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IDProjectCategoryView StatusLast Update
0002008OpenFOAM[All Projects] Bugpublic2016-06-03 16:47
Reporterthings.are.clearAssigned To 
PriorityhighSeveritymajorReproducibilityalways
Status newResolutionopen 
PlatformLinux x64OSOpenSuSEOS Version13.2
Product Version 
Fixed in Version 
Summary0002008: viewFactorsGen produces incorrect view factors in a very thin gap
DescriptionThermal radiation between two concentric spheres are examined. Consider three cases distinguished from one another only by the gap between the spheres: 1 mm, 10 mm, and 100 mm. The units are unimportant here; it’s the order of magnitude that is really relevant to the problem. Mesh cells' characteristic size is 10 mm for all the cases in question.

Here are ViewFactorsGen output for 3 cases

1 mm case
ViewFactorsGen output:
F00: 0
F01: 24.05267
F10: 21.62112
F11: 0.01200849

10 mm case
ViewFactorsGen output:
F00: 0
F01: 1.01395
F10: 0.8505047
F11: 0.1242975

100 mm case
ViewFactorsGen output:
F00: 0
F01: 1.001733
F10: 0.2489729
F11: 0.7506583

As you can see, in case 1 (gap 1 mm) view factors are totally wrong. The error is enormous! The results in case 2 are tolerable. The last case (100 mm) looks OK! The question is Why?
Steps To ReproduceThe simulation case can be downloaded from: https://drive.google.com/file/d/0Bz9jCmNem157a18zNmpISjRNRDA/view?usp=sharing.
"mesh" folder contains 3 sub-folders with mesh files.
Run in terminal:
star4ToFoam -ascii -solids mesh/001mm/star // mesh/001mm/star or mesh/010mm/star or mesh/100mm/star
splitMeshRegions -cellZonesOnly -overwrite
then:
faceAgglomerate -region cellZone_2 -dict constant/viewFactorsDict > log.faceAgglomerate.$i 2>&1
viewFactorsGen -region cellZone_2 > log.viewFactorsGen.$i 2>&1
Additional InformationI started a thread on CFD-Online forum to discuss the problem: http://www.cfd-online.com/Forums/openfoam-pre-processing/166634-viewfactorsgen-produces-incorrect-view-factors-very-thin-gap.html#post585056. There you might find some additional information like, e.g., pictures with view factor field on a surface.
Tagsthermal radiation, view factors, viewFactorsGen

Activities

henry

2016-02-28 17:19

manager   ~0005985

Can you provide a patch to fix the problem?

things.are.clear

2016-03-05 17:44

reporter   ~0005994

Last edited: 2016-03-05 17:44

View 2 revisions

No, I can't. Unfortunately, my expertise is not equal to the task of developing a patch. I might try to look into the source code to see where the issue comes from.

MattijsJ

2016-06-03 16:47

reporter   ~0006364

If you switch on in the viewFactorsDict the

debug 1;

//- Dump connectivity rays
dumpRays true;


you can load in the ./allVisibleFaces.vtk and see that you only get a few rays in between the two shells. Since the shells are so close together and the agglomeration centres on inside and outside shell are not synchronised you will not get any perpendicular rays; instead all your rays are very shallow. I assume this causes the error.

Issue History

Date Modified Username Field Change
2016-02-28 17:12 things.are.clear New Issue
2016-02-28 17:15 things.are.clear Tag Attached: thermal radiation
2016-02-28 17:15 things.are.clear Tag Attached: view factors
2016-02-28 17:15 things.are.clear Tag Attached: viewFactorsGen
2016-02-28 17:19 henry Note Added: 0005985
2016-03-05 17:44 things.are.clear Note Added: 0005994
2016-03-05 17:44 things.are.clear Note Edited: 0005994 View Revisions
2016-06-03 16:47 MattijsJ Note Added: 0006364