Unique behaviors of a backward-facing step flow at microscale

The direct simulation Monte Carlo (DSMC) method is used to simulate micro backward-facing step flows in both slip and transition flow regimes. The effects of rarefaction on flow characteristics are analyzed and discussed. It is found that flow separation, recirculation, and reattachment will disappear as Knudsen number Kn exceeds 0.1. The stability of the vortex behind the step relies on the magnitudes of mean and thermal velocities in the region, which are closely related to the Kn number, local temperature, and driving pressure ratio. A highly intensified pressure and velocity region behind the step is identified in the transition flow regime. The mechanism leading to the significant increase of velocity and pressure is discussed. The effect of rarefaction on the surface shear stress and friction coefficient is also studied.