft-sphere model by reproducing successfully the dynamics of flows in a cylindrical hopper. Other tests will be performed in the future for different dynamical contexts, including the presence of external and self-gravity, as our code also includes interparticle gravitational force computations. This will then allow us to apply our tool with confidence to planetary science studies, such as those aimed at understanding the dynamics of regolith on solid celestial body surfaces, or at designing efficient sampling tools for sample-return space missions.