On the Role of Collisions in the Ejection of Lunar Regolith during Spacecraft Landing

Gas-solid flows are ubiquitous in nature and industry and, despite being widely studied, are still not well understood. One example of such a flow is the spraying of lunar regolith (soil) from a rocket landing on the Moon. Such spray poses a danger to both equipment and personnel for future missions. Previous researchers, using models such as single particle trajectory models and direct simulation Monte Carlo (DSMC), have not directly studied the erosion from the surface or modeled the collisions directly. The goal of this work is to improve upon prior models by developing and validating a new model that can be used to design mitigation systems such that future missions will not be endangered. For this purpose, the discrete element method (DEM) is first used to examine the erosion from the surface and to probe the relevant erosion mechanisms in order to better understand the erosion process. This work is performed using a variety of particle size distributions (PSDs), including monodisperse, binary, and lognormal. The results show that collisions are crucial in correctly modeling both the near-field (surface erosion) and far-field (downstream) effects. However, the DEM model is too computationally expensive to be used for the entire lunar system. Thus, the erosion results from the DEM model are used in a kinetic-theory-based continuum model, similar to the Navier-Stokes model for traditional fluids, using a discretized PSD. Validation of this model is performed against Apollo data and shows discrepancies between the observed and predicted particle velocities. Further work is required to resolve this discrepancy, along with additional validation due to difficulty of obtaining experimental/field data for lunar conditions. In addition, the validity of using a single-particle drag law in rarefied conditions is evaluated using the lattice-Boltzmann method (LBM) to simulate periodic arrays of spheres. The results suggest that such an assumption may be valid if the Knudsen number (ratio of mean free path of the gas to particle diameter; used to measure rarefication) is sufficiently large, as it may be for the lunar case. However, additional work is needed to fully understand the implications of such an assumption and develop a multi-particle drag law that considers rarefication