X. Li’s scientific contributions

The concept of "low Knudsen number characteristic region", which is [Kn1, Kn2], in the process of reentry, has been presented for the first time, and the fluid in the region has been studied using direct simulation Monte Carlo (DSMC) and Navier-Stokes model respectively. It has been shown by the numerical computation that both of the DSMC code and weak conservative Navier-Stokes code with multi-species, thermodynamic non-equilibrium and chemical non-equilibrium compiled by Professor Wang Baoguo and his group can complete the computation in this region. The distribution of flowfield shows agreement with each other, and values of corresponding parameter only have very limited difference. As we can see in the computation: when DSMC is being used, the computation time increases with the decrease of Knudsen number. Value of Kn1 can be got from this code, which is near 0.0019. When Navier-Stokes is being used, the computation is more difficult to get convergence with a higher Knudsen number. Value of Kn2 can be got from this code, which is near 0.0125. It has also been found that during the computation, for Knudsen number closed to Kn1 and Kn2 respectively, the use of DSMC and Navier-Stokes code will lead to a decrease of the convergence rate. There are three measures to accelerate the convergence which will be introduced in this paper. Because of the low Knudsen number characteristic region, all the flight cases of the reentry process could be completed by appropriate selection between DSMC and Navier-Stokes code. Obviously, this conclusion is very important for the numerical computation in reentry flight process.

Analytical study of thermodynamics and heat transfer of high-temperature and high-velocity flow field behind shock waves is an important problem involving the design and heat transfer analysis of vehicle thermal protection system (TPS).The flow fields of the Apollo AS-202 return capsule aviating in the earth atmosphere of six cases (the Mach number varying from 15.52 to 22.63) and the Huygens vehicle aviating in the Titan atmosphere of six cases (the Mach number varying from 17.29 to 24.47) were simulated by using conservation integral form of Navier-Stokes equations with multi-species, nonequilibrium molecular thermal excitation and chemical reactions, based on the high-resolution total variation diminishing (TVD) scheme. Thermodynamics and heat transfer of high-temperature and high-velocity flow field behind bow shock were studied, and the wall heat flux density, wall temperature and Stanton number were in good agreement with the flight data. These results are useful for design of correlative vehicles'TPS.