Gravity-dependent rate sensitivity in granular intrusion: microgravity experiments and simulations.

Understanding the shear and intrusion rheological behavior of granular materials under reduced gravitational conditions is crucial for applications in planetary exploration and submarine earthquake engineering. To this end, it is important to understand whether gravity affects the drag forces on objects intruding granular media and, if so, quantify these effects. We have studied this issue experimentally in 1 g and 0 g conditions, the latter using the Beijing Drop Tower. Measuring the resistive forces on a cylinder moving at a constant speed through a granular bed, we find that gravity plays a significant role - the resistive forces increase significantly with cylinder speed in 0 g, while increasing much more slowly in 1 g. We use Coupled Eulerian-Lagrangian (CEL) simulations that support our results. We attribute this behavior to the increasingly dominant effect of pressure-sensitive frictional forces in microgravity with increasing fluidity in these conditions. Other than the significant implications for extraterrestrial exploration, our findings suggest that constitutive modeling of flow in microgravity should differ significantly from that in 1 g.