The Navier-Stokes equations, which are the basis for many commercial CFD programs, including SW Flow Simulation (aka Cosmos FloWorks), requires that the Knudsen number be below 0.01 (and better if below 0.003) because it assumes a continuum in the fluid.
You might want to check out codes that support the Lattice Bolzmann Method, which "demonstrates promising results in the area of high Knudsen number flows." You can find a list of some software codes here including open source ones.
Thank you for your reply!
I think the best solver in order to slove reliably and correctly transition gas flow is the Direct Simulation Monte Carlo.
I 'll check out the Lattice Bolzmann Method!