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2 different time steps in Fluent's VOF??

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Old   March 15, 2010, 19:36
Default 2 different time steps in Fluent's VOF??
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Hi
i have red in Fluent's manual that in VOF, Fluent uses 2 different time steps. The first for the volume fraction equation, and the second for the rest of transport equations.....
I didnt understood this at all
Can anyone help please???
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Old   March 16, 2010, 04:13
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There is one timestep for the Navier-Stokes equations and one for the VOF-equation.

The VOF-equation is solved with a smaller timestep so it will be solved multiple times inside a single Navier-Stokes timestep.
Thus you will get a stable solution for your interface without increasing computational times for pressure and velocity.
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Old   March 16, 2010, 15:13
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Thank u a lot my friend.
I will allow myself to ask u some more questions please, and hoping it is not bothering u:
1-Is the interface reconstructed at each of those VOF time-steps (the small ones)??? or once each big time-step?
2-Why when i am solving in VOF model , Fluent does not plot residuals for Volume fraction equation?????

Thank u and best regards
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Old   March 16, 2010, 16:37
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Quote:
Originally Posted by thecfduser View Post
Thank u a lot my friend.
I will allow myself to ask u some more questions please, and hoping it is not bothering u:
1-Is the interface reconstructed at each of those VOF time-steps (the small ones)??? or once each big time-step?
2-Why when i am solving in VOF model , Fluent does not plot residuals for Volume fraction equation?????

Thank u and best regards
Sorry, I can't tell you that.
I'm familiar with the theory about VOF subcycles from OpenFOAM, but not from FLUENT.

I'm afraid this is up to someone else who is more into FLUENT.
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Old   March 16, 2010, 17:09
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If u can tell me how it is done in Opanfoam, it will be very good .
It must be done by a similar way in Fluent i think.
thanks
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Old   March 17, 2010, 16:28
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If u can tell me how it is done in Opanfoam, it will be very good .
It must be done by a similar way in Fluent i think.
thanks
I think a "reconstruction" is done with every converged time step.
And OpenFOAM is plotting residuals for the VOF-equation.

Why do you bother? Something wrong with your calculations?
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Old   March 17, 2010, 17:05
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Hi and thanks for ure answer. In fact i have a problem in my simulations, and i was trying to completely understand the VOF model to see if it can be due to the VOF model or to other things...
I will tell u my problem very brievly, and i hope that u can help...
I am simulating a stratified gas-liquid flow in pipes. In simulations, the height of the liquid phase is always less than in experiments. I tried all turbulence models.
The result are not very good, since the water velocity is overestimated and gas velocity underestimated, because of this problem.
I tested on 2 diferent grids: same result.
I tried to refine (adapt function in Fluent) the interface cells: same results.

I the problem du to the VOF technique????

Thanks in advance for ure help
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Old   March 17, 2010, 18:59
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This could really be anything, not only the VOF.

I suggest you start a new discussion special to your problem and give more details about the whole situation.
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