# help with understanding transient iterations and time steps

Question asked by Dan Hofstetter on Oct 8, 2012
Latest reply on Oct 8, 2012 by Bill McEachern

Hi all,

I'm hoping someone here can help me to better understand how Flow Simulation treats iterations and time steps in transient analyses.  I thought I was doing OK until I recently ran Phoenics for the same problem and noticed that it was performing many more iterations to reach the solution than Flow Simulation (as set up by me).  I read in the help file that for transient simulations, the number of iterations is equal to the number of time steps.  However, when looking at other CFD solvers, I notice that many of them may use hundreds of iterations per time step in order to reach convergence, and at the end of each time step the calculation continues but with variables that must converge again for that step.

I am working on a simulation that requires time accuracy for gas concentration decay, and want to make sure I am doing everything correctly.  When I specify a 10s time step, which was used in Phoenics and resulted in a close match to measured data, Flow Simulation solves the simulation very quickly but with only one iteration per 10 seconds.  I have obtained some results that match the measured data pretty closely, but I had to try many different combinations of fan inlet concentration values, and without convergence I suspect the solutions may not be valid.  I am only trying to mimic Phoenics to prove that Flow Simulation can produce acceptable results when running the same model - after I achieve this I will be running a model with an internal fan that takes care of recirculation for me, and will probably use automatic settings with flow freezing to save computing time.

To try to mimic the way Phoenics solves the problem, here is what I have tried:

1.  Set a manual time step of 0.018 seconds.  This results in ~556 iterations per 10 seconds of simulation time.

2.  Set the maximum simulation time to 10 seconds.

3.  Run the solver.  The variables for Pressure, Vx, Vy, Vz, KE, and EP are used for convergence with the "if all are true" calculation control option.  These values typically converge by iteration 217.  The remainder of iterations are used for gas concentration decay.

4.  When the solver finishes, I change the fan inlet concentration based on the outlet concentration.

5.  I increase the maximum simulation time by 10 seconds (to 20 seconds in this case) and run the solver again, continuing the calculation.  The variables have to converge again, and when the solver is finished I repeat steps 4 and 5 until I reach 90 seconds.

From what I know about other finite numerical methods (FEM, FDM, BEM), the time step is a carefully chosen period used to march the simulation ahead in time, and a certain number of iterations are required for convergence during each time step.  It seems like Flow Simulation doesn't treat time step and iterations in this way.  Rather, it seems that the maximum simulation time really acts like the time step, and the manual time step controls how many iterations the solver will perform during that time.  Am I wrong?  Is there a better way to do what I am attempting?

Thanks!

- Dan