AbstractA numerical study is conducted to investigate the effects of the inflow Knudsen number (Kn) on hypersonic chemical nonequilibrium rarefied flows using the unstructured direct simulation Monte-Carlo (DSMC) method, and an empirical formula for fast prediction of flow-field structure for different values of Kn is proposed and verified. First, the flow around a vertical plate is analyzed at a free-stream velocity of 7,500 m/s using a perfect-gas model with inflow Kn values ranging from 0.035 to 13.36. Through analysis of the flow-field characteristics with varying Kn values, a linear expansion law of flow-field versus the square root of Kn is discovered and an empirical formula based on least-squares fitting is developed for fast prediction of the flow structure at different values of Kn. Further, the effects of chemical nonequilibrium on hypersonic rarefied flow-field structure are investigated to verify the applicability of the empirical formula. Finally, cases of the cylinder flow are simulated in...