bubbleFoam  behavior on finer mesh
5 Attachment(s)
Hello foamers,
I am investigating a case of completely closed domain where a large bubble is rising and spreading across the ceiling. The domain is a cylinder "on side" and the calculation works quite correctly for cruder hexahedral mesh. Attachment 10058 Attachment 10059 But the target is to use a polyhedral mesh, but the calculation doesn't converge on it... so I tested a finer hexahedral mesh and finer polyhedral mesh and I have got those strange results... Crude Hexahedral mesh Attachment 10060 Fine Hexahedral mesh Attachment 10061 Fine Polyhedral mesh Attachment 10062 I am quite new to the CFD solving... and I understand that this is not correct. But why? Thanks in advance for all hints you can offer Martin 
Convergence
3 Attachment(s)
So, in the first step I examined the convergence and it is as those attached graphs shows.
The most correct calculation, with the crude hexahedral mesh looks like this: (convergence of p in individual time steps) Attachment 10068 The finer hexahedral mesh then looks like this Attachment 10069 And the finer polyhedral mesh like this Attachment 10070 So the convergence is poor in the beginning in all the calculations... And it is similar in Hex Crude and Hex Finer mesh... I assume that the bad convergence in the polyhedral model is in the moment the model touches the wall... 
fvSolution
Hello all,
In the next step, I am going to try to change the fvScheme file... but I have no clue which is good to modify first... I do know what the lines in fvScheme stand's for and I do know how each scheme looks, but I am not sure how to guess the influence on the result. Is there any hint? What to start with, which literature read, some similar cases etc? Thanks again for any help or hint... Martin. 
First of all, did you reduce the maximum time step after refining the mesh?

I think a Courant number of 1 is already pretty large for multiphase flows; 2 is just asking for trouble. Definitely check if your results change with smaller time steps.
MaxDeltaT is not always necessary, but more strictly enforced than maxCo, so if you notice the solver jumping over the maxCo number it helps to limit maxDeltaT as well. 
I agree with Anton. An Euler/Euler multiphase flow calculation needs a small courant number. In my different tests, I used to bound the Courant number (based on Ur) by 0,4.

Courant could be set upto 0.5 maximum in EulerEuler multiphase flows, beyond this value is not recommended

Final Results
4 Attachment(s)
Hello All,
Thanks again for the recommendations. You were right... the Time step has quite influence on the convergence / divergence of the whole model. I tried the "adjustTimeStep" switch (added to bubbleFoam) to correct this problem, but the calculation turned out to be still dependent on the time step. To save the calculation time, I examined following rhetorical question: Why do we have the calculated Courant Number for each time step when we need to limit the deltaT anyway? If I force the calculation to push the Courant number (with maxCo constant) down do I still need the maxDeltaT? What if the problems with divergence are only in the beginning of the calculation cycle before the timeStep is pushed down by the MaxCo criteria? In this case, it would be enough to set maxCo (0.4) and Calculate sufficient deltaT for T=0... and let the maxDeltaT to be higher... because the limitation by maxCo should be enough. For now I have some results on finer mesh (T = 0.5 s ) (120 000 polyhedral cells)... fixed dT = 0.1 ms, maxCo = 0.4 Attachment 10170 Attachment 10172 variable dT = 0.5 ms,dT(T=0) = 0.1 ms, maxCo = 0.4 Attachment 10171 Attachment 10173 As it looks, the results are similar, so the convergence problem was in the first part of the calculation... BUT the results does differ quite a bit, and the residua (thick green results in graph) are quite worse for the variable dT. The scary thing is the yellow value of the Courant number... even the parameter maxCo is 0.4 the calculated maxCo is in one moment little bit over 4. So, the problem isn't fully solved. When the calculation reaches a problematic place it fails (if the time step is fixed) or it creates unrealistic results, if the time step is variable. But the first question, the divergence at the beginning is now solved. I will create necessary graphs and submit the other problematic place in the calculation. Thanks again Anton, Aurelien and Naveed a lot for the hints... fluid dynamics are now again a little bit clearer for me. Martin. 
All times are GMT 4. The time now is 03:34. 