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|ID||Project||Category||View Status||Date Submitted||Last Update|
|0000656||OpenFOAM||[All Projects] Bug||public||2012-10-04 14:49||2013-06-20 14:04|
|Target Version||Fixed in Version|
|Summary||0000656: View Factor Generation Inaccurate|
|Description||the pre-processing utility viewFactorsGen is inaccurate. This is because the function calculateViewFactorFij (in viewFactorsGen.C) that calculates the view factor of two faces, uses the formula for the view factor of two infinitesimal faces. Instead, it should use a double integration over the face area or a line integration around the borders.|
|Steps To Reproduce||run viewFactorsGen on any geometry|
|Additional Information||The computed radiative heat flux using the radiosity method and view factors is highly sensitive to the precision of the view factor. The method currently used in OpenFOAM is unusable and can lead to errors up to a factor 1000 on coarser grids. However, due to the nature of the view factor calculation used now, the view factor converges to the correct value when the cell size goes towards 0.|
Tests were made to compare OpenFOAM with View3D and an analytical solution.
|Tags||No tags attached.|
|Attached Files|| viewfactors_sphere.jpg [^] (40,714 bytes) 2013-03-29 09:34
viefactors_rays_default.jpg [^] (355,964 bytes) 2013-04-04 12:55
viefactors_rays_modified.jpg [^] (365,241 bytes) 2013-04-04 12:56
viewfacctorgen_sphere_bug.tar.gz [^] (58,556 bytes) 2013-04-08 11:34
We are aware of the drawbacks of this method. It was a design decision fit for the project in which this model was developed.
If you have the code or reference for the model you are referring I could have a look at it.
|http://www.bfrl.nist.gov/IAQanalysis/docs/NISTIR-6925.pdf [^] provides an overview over different integration techniques. For example, the double line integration seems suitable (eq. 4), or the single area integration methode (eq. 7). A similar methode is implemented in View3D (http://view3d.sourceforge.net/ [^])|
I tried to get viewfactors for a sphere. Without success.
I uploaded an image of the resulting unusable viewfactorfield.
I was able to locate the problem.
A double integral procedure is used and not causing the problem.
For debugging I added the following lines to viewFactorsDict:
The "zero entries" are due to aglomeratedFaces that are choosen to have no visble faces in shootRays.H. (This can als be checked in globalFaceFaces in the constant directory. There should be faces belonging to the coarse faces)
Changing the following lines in shootRays.H as done here shows interesting results:
00043 start.append(fc ); 00045 end.append(fc );
Due to that change I have no more zero entries but now there are rays entering the sphere and I have an overestimation of the surrounding patch.
I hope a specialist can give a good fix now.
Could you upload the mesh you're using then we can check up the view factors
I uploaded a test case.
The debug output is for now looking like in the following:
Calculating view factors...
Writing view factor matrix...
But it should be (ideal):
I think I solved the problem:
After checking visibility using the inner product in line 41 of shootRays.H the connecting vector is reduced in line 43 (start.append(fc + 0.0001*d);) and 45 (end.append(fc + 0.9999*d); ) for not having start/end-points on the face anymore so that they are not detected as intersection in line 73 (if (!hitInfo[rayI].hit())). But the surfaces used for checking intersection are coming from a triangulization in searchingEngine.H and are not the original faces. This results in a lot of faultily detected intersections due to insufficient length reduction in line 43 and 45.
A secure setting could be:
line 43: start.append(fc + 0.1*d);
line 45: end.append(fc + 0.9*d);
Now the rays are not sorted out faultily any more.
If the reduced line is used as distance for the viewfactor calculation as well maybe 0.99 and 0.01 is better.
|2012-10-04 14:49||marc.immer||New Issue|
|2012-10-05 10:35||user21||Note Added: 0001702|
|2012-10-05 10:50||marc.immer||Note Added: 0001703|
|2013-03-29 09:34||eddi0907||File Added: viewfactors_sphere.jpg|
|2013-03-29 09:36||eddi0907||Note Added: 0002080|
|2013-04-04 12:55||eddi0907||File Added: viefactors_rays_default.jpg|
|2013-04-04 12:56||eddi0907||File Added: viefactors_rays_modified.jpg|
|2013-04-04 13:24||eddi0907||Note Added: 0002086|
|2013-04-08 11:06||user21||Note Added: 0002096|
|2013-04-08 11:34||eddi0907||File Added: viewfacctorgen_sphere_bug.tar.gz|
|2013-04-08 11:43||eddi0907||Note Added: 0002097|
|2013-04-09 09:09||eddi0907||Note Added: 0002101|
|2013-06-20 14:04||user21||Status||new => resolved|
|2013-06-20 14:04||user21||Resolution||open => fixed|
|2013-06-20 14:04||user21||Assigned To||=> user21|