CFD Online Discussion Forums (https://www.cfd-online.com/Forums/)
-   OpenFOAM (https://www.cfd-online.com/Forums/openfoam/)
-   -   Does MULES::implicitsolve allow for Courant numbers > 1? (https://www.cfd-online.com/Forums/openfoam/88938-does-mules-implicitsolve-allow-courant-numbers-1-a.html)

 Philip May 31, 2011 03:00

Does MULES::implicitsolve allow for Courant numbers > 1?

Hello,

although there are several threads concerning the time step size for MULES VOF (in interFoam and/or interDyMFoam) I have not found a definitive answer whether you (I/OpenFOAM) can safely increase the time step size, or that anyway the max Courant number should still remain < 1?

thanks,

Philip

 akidess May 31, 2011 05:09

No, you are still bound by the PISO algorithm to Co < 1. However, using explicit MULES you are bound by the capillary wave length, which usually gives a time step criterion dt << (Co < 1). MULES implicit somewhat relaxes that time step criterion.

 Philip May 31, 2011 05:37

Hi Akidess,

thanks for this fast reply. I am an openfoam beginner and I did not know that all PISO (so all transient Navier-Stokes) calculations were limited by Co < 1. Do you know why this is so? Apart from losing accuracy for larger steps, I am not aware of stability issues with eg backward Euler time integration and SIMPLE (or PISO).

thanks,

Philip

 akidess May 31, 2011 05:54

The restriction is not OpenFOAM specific, but tied to the PISO algorithm itself. If you read the derivation of the algorithm, there is a step where it is assumed that the velocity only slightly changes between time steps. This is the reason why PISO does not require relaxation and usually converges within 2-3 iterations. With SIMPLE you don't have the same time step restriction, but you'll have to do a lot more work to get a converged solution for a given time step.

***
R. I. Issa, Solution of the implicitly discretised fluid flow equations by operator-splitting, Journal of Computational Physics, Volume 62, Issue 1, January 1986, Pages 40-65, ISSN 0021-9991, DOI: 10.1016/0021-9991(86)90099-9.
(http://www.sciencedirect.com/science...21999186900999)

 Philip May 31, 2011 06:25

Hi again,

Thanks. I will look up the original article of Issa.
I found a similar discussion under the title "Courant number and implicit treatment thread":

http://www.cfd-online.com/Forums/ope...treatment.html

that make things a bit clearer. At this moment I still wonder if this has been tried: Looping (within a time step) over the momentum solver + corrections does not allow for a larger time step?

Philip

 All times are GMT -4. The time now is 17:48.