A rarefied gas flow into vacuum through a tube of finite length is investigated over the whole range of gas rarefaction by the direct simulation Monte Carlo method. The nonequilibrium effects at the inlet and outlet of the tube have been considered by including in the computational domain large volumes of the upstream and downstream reservoirs. Results for the dimensionless flow rate and for the flow field are presented for a wide range of the gas rarefaction and for various values of the length to radius ratio in the range from 0 to 10. The influence of the gas-surface interaction model, as well as the effect of the intermolecular potential model on the gas flow, is examined. A good agreement has been obtained between the present numerical results and the corresponding experimental ones available in the literature.

Gas flow through a two-dimensional slit into a vacuum is investigated by a direct simulation Monte Carlo method. Results for the mass flow rate are obtained as a function of the rarefaction parameter, which is inversely proportional to the Knudsen number. The distributions of density, temperature and mass velocity, and streamlines are presented. In the free molecular flow regime and in the hydrodynamic limit, our results agree with theoretical asymptotes, and in the transition regime, they compare well with numerical simulations by other authors.