# VOF : reproduce Variable Time Stepping Algorithm

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February 22, 2016, 10:59
VOF : reproduce Variable Time Stepping Algorithm
#1
New Member

Lars
Join Date: Jul 2015
Posts: 8
Rep Power: 10
Hello,

I am simulating a two-phase 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/mass-flow`
which will give me the mass-flows for my four cell zones (i.e. zone1, zone2, zone3, zone4) in my domain.

And
Code:
`/report/volume-integrals/mass`
will give me the mass for each of my two phases in the four cell zones. (I didn't find any command to report the volume fluxes, so I think using the mass fluxes divided by the mass will work)

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:
 where the ratio is calculated for each cell.
does it mean for each cell zone or for each of the finite volume elements?

Thank you!

 February 23, 2016, 07:12 #2 Senior Member   Join Date: Mar 2014 Posts: 375 Rep Power: 13 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
New Member

Lars
Join Date: Jul 2015
Posts: 8
Rep Power: 10

Quote:
 Originally Posted by hwet do you specify a courant number as well?
In Fluent as well as in my Excel calculation I specify a courant number (CFLglobal) of 2.

Quote:
 Originally Posted by hwet Maybe it is just trying to keep to the maximum courant number value you have specified by keeping the time step small
Yes, the variable time stepping method of Fluent tries to stay at CFLglobal = 2. In order to do so, Fluent uses the equation .

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:
 Originally Posted by Fluent Documentation The variable time step is based on the maximum Courant number near the VOF interface. To calculate that Courant number, ANSYS Fluent uses a flux-based definition where, in the region near the fluid interface, ANSYS Fluent divides the volume of each cell by the sum of the outgoing fluxes.
it seems, that Fluent uses the "outgoing fluxes" and "volume" of selected finite volume elements (called "cells" in this case), maybe something like VOFwater = 0.4 ... 0.6, near the water-air-interface and not the whole cell zones.

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 Senior Member   Join Date: Mar 2014 Posts: 375 Rep Power: 13 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