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Anna August 1, 2003 18:29

time step size
Hi guys, who can tell me why time-step-size setting changes the mass-flowrate result so much? I am doing a 2D two-phase(oil-gas) UNSTEADY flow model simulation which employed VOF. Because the final flow rate cann't match the experiment data very well, I set different conditions and whatever to try to solve out. Usually I set time-step-size as 1, when I set 0.5, the mass flow rate at outlet doubled, moreover when I set o.1, the result is double of the 0.5 one. I guarantee other settings are absoutly the same.My model's mass flow rate is 1e-6~2e-6kg/s usually, quite slow. It shouldn't happen from the tutorial statement. I am so appreciated for any answer.

ap August 4, 2003 08:18

Re: time step size
A time step of 1 sec seems too high to obtain accurate results. Try reducing it to 0.001 or so, and activate the adaptive time stepping available in FLUENT.



Anna August 4, 2003 21:26

Re: time step size
Thank you ap. But what is the rule for seting the time step size? I have 9 models, 9 level temperatures and 3 kinds of oil to simulate. It seems I can't crack within one month.

ap August 5, 2003 10:02

Re: time step size
There is no exact rule to determine the time step size. It depends on the quality of your spatial discretization and on your system behaviour and properties.

For example, if your flow oscillates with a period T, if you want to capture the oscillation, you need a time step smaller than T.

If you use the coupled solver, you can choose between the implicit and explicit time stepping, while if you use the segregated solver, you have to use the implicit time stepping.

See the manual for differences between the two schemes and for the Courant number criterion.

Implicit time stepping is usually better, because it is always stable. However stable doesn't mean accurate :)

So you have to choose a time step which is small enough to allow your solution to converge in a small number of iterations (usually 10-20), and to capture your system behaviour.

I suggest you to start with a small time step and to use the adaptive time stepping.



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