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On the Outflow Boundary Conditions for Subsonic Rarefied External Flows

The effects of outflow boundary conditions are investigated on subsonic external flows in the transitional regime. Calculations have been performed for two-dimensional flows over a finite flat plate using the direct simulation Monte Carlo method, concentrating on Knudsen numbers of 0.02 and higher. At the outflow boundaries, the incoming molecules are determined by using a model which adjusts the outflow boundary conditions according to the local rarefaction assuming equilibrium. The results show sensitivity to the model at low Mach numbers but as the speed increases the sensitivity decreases. The effects of domain size, sample size and doing the extrapolations from the adjacent boundary cells rather than following the streamline trajectories are shown.