CFL number limit for InterFoam
 

Hello everybody,
I have been using lesinterFoam for studying the breakup of liquid sheets. I have also been testing the solver for the CFL limit. Recently I read an article "Volume of fluid methods fro immiscible-fluid and free-surface flows" by Vinay R.Gopala, Berend G.M. van Wachem.Chemical Engineering Journal 141 (2008) 204-221

In this paper it has been specified a couple of times that for the inter gamma scheme that is implemented in interFoam it is required to have very small time steps, CFL<0.01 for keeping a sharp interFace. Based on tis comments I have done a comparative study of different CFL numbers for the same mesh and same boundary conditions.

I studied CFL numbmers of 0.2,0.1,0.05,0.01 and 0.0075 for the liquid sheets.

I found out that the cases are very much dependent on the CFL numbers, but the cases with CFL nummbers 0.01 and 0.0075 are closer to each other. Then I alos found in article by Henry Weller " Interface tracking capabilities of the Inter-Gamma Differencing scheme" that it ir required to use Co<1/2 for 2d cases and Co<1/3 for 3d cases.

Then it is also specified in Gopalas paper that in order to use larger time steps subcycles are used to solve the VOF equation. For a given Courant number, the flow equations are solved initially and later the VOF equation is fractionally updated n times, where n represents the number of subcycles predefined in the code.

Now my question is why has the CFL limit been specified differently in these two literatures, which one should I follow, should I follow the limit specified by Gopala in his paper to keep a sharp interface or the one specified by Henry in his article which looks quiet old to me.

My second question what is the limit to which i can vary my CFL number if I can use more n gamma subcycles.

Has anybody studied the accuracy of the solution by using higher CFL number basically higher time steps and increasing the ngamma subcycles.
I mean if want to use a CFL number of 0.2 is it sufficent that I use 4 ngamma subcycles or shouldi use more.
Is there any reference for this CFL limit and the ngamma subcycles.

bye
with regards
K.Suresh kumar