Granular dynamics vs. fluid dynamics: similarities and differences

In understanding the dynamics of granular systems, a discrete modeling approach tracks the motion of all particles. Such an approach is computationally demanding especially when the number of particles is large, e.g., when going beyond tens of millions of particles. In these cases, one can contemplate switching to continuum models, which are computationally less expensive. In order to assess when such a discrete to continuum switch is justified, we report herein results for a comparison between the dynamics of granular and fluid flows obtained with an open source code that scales to handle granular systems with more than 1 billion degrees of freedom (DOFs); i.e., two orders of magnitude higher than the state of the art. On the one hand, we solve the Newton-Euler equations of motion, which govern the time evolution of the granular system. On the other hand, we solve the Navier-Stokes equations that describe the time evolution of the non-Newtonian fluid model. Both the multibody and fluid dynamics solvers leverage parallel computing on the Graphics Processing Unit (GPU). We report similarities and differences between the dynamics of the discrete, fully resolved system and the continuum granular material model via a set of numerical experiments that include both static and highly transient scenarios. The simulation platform that anchors this contribution is publicly available on GitHub and is part of an open source code called Chrono.