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robin781105 December 18, 2013 06:25

Time-step setting
Dear all

I have a quick equation to ask.

How to set the "delta t" in the Flow-3D or if the model will automatically set during the simulation ?:confused:

Thanks all

by Robin

JBurnham December 18, 2013 21:34

It will automatically set delta t (dt), and dt can change throughout the simulation. The setting of dt depends on what physics models you have active, the velocity of the flow, and the cell size. You can also limit dt to a maximum value from the Model Setup > Numerics tab, but usually I just let FLOW-3D pick dt for me.

robin781105 December 19, 2013 07:45

Learn a new lesson, thank you Jeff.:)

Can I ask another question?
So, If we let the model automatically setting dt then we just need to change the cells size for simulation?

JBurnham December 22, 2013 03:01

Yes. The time step size will change when you change the cell size. Other things that affect the time step size are:

1) too many pressure iteration failures (check boundary conditions, initial conditions, mesh resolution and aspect ratio.stl files for correctness, and that gridlines match at mesh block interfaces).

2) sudden changes in flow direction, which may give a 'convective volume flux' error, which is harmless (the time step is just reduced and the cycle is run again).

3) splashing or droplets may reduce the time step because they have high velocity. You can remove flying droplets by setting FCLEAN = 0.01 to 0.05 in the Numerics tab, or set initial conditions so that fluid is not splashing on solid surfaces.

robin781105 December 27, 2013 01:21

Thanks Burnham :)

Can I ask a new question about time step?

I set my model's finish time at 100s and the simulation process is all fine, but when I change the finish time to 4500s then I got the "time step size < dtmin" problem at 400s(The time step setting is automatically), how does it mean? If I need to change the time-step size artificially?

JBurnham December 31, 2013 12:41

DTMIN is based on the finish time TWFIN. It's different in different versions, but for example it might be DTMIN = TWFIN/1000. You can check DTMIN value in Diagnostics > Preprocessor Summary (or file If DTMIN > 1E-6, I would set DTMIN = 1E-6 or 1E-7 for most hydraulics problems (and smaller for microfluidics).

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