The concept of “low Knudsen number characteristic region”

which is

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 0.001 9.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 0.012 5.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.