FLOW CHARACTERISTICS OF THREE-DIMENSIONAL MICROSCALE BACKWARD-FACING STEP FLOWS

摘要

The flow in three-dimensional ( 3-D) microscale backward facing step is investigated using the direct simulation Monte Carlo method. To ensure the rationality of two-dimensional (2-D) simplification for a 3-D structure, the flow characteristics of the 3-D microscale backward-facing step flows with various cross-section aspect ratios are compared with those of 2-D simplification. When the cross-section aspect ratio of 3-D flow cases is greater than 5, the approaching level of 3-D flows to 2-D simplification is over 98% for the nitrogen flow with inlet Knudsen number ( Kn) of 0.04. Moreover, it is found that the approaching level decreases as Kn increases. The dependence between the vortex stability and the cross-section aspect ratio is studied. It is found that the flow separation, recirculation, and reattachment will disappear as the cross-section aspect ratio is less than 1 for flows with inlet Kn of 0.04.