We provide the first direct calculation of the Knudsen layer thickness in rarefied gas flows based on the ballistic molecular motions in the direct simulation Monte Carlo (DSMC) simulations. Calculations reproduce a linear relation between the Knudsen layer (KL) thickness and the mean free path (MFP) of the gas, which agrees with the classical gas kinetic theory statement. The ratio of the KL thickness and MFP is only a function of gas molecular model at slight flow gradients. However, high flow gradient and high surface curvature effects destroy the linear relation between the KL thickness and MFP.

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