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 
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.

Hi Akidess,
thanks for this fast reply. I am an openfoam beginner and I did not know that all PISO (so all transient NavierStokes) 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 
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 23 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.
You can read more about this in the original paper of the PISO algorithm: *** R. I. Issa, Solution of the implicitly discretised fluid flow equations by operatorsplitting, Journal of Computational Physics, Volume 62, Issue 1, January 1986, Pages 4065, ISSN 00219991, DOI: 10.1016/00219991(86)900999. (http://www.sciencedirect.com/science...21999186900999) 
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.cfdonline.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 
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