We investigate the gas flows near to solid surfaces in terms of the local spatial variation in the molecular mean free path (MFP). Molecular dynamics (MD) is the appropriate scientific tool for obtaining molecularly-accurate dynamic information in micro and nano-scale gas flows, and has been used to evaluate the molecular mean free path of gases. In the calibration procedure, the viscosity of a gas in the homogeneous case can be recovered in our MD simulations and reach good agreement with the theoretical prediction and data from NIST. In surface-bounded gas flows, if the collisions between gas molecules and walls are counted, a spatially-varying mean free path is presented, and for the first time we have observed that the distribution of the free paths deviates from the exponential one and spikes appear in their distributions at larger Kn, i.e. in the transition flow regime. Based on elementary kinetic theory, the effective viscosity of the gas derived from the mean free path has been incorporated into the framework of the continuum-fluid dynamics equations, and micro-Couette flows are performed to demonstrate this potential application.

中文翻译：

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微/纳通道气流的平均自由程方法

我们根据分子平均自由程（MFP）的局部空间变化研究了靠近固体表面的气流。分子动力学（MD）是用于获得微米级和纳米级气流中分子精确的动态信息的适当科学工具，并已用于评估气体的分子平均自由程。在校准过程中，均质情况下的气体粘度可以在我们的MD模拟中恢复，并且与NIST的理论预测和数据非常吻合。在以表面为界的气流中，如果计算出气体分子与壁之间的碰撞，就会显示出空间变化的平均自由程，并且我们首次观察到自由程的分布偏离指数分布和尖峰。在它们的分布中以较大的Kn出现，即 在过渡流态。基于基本动力学理论，从平均自由程获得的气体有效粘度已被纳入连续流体动力学方程式的框架中，并进行了微库特流分析以证明这一潜在应用。