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viewFactorsGen produces incorrect view factors in a very thin gap

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Old   February 13, 2016, 17:40
Question viewFactorsGen produces incorrect view factors in a very thin gap
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Sergei
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Formulation
The problem I am going to talk about was examined in the thread chtMultiRegionSimpleFoam: Thermal Conduction + Surface-To-Surface Radiation
in a quite broader context. This thread is all about one particular aspect of the problem - the way utility viewFactorsGen works.
A sketch of the simulation domain:

Simple heat conduction equation is solved in all regions. In addition, thermal radiation is modeled in the central region cellZone_2, with the radiation model being viewFactor (some sources refer to it as surface-to-surface radiation).
Well, consider 3 cases distinguished from another only by the width of the central region (cellZone_2 in the picture): 1 mm, 10 mm, and 100 mm. The units are unimportant here; itís the order of magnitude that is really relevant to the problem. Characteristic size of mesh cells is 10 mm for all the cases in question.

Results

Region width 1 mm
ViewFactorsGen output:
Code:
F00: 0
F01: 24.05267
F10: 21.62112
F11: 0.01200849


Region width 10 mm
ViewFactorsGen output:
Code:
F00: 0
F01: 1.01395
F10: 0.8505047
F11: 0.1242975


Region width 100 mm
ViewFactorsGen output:
Code:
F00: 0
F01: 1.001733
F10: 0.2489729
F11: 0.7506583

As you can see, in case 1 (region width 1 mm) view factors are totally wrong. The error is enormous! The results in case 2 are tolerable but there are some faces with values as low as 0.74 (not good at all). The last case (100 mm) looks OK! The question is Why?

Last edited by wyldckat; February 21, 2016 at 17:09. Reason: repaired link
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Old   February 21, 2016, 17:18
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Bruno Santos
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Quick question: Can you provide the test case(s) that you've used for reproducing these results?

I ask this because if we use Google with following search string:
Code:
site:openfoam.org/mantisbt "viewFactorsGen"
there have already several bugs fixed in this utility and there are possibly still a few more to be solved, probably including this issue that you're pointing out.

Therefore, if you can provide these test cases, along with these details that you've provided, than it's also probably best to report this on the bug tracker: http://www.openfoam.org/bugs
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Old   February 28, 2016, 13:22
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Quote:
Originally Posted by wyldckat View Post
Can you provide the test case(s) that you've used for reproducing these results? Therefore, if you can provide these test cases, along with these details that you've provided, than it's also probably best to report this on the bug tracker: http://www.openfoam.org/bugs
Bruno, thank you for your reply.
The simulation case can be downloaded from: https://drive.google.com/file/d/0Bz9...ew?usp=sharing
"mesh" folder contains 3 sub-folders with mesh files.
Run in terminal:
Code:
star4ToFoam -ascii -solids mesh/001mm/star // mesh/001mm/star or mesh/010mm/star or mesh/100mm/star
splitMeshRegions -cellZonesOnly -overwrite
then:
Code:
faceAgglomerate -region cellZone_2 -dict constant/viewFactorsDict > log.faceAgglomerate.$i 2>&1
viewFactorsGen -region cellZone_2 > log.viewFactorsGen.$i 2>&1
And I also submitted a bug report: http://www.openfoam.org/mantisbt/view.php?id=2008
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Old   March 6, 2016, 11:20
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Hi Sergei,

Many thanks for the cases and the details on how to use them! And for reporting this on the bug tracker!
Hopefully I'll be studying this week the ins and outs of Radiation in OpenFOAM at work and hopefully I'll start getting a better notion of what's going on here.

I already have a feeling that the issue is related to the view factors being designed to work with a uniform mesh, but I still need to double-check if this is the case in the code itself. For example, it's possible that the view factors are currently designed to be calculated before generating the boundary layer mesh.

Best regards,
Bruno
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Old   March 6, 2016, 16:41
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Quote:
Originally Posted by wyldckat View Post
Hopefully I'll be studying this week the ins and outs of Radiation in OpenFOAM at work and hopefully I'll start getting a better notion of what's going on here.
I already have a feeling that the issue is related to the view factors being designed to work with a uniform mesh
Bruno, actually I have already tried view factors radiation model to solve a case with structured uniform pure hexa mesh and got the same - wrong results.
It would be really nice of you If you could resolve the issue. Looking forward to hearing from you on the progress.
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