Abstract: A modified treatment for pressure boundary conditions in DSMC is shown, The microscale backward-facing gas flow is computed by a partitioned domain DSMC method and a general flow construction is obtained. It is shown that the recirculating flow region attached to the backward-facing step exists in the microscale flow with proper conditions while the region of separation on upper side does not. A discussion about the effect of flow parameters is given and a relationship between reattachment length and Reynolds number is obtained.

Key words: DSMC, pressure boundary condition, microscale backward-facing, reattachment length