Simulation Study On Non-equilibrium Flow Field And Wall Catalytic Effect For Reentry Vehicle

When the near-space vchicle re-enters the Earth’s aimosphere,the inflow of air is rapidly compressed owing to the rapid reentry speed,causing intense friction with the vehicle wall.The kinetic energy of the vehicle is converted irreversibly into the internal energy of the gas in theflow field,resulting in a rapid increase in temperature near the vehicle wall and the formation of an intense shock layer at the front of the vehicle.Within the shock,the inflow air undergces processes such as high-temperature ionization,catalytic recombination,and internal energyexchange,accompanied by obvious thermochemical non-equilibrium effects.Eventually,a hightemperature gas sheath with large amounts of free elections and cations forms around the vehicle’s surface,i.c.,a plasma sheath.This sheath has the effect of reflecting,absorbing,and scattering electromagnetic waves and is the root cause of communication interruption between vehicle and ground.The communication interruption means that ground stations are unable to receive flight data in a timely manner.making it difficult to track the position of the vehicle and inereasing the uncertainty of the landing siteprediction and recovery cost,which seriously threatens the safety of astronauts and the success of space exploration missions.Therefore,this paper takes an atmospheric reentry demonstrator the research object and uses a multi-physics numerical model covering flow field,catalytic chemical field,and thermal field to study the thermochemical non-equilibrium flow field and wall catalytic characteristics of the plasma sheathIn this study,a multi-physics coupled model is established to simulate the thermal-chemical non-equilibrium flow field of the plasma sheath of a reentry vehicle,which considers NavierStokes equations.54 chemical reactions of air,a thermodynamic non-equilibrium fourtemperature model,and an internal energy exchange model.The flow field characteristics of the plasma sheath under different flight conditions were studied using the above numerical model.It was found that there is a strong non-equilibrium phenomenon in the shock layer flow field at 80 km.Conversely,when the vehicle is at 65 km,the shock layer flow field tends to be locally thermally equilibrated.When the air does not pass through the shock layer,O2 and N2 are the main species of the inflow.The temperature of the flow field is low,no chemical reaction takes place,and the molar fraction of the two species changes slowly.When the inflow passes through the shock layer,the mole fractions of O2 and N2 molecules decrease,and the molar fraction of O2 decreases more rapidly.The dominant chemicals are N2,N,and O in the shock layer.The flow field temperature drops sharply,and the mole fractions of N2 and O2 increase slightly near the vehicle wall.As the free stream passes through the shock layer,the gas pressure increases rapidly and eventually reaches a maximum near the vehicle wall.The high-pressure region of the wall is concentrated near the vehicle’s head.The wall pressure decreases rapidly in the rear cone and back cover of the vehicle.The distance from the shock layer to the stagnation point decreases with a decrease in the reentry altitude from 90 to 65 km but increases with a decrease from 65 to 40 km.The electron number density in the shock layer is the highest and generally exceeds the critical value.The electron number density in the wake region will vary significantly due to different flight altitudes.Based on the aforementioned model,a catalytic recombination reaction model is introduced to study the catalytic characteristics of the vehicle’s wall.The mechanism of the surface catalytic effect on the plasma sheath is analyzed and revealed.The study found that wall catalytic effects have a significant effect on the flow field temperature and chemical species near the wall of the vehicle.The higher the catalytic recombination coefficient,the higher the flow field temperature near the wall of the vehicle,the lower the mole fraction of N,O,N+,and O+,and the higher the mole fraction of N2,O2,and NO molecules.In addition,although the wall catalytic effect has little effect on the electron number density in the shock layer,it can effectively reduce the electron number density in the wake region of the vehicle,achieving the goal of mitigating communication blackout.Furthermore,the numerical model established in this study is used to simulate the RAM-C-Ⅱ and OREX vehicles.By comparing it with publicly available data,the accuracy of the model presented in this article is verified.