2 different time steps in Fluent's VOF??

thecfduser · March 15, 2010, 19:36

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???

sega · March 16, 2010, 04:13

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.

thecfduser · March 16, 2010, 15:13

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

sega · March 16, 2010, 16:37

Quote:

Originally Posted by thecfduser

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.

thecfduser · March 16, 2010, 17:09

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

sega · March 17, 2010, 16:28

Quote:

Originally Posted by thecfduser

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?

thecfduser · March 17, 2010, 17:05

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

sega · March 17, 2010, 18:59

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.

March 15, 2010, 19:36	2 different time steps in Fluent's VOF??	#1
thecfduser Senior Member karine Join Date: Nov 2009 Posts: 158 Rep Power: 17	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???

March 16, 2010, 04:13		#2
sega Senior Member Sebastian Gatzka Join Date: Mar 2009 Location: Frankfurt, Germany Posts: 729 Rep Power: 20	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. __________________ Schrödingers wife: "What did you do to the cat? It's half dead!"

March 17, 2010, 18:59		#8
sega Senior Member Sebastian Gatzka Join Date: Mar 2009 Location: Frankfurt, Germany Posts: 729 Rep Power: 20	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. __________________ Schrödingers wife: "What did you do to the cat? It's half dead!"

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March 16, 2010, 15:13		#3
thecfduser Senior Member karine Join Date: Nov 2009 Posts: 158 Rep Power: 17	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

March 16, 2010, 17:09		#5
thecfduser Senior Member karine Join Date: Nov 2009 Posts: 158 Rep Power: 17	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

March 17, 2010, 17:05		#7
thecfduser Senior Member karine Join Date: Nov 2009 Posts: 158 Rep Power: 17	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