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Mavinakere September 3, 2002 15:29

time steps
Hi all, i'm workin on multiphase simulation with air and water using VOF .I have problems choosing the right time steps.i need to choose the time steps according to the maximum velocity around the flow but to do that i need to write a script.which i do not know how to??. I have 700,000 cells in my domain and the largest time step FLUENT allows me to choose is 10^-5 and this takes me for ever to get the results.So,i need a UDF which uses the starting time step as 10^-5 and gradually change the step as required by the flow. I did also change the courant number to maximum 1 using geo-reconstruct option,but then,results blew up after 20 iterations.Can anybody plz show me some light on this. Mavinakere.

sri September 5, 2002 12:29

Re: time steps

The adaptive time-stepping method in fluent should help you.


Mavinakere September 5, 2002 13:54

Re: time steps
Hi Sri, thanks for you reply..but can you be more specific as to where do i click buttons to get there???and what setup with the FLUENT it needs to use adaptive time steps ?? Thankyou, Mavinakere.

Atholl September 6, 2002 03:56

Re: time steps
Hi, unfortunately, the adaptive time stepping option isn't available in conjunction with the VOF model. Are you calculating your time step correctly?

Co = time step/delta x cell / average fluid velocity


time step = Co * delta x cell / average fluid velocity

I report the volume average velocity after converging for flow & turbulence before I enable VOF. Delta x cell is the number of cells in the flow direction of your mesh divided by the length in that direction.

The time step this gives me is usually pretty reasonable, providing a balance between good & fast convergence.


sri September 6, 2002 05:54

Re: time steps

You will the option in in iterate panel.There are two options :

fixed time step adaptive time step

by default fixed is switched on.


Atholl September 6, 2002 08:01

Re: time steps
Fluent help files quote "Adaptive time stepping...cannot be used with the VOF or discrete phase model"

sri September 6, 2002 11:27

Re: time steps
In that case,the adaptive time stepping should be blocked out once the vof method is selected.I think we should report this to fluent people.


Mavinakere September 6, 2002 12:32

Re: time steps
Hi,thanks for all the comments what is Co? and what do you mean by "I report the volume average velocity after converging for flow & turbulence before I enable VOF" and how do i do it? i have,155 cells in flow direction and the length is 1.23m....and i'll know the average velociy only when i run few iterations so,how long should i run to get a approximate avg velocity? Mavinakere.

mav September 7, 2002 23:07

Re: time steps
This all sounds rather painful!

I hear that CFX-5 has a much more robust multiphase model and because of their coupled-multigrid solver, the timestep is based on the actual physics, and not limited by cell size.

Has anyone else tried this?


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