Gravity sensitivity of continuum numerical solvers for granular flow modeling

One of the major challenges in space exploration robotics is understanding the interactions between robot wheels and planetary terrains consisting of granular regolith in reduced gravity. A key factor, and the focus of this work, is the effect of gravity. Experimental results from the literature for a Taylor Couette cell, with granular material between a rotating inner and a fixed outer concentric cylinder, flown aboard a reduced-g aircraft are taken as a baseline. In this research, granular flow is modeled using continuum methods to capture complexities neglected by terramechanics models without the computational expense of discrete element method (DEM). This research identifies material point method (MPM) as an appropriate continuum solver to model granular flows under the influence of gravity, as it generates both absolute velocity values and trends more consistent with the variable-g experiments than do analytical methods or finite element method (FEM). The modeling of stress-free particles in the shear band by MPM generates a pressure field that leads to the desirable results. This research focuses on quasi-static and intermediate flow regimes, as a survey of the present and past rovers shows these are the most important regimes in planetary applications. Improved flow modeling can contribute to advancing future robot wheel design and mobility control.