I'm new to flow simulation and think I have the problem set up properly (have tried several geometries, meshing strategies, boundary conditions, etc.).  I have a feeling and have been told that flow simulation may not be able to do this type of problem but didn't really get an explanation as to why.

The problems seems simple enough:

A cylinder filed with gas at an initial temperature and pressure being fed by a constant pressure source flowing though a converging nozzle. 

I would like to be able to plot the mass/volumetric flow and tank pressure/temp as a function of time.  With the pressure differential I'm using (450psia inlet pressure, tank at 100psia initial pressure)  I would expect the mass flow to be choked initially and then taper off after the critical pressure ratio is reached.

The simulation shows that the tank approaches inlet pressure in a fraction of a second (around 0.1s) which I know is incorrect as I'm feeding a 450 ft^3 cylinder through a 0.25 in nozzle.

I have tried the problem with one inlet, and also with one inlet and one outlet with a very small mass flow rate on the outlet.  Have also tried ramping in the inlet pressure starting at tank pressure and ramping up to the final inlet pressure at different rates as well as a static pressure boundary condition, etc.  I have also tried changing the value for periodic saving which does seem to have an effect on the results (down to 0.001 s) but for a process that should take several seconds or even minutes I would think this overly sufficient.

I have been told that i shouldn't set up the problem with only an inlet lid. 

Looking for any insight as I haven't found any helpful sw tutorials for transient flow analysis.

Thanks in advance.

Max