Time-step setting

robin781105 · December 18, 2013, 06:25

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 ?

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 prpout.xxx). If DTMIN > 1E-6, I would set DTMIN = 1E-6 or 1E-7 for most hydraulics problems (and smaller for microfluidics).

December 18, 2013, 06:25	Time-step setting	#1
robin781105 New Member tsung han yang Join Date: Dec 2013 Posts: 12 Rep Power: 13	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 ? Thanks all by Robin

December 22, 2013, 03:01		#4
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	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. zhouyu3092070, lamnguyen11b2 and Ethon like this.

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December 18, 2013, 21:34		#2
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	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.

December 19, 2013, 07:45		#3
robin781105 New Member tsung han yang Join Date: Dec 2013 Posts: 12 Rep Power: 13	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?

December 27, 2013, 01:21		#5
robin781105 New Member tsung han yang Join Date: Dec 2013 Posts: 12 Rep Power: 13	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?

December 31, 2013, 12:41		#6
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	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 prpout.xxx). If DTMIN > 1E-6, I would set DTMIN = 1E-6 or 1E-7 for most hydraulics problems (and smaller for microfluidics).