CFL drops when using fieldAverage

me3840 · December 3, 2017, 01:40

Hey there. I'm having an odd problem I think with the fieldAverage utility.

I have an LES case I'm running which runs fine by itself with a max CFL of 2. The timestep is chosen based on the CFL. However when I enable the fieldAverage in my controlDict, for a short time the CFL stays at 2. After some time, though, the CFL starts plummeting and my timestep drops to 1e-17 for seemingly no reason. If I run the case without fieldAverage on, this doesn't happen.

Anyone encountered this before? Is this a bug?

me3840 · December 4, 2017, 23:44

I noticed that the fieldAverage was the only dictionary in the system directory which was set to writeTime instead of adjustableRunTime. That sounds like it could be the cause, so I'll try setting the average interval correctly to match the rest of the dictionaries...

me3840 · December 5, 2017, 08:48

Apparently that's not the problem.

It also appears that no matter what writeControl/writeInterval I use, and no matter what startTime I choose, the timestep drops starting at startTime+0.015s. So strange..

pbachant · December 6, 2017, 07:52

So you've determined that it isn't the field averaging causing the solver to diverge? That would have been very strange.

me3840 · December 6, 2017, 08:27

The solver doesn't diverge. The solution always looks as expected, but for some reason the timestep it chooses just drops. Even though maxCo is set to 2, it's computing a timestep at nowhere near that CFL. If I don't use the fieldAverage, the CFL stays at 2 as specified and everything runs as I would expect.

None of this happens if I use a static timestep, either.

pbachant · December 6, 2017, 20:07

Interesting. Sounds like a bug that should be reported at https://bugs.openfoam.org.

me3840 · March 5, 2018, 19:50

This was indeed a bug, fixed with
https://bugs.openfoam.org/view.php?id=2820

December 3, 2017, 01:40	CFL drops when using fieldAverage	#1
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 24	Hey there. I'm having an odd problem I think with the fieldAverage utility. I have an LES case I'm running which runs fine by itself with a max CFL of 2. The timestep is chosen based on the CFL. However when I enable the fieldAverage in my controlDict, for a short time the CFL stays at 2. After some time, though, the CFL starts plummeting and my timestep drops to 1e-17 for seemingly no reason. If I run the case without fieldAverage on, this doesn't happen. Anyone encountered this before? Is this a bug?

December 6, 2017, 07:52		#4
pbachant Senior Member Pete Bachant Join Date: Jun 2012 Location: Boston, MA Posts: 173 Rep Power: 13	So you've determined that it isn't the field averaging causing the solver to diverge? That would have been very strange. __________________ Home \| Twitter \| GitHub

December 6, 2017, 20:07		#6
pbachant Senior Member Pete Bachant Join Date: Jun 2012 Location: Boston, MA Posts: 173 Rep Power: 13	Interesting. Sounds like a bug that should be reported at https://bugs.openfoam.org. __________________ Home \| Twitter \| GitHub

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December 4, 2017, 23:44		#2
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 24	I noticed that the fieldAverage was the only dictionary in the system directory which was set to writeTime instead of adjustableRunTime. That sounds like it could be the cause, so I'll try setting the average interval correctly to match the rest of the dictionaries...

December 5, 2017, 08:48		#3
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 24	Apparently that's not the problem. It also appears that no matter what writeControl/writeInterval I use, and no matter what startTime I choose, the timestep drops starting at startTime+0.015s. So strange..

December 6, 2017, 08:27		#5
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 24	The solver doesn't diverge. The solution always looks as expected, but for some reason the timestep it chooses just drops. Even though maxCo is set to 2, it's computing a timestep at nowhere near that CFL. If I don't use the fieldAverage, the CFL stays at 2 as specified and everything runs as I would expect. None of this happens if I use a static timestep, either.

March 5, 2018, 19:50		#7
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 24	This was indeed a bug, fixed with https://bugs.openfoam.org/view.php?id=2820