2 Replies Latest reply on Oct 24, 2013 1:04 PM by Max Wilson

    tank filling problem - transient analysis

    Max Wilson

      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.



        • Re: tank filling problem - transient analysis
          Jared Conway

          have you checked out: S-056539 in the solidworks KB? i think there is a post about this in the forum as well.


          it sounds like you may have set a manual time step that is too large because my experience with these types of problems is that it requires a really small time step and really well defined mesh to make it work properly. if you are really new to flow simulation, I would start by going through the standard tutorials to get a hang for the software and then move onto this problem. i think the setup sounds reasonable but would need to dig into it further to know where things might not be working.

            • Re: tank filling problem - transient analysis
              Max Wilson

              Thanks Jared.  How did you guess...this was the first problem that I've had to work on using flow...at least they gave me an easy one


              You are correct about the time step; I had been using 0.01 s which according to the KB doc needs to be in the range of 1e-7 s.  I think it really comes down to the fact that the characteristic time for wave propagation is so much smaller than that of the tank filling process.


              There's also additional explanation in the flow help index under "tank evacuation" for reference if anyone else is interested.  Thanks again.