The Effect of a Binary Size Distribution On Lunar Regolith Ejection

Upon landing of a spacecraft on the Moon (or other extraterrestrial body), the rocket exhaust plume causes regolith (soil-like material) to be blown in all directions. The lack of atmosphere and corresponding drag on the regolith motion causes the ejection to be extremely hazardous to objects both near and far. Work was previously performed on this system to determine the role of collisions in the erosion and ejection of the regolith by investigating a monodisperse system using the discrete element method (DEM), which incorporates collisions explicitly. Following up on this study, we seek to perform a similar investigation using a binary-sized system. In order to do this, we utilize two version of DEM. Namely, one which resolves collisions using a soft-sphere model and the other which ignores collisions and allows for the (unrealistic) crossing of particle trajectories. The system simulated is located 6m from the impingement point of the rocket and lift and drag correlations are used to simulate the forces on the particles. A one-way particle-gas coupling is used such that the particles are affected by the gas but the particles do not influence the gas. The binary mixtures are chosen such that we vary the size ratio and composition. The results show that the initial erosion rate tends to drop significantly during the onset of collisions above the surface when smaller sizes are present, but the steady-state erosion rate appears to differ only slightly with composition and size ratio for the systems investigated. In addition, the character of the collisions (small particle-small particle, small-large, large-large) as they change with size ratio and composition is investigated.