In this paper

numerical simulations are presented about the effects of gas rarefaction on hypersonic flow field. Due to the extremely difficult experiment

limited wind-tunnel conditions and high costs

most problems in rarefied flow regime are investigated through numerical methods

in which the Direct Simulation Monte Carlo method (DSMC) is widely used. And the unstructured DSMC method is employed here. Flows around a 2D vertical plate at a given velocity 7500m/s are simulated. For gas rarefaction is judged by the free-stream Knudsen number Kn

two vital factors are considered: molecular number density and the plate’s length. Cases in which Kn varies from 0.035 to 13.36 are simulated. Flow characters in the whole rarefied regime are described

and flow field structure affected by Kn is anaylyzed. Then

the dimensionless position D* of a certain velocity in the stagnation line is chosen as the marker of flow field to measure its variation. Through flow field tracing and least-square numerical method analyzing

it is proved that hypersonic rarefied flow field expands outward linearly with the increase of the square of Kn. At last

an empirical method is established

which can be used for the prediction of the hypersonic flow field structure at a given inflow velocity

especially the shock wave position.