I'm trying to setup and run a flow simulation that is basically a 'nozzle' discharging into a vacuum chamber. We maintain a vacuum chamber pressure of 100 mtorr and are trying to evaluate the pressure in the tube upstream from the nozzle, which in this case is an orifice, 1-2mm in diameter when we introduce an inlet mass flow of argon at 100 SCCM. Basic choked flow calculations are not matching the flow simulation results and I am struggling to understand why. Any guidance would be welcome. Thanks in advance.