Investigating collision effects on lunar soil particles ejected under rocket plumes

• Published in: Acta astronautica 2024-05, Vol.218, p.114-125
• Main Authors: Sarwar, Shah Akib, Hasnain, Zohaib
• Format: Article
• Language: English
• Subjects: DSMC; Lagrangian particle tracking; Particle–particle collision; Plume-surface interaction; Soft sphere method
• ISSN: 0094-5765; 1879-2030
• DOI: 10.1016/j.actaastro.2024.02.014

As a lunar lander attempts to make a soft landing on the moon, the supersonic exhaust plumes from the descent engine can liberate a large quantity of regolith grains off the ground surface. In this work, we investigate the particle–particle collision phenomenon inside this regolith spray with the soft sphere method — an alternate to the generally used hard sphere approach. Individual collisions are resolved by generating spring and damping forces at the contact point of particles that are already moving under fluid drag and buoyancy. To reduce the computational burden, a 1:100 scale model of the real flow domain is used. We insert 60,000 numerically representative regolith particles through random locations on the ground to yield an initial solid grain density of ∼108 particles/m3. Three different particle diameters, ranging from small to moderately large, are considered to account for the wide size variation of lunar regolith, and observe the relationship between inter-particle collisions and particle size. The coefficient of restitution is varied to model completely inelastic collisions to highly elastic collisions. Results indicate that collisions between larger regolith grains with high elasticity cause a significant portion of particles to reach high elevations inside the domain and leave at large angles. However, the majority of the ejecta spray is bound within a shallow region (< 3°) above ground where they move slowly due to weak plume-generated forces. On the other hand, inelastic collisions tend to significantly decrease the maximum particle height within the domain, but the main particle stream is more uniformly distributed in the flow domain and average particle speed increases. Maximum particle speed shows an inverse relation with energy depletion in collisions, but this relation becomes less remarkable for larger (100 and 150μm) grains. Overall, particle trajectory results in this work align with the 1–3° angles measured for the visible regolith sheet in Apollo landing videos. However, the possibility of many grains ejecting at much higher angles due to collisions is also revealed. These high-angle particles can be problematic for relying on berms on the lunar surface to block the regolith spray. •Collisions inside plume-driven lunar regolith are studied with the soft sphere method.•Highly elastic collisions between large grains decrease their average speed.•The same collisions increase maximum particle height and speed in the flow domain.•Tr