# Search idea for solving a transient problem

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 May 13, 2011, 04:36 Search idea for solving a transient problem #1 Senior Member   Join Date: Mar 2011 Posts: 155 Rep Power: 4 Hi there, I need a good idea for solving a case transient. The problem is that the case consists of about nearly 2 mio elements. And I need to solve 285 seconds transient. The courant criteria calculated time steps from ~10^(-5) seconds. Now I need a good idea how to solve the case transient. What I still have tried: - I still have reduced the element number from 9mio to 2mio elements. - I have tried to run the case with a fixed time step 0.0001s, without success. -> I have a question to that. If I choose to let the solver use the courant criteria for calculating the time steps, does the solver use constant time steps or can it be that sometimes he calculated a time step of deltat=10^(-5)s and sometimes deltat=10^(-3) seconds? I want to know this because if the solver calculates the courant criteria only at time t=0 and uses than constant time steps, I think I can halve the time calculated by courant criteria. Because I think that the implementation of courant criteria in the solvers is configured conservative. I think that on every time step the courant criteria is used to calculate the next time step, but I'm nut quite sure. Maybe you need the information that I use chtMultiRegionFoam. Best Regards, tH3f0rC3

 May 13, 2011, 06:45 #2 Senior Member   Bernhard Join Date: Sep 2009 Location: Delft Posts: 696 Rep Power: 11 You can easily see from the output of the solver, whether your timestep changes or not. You can use adjustTimeStep in controlDict to recalculate the timestep based on the Courant number limit.

May 13, 2011, 11:22
#3
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Join Date: Mar 2011
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Quote:
 Originally Posted by Bernhard You can easily see from the output of the solver, whether your timestep changes or not. You can use adjustTimeStep in controlDict to recalculate the timestep based on the Courant number limit.
so adjustTimeStep is for the recalculation after every time step?
So if I switch the setting to "no;" the courant number is only calculated at time t=0 and this time step is than used for every following time step?

Best Regards,
tH3f0rC3

 May 13, 2011, 16:01 #4 Senior Member   Bernhard Join Date: Sep 2009 Location: Delft Posts: 696 Rep Power: 11 If you set it to no, then the deltaT that you set in controlDict is used. The Courant number is only calculated, but not used later on.

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