Lunar Dust Particles Blown By Lander Engine Exhaust in Rarefied and Compressible Flow

Abstract

Previously we presented a numerical model that predicts trajectories of lunar dust, soil, and gravel blown by the engine exhaust of a lunar lander. The model uses the gas density, velocity vector field, and temperature predicted by computational fluid dynamics (CFD) or Direct Simulation Monte Carlo (DSMC) simulations to compute the forces and accelerations acting on the regolith particles, one particle at a time (ignoring particle collisions until more advanced models are developed). Here we present significant improvements to the model, including the implementation of particle drag and lift formulas to account for the rarefaction and compressibility of the flow. It turns out that the drag force is reduced due to the rarefaction, but the lift is increased due to several effects such as particle rotation. A data matrix of particle sizes, engine thrusts (descent and ascent values for Altair), horizontal and vertical starting distances, and lander height above ground, have been tested using the latest version of the software. These results suggest that the previously reported 3 degree trajectory angle limit can be exceeded in several cases by as much as a factor of five. Particles that originate at a height of 1 cm above the surface from an outer crater rim can be propelled to angles of 5 degrees or more. Particles that start at 10 cm above the surface can be ejected with angles of up to 15 degrees. Mechanisms responsible for placing particles at starting heights above the surface may include the kinetics of horizontal collisions, as suggested by Discrete Element Method (DEM) simulations. We also present results showing the distance particles travel and their impact velocities. We then use the model to evaluate the effectiveness of berms or other methods to block the spray of soil at a lunar landing site.