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- - **chtMultiRegionFoam - Courant number, deltaT, convergence criterion**
(*https://www.cfd-online.com/Forums/openfoam/105903-chtmultiregionfoam-courant-number-deltat-convergence-criterion.html*)

chtMultiRegionFoam - Courant number, deltaT, convergence criteriondear all,
I hope everybody is doing good I came across something quite weird and I would like to share it with you in order to listen to the experience and suggestions of other people. I simulate a laminar flow for a quite simple (though large) geometry, I use chtMultiRegionFoam of the latest version OF-2.1.1. It is a transient conjugate heat transfer problem. Everything is good but I would like to speed up the whole simulation because I need to simulate a lot of hours and many times. I use adjustable timestep and maximum Co 5 (have tried 10 and 15) because higher values lead to divergence (weird, considering that Crank Nicholson should work for any timestep). Therefore I am limited to a very small timestep which will take me ages to get my results. So I have decided to change the convergence criteria in order to have lower computational time per time step (very desperate?). Anyway when I reduce the convergence criteria by one order of magnitude (1e-07 to 1e-06) this influences the deltaT, which drops from 4-5seconds to 0.2 seconds. So the question is: What is the relation between the convergence criteria and the delta? Are there any ideas in order to speed up my simulations?Thanks a lot in advance!cordially, Giorgos |

hello,
You can't get faster since chtMultiRegionFoam is a weakly coupled solver, and trying to solve with higer tolerance residual will not help you. You may take a look at conjugateHeatFoam (1.6-ext). regards, olivier |

thank you Olivier for your response!
what do you mean by take a look at conjugateHeatFoam. It is not available in the version that I am using. Can I still use it? And if yes, how can I use larrger time step?thanks a lot |

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