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VOF : reproduce Variable Time Stepping Algorithm 

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February 22, 2016, 10:59 
VOF : reproduce Variable Time Stepping Algorithm

#1  
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Lars
Join Date: Jul 2015
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Hello,
I am simulating a twophase flow (water and air) and in order to understand the variable time stepping algorithm better, I try to calculate the next physical time step FLUENT will use following the equation given in the manual (Link) So I found the tui command Code:
/report/fluxes/massflow And Code:
/report/volumeintegrals/mass So I set up an excel sheet to calculate myself with the values given by the aforementioned tui commands: click for larger image For this arbitraty state in the simulation process I get a physical step size of 0.006746321 seconds for the next iteration, but when I start the iteration in FLUENT, it defines 0.0013777 seconds. I defined high values for the maximum time step size and the maximum step change factor and low values for the minimum time step size and minimum step change factor respectively. So none of these boundaries will take effect. Can you please point out, where I am missing something? When the manual says Quote:
Thank you! 

February 23, 2016, 07:12 

#2 
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Join Date: Mar 2014
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do you specify a courant number as well? Maybe it is just trying to keep to the maximum courant number value you have specified by keeping the time step small


February 23, 2016, 07:52 

#3  
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Lars
Join Date: Jul 2015
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Thank you for your reply.
In Fluent as well as in my Excel calculation I specify a courant number (CFLglobal) of 2. Quote:
But when I calculate myself, I get a different value. So the values for "outgoing fluxes" and "volume" I extract from Fluent seem to be different from the values, which Fluent uses to calculate . Based on the statement Quote:
So when I use the cell zone values, which are averages of all containing finite volume elements, the fluxes are not as critical and therefore leading to a bigger in my manual calculation. Is there a way in Fluent to report the element based volume fluxes? Preferably of a preselected set of elements (e.g. VOF = 0.4 ... 0.6) ? . 

February 24, 2016, 17:37 

#4 
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Join Date: Mar 2014
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in the vof window there is a courant number option as well which is for the phase and different from the global courant number, check that out as well.


Tags 
cfl, courant, step size, vof 
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