Numerical Study On Diffusion Problem And Collision Problem Of Lunar Soil Particles In A Vacuum Environment Under Engine Plume

During the landing of lunar probes on the Moon surface,the gas injected by the engine expands rapidly in the vacuum environment of the lunar surface and forms an engine plume.The loose lunar soil particles interact with the plume,causing them to move in different directions.The raised lunar soil particles diffuse from the high concentration area to the low concentration area on the lunar surface.Meanwhile,the lunar soil particles in the high number density posittion inevitably collide with each other.In other word,the movement of lunar soil particles is not only affected by the engine plume,but also by the diffusion and collision phenomena of lunar soil particles.The floating lunar soil particles can affect the astronauts’ observational visibility,adhere to and contaminate the surface of the lunar probe,jam the sensitive mechanisms of the lander,wear out the lander,cause the failure of thermal-control systems and sealing mechanisms,and even affect the extravehicular activities and the health of the astronauts.Therefore,studying the diffusion and collision of lunar soil particles under the engine plume is an important issue that cannot be ignored.In this paper,a dynamic model of lunar soil particles diffusion under the engine plumes is established,and the discrete element method based on mass-and energy-conservation equations and the finite difference method based on convection-diffusion equation are proposed.The influencing factors and mechanism of lunar soil particles diffusion under the engine plume are analyzed by considering the injection angle,the percentage of space occupied by particles,the speed of particles,the trajectoty of partilces and the motion stabilization time.First,the discrete element method of conservation equations is proposed to study the influencing factors and mechanism of lunar soil particles diffusion under the plume(including drag and lift),temperature and lunsr surface gravity.Then,the finite difference method of convection-diffusion equation is proposed to analyze the influencing factors and mechanism of lunar soil particles diffusion under the engine plume.The numerical results show that lunar soil particles with small diameters have obvious diffusion phenomenon.After considering diffusion problem,the diffusion speeds of the 10-and 40-micron lunar soil particles are 3.65–4.45 times of those without the diffusion problem;the spatial distribution range is increased by 4.46 times.The maximum lifting angle is increased by 1.0°–1.6°.However,lunar soil particles with diameters of 70-and 100-microns are not considerably affected by diffusion.The calculation results also show that energy loss has a significant impact on the diffusion phenomenon of small-diameter lunar soil particles,and the more energy loss,the more obvious the weakening trend of diffusion phenomenon.When the energy recovery coefficient is 0.1,the diffusion speeds of the 10-and 40-micron lunar soil particles are 2.23–3.23 times of those without the diffusion problem;the spatial distribution range is increased by 3.45 times.The maximum lifting angle is increased by 0.81°–1.26°.Meanwhile,a mechanical model of the lunar soil particle collision problem is established,and a hard-ball model based on the conservation of momentum and energy is given.The mechanism of impact of particle collision on the spatial distribution of lunar soil particles is analyzed,especially the collision between particles of different diameters.First,the influencing factors and mechanisms of lunar soil particles collision under the engine plume are analyzed by considering the number of collision times,the injection angle,the percentage of space occupied by particles,the speed of particles,the trajectoty of partilces and the motion stabilization time.Furthermore,the influence of collision recovery coefficient on lunar soil particle collision is analyzed.Assumed no collision energy loss(collision restitution cofficient is 1.0),the lunar-soil particles with different diameters collide 36.8–153.2times on an average,and the average speed is 50.4–426.7 m/s.When the restitution coefficient is reduced from 1.0 to 0.1,the average number of collisions increases by16.3%–65.7%,and the average speed decreases by 24.2%–49.8%.With the decrease of collision restitution cofficient,the number of particle-occupied space grids increases for small-diameter lunar-soil particles,but decreases for large-diameter particles.Finally,the numerical results of this paper are compared with the landing data of Apollo’s missions.The results show that the numerical results considering the diffusion and collision of lunar soil particles are in good agreement with the lalnding data,which not only verifies that the research methods in this paper are reliable,but also confirms that diffusion and collision problems are the important factors that cannot be ignored in the study of the interaction between lunar soil particles and engine plume.