# viewFactorsGen produces incorrect view factors in a very thin gap

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August 6, 2019, 10:04
#21
Senior Member

Raza Javed
Join Date: Apr 2019
Location: Germany
Posts: 183
Rep Power: 7
Hello Everyone,

As you all are discussing about viewFactors etc, so I thought to ask some basic questions.

I am trying to implement view Factor radiation model in chtMultiRegionSimpleFoam, I have a simple geometry of two solid regions.

As you can see in the attached figure:

1. small green box is a heater with 500K temperature
2. Big blue box is the room with 300K

So heat transfer through radiation will take place from the green small solid to big blue solid.

Now, I implemented view Factor model. and my case is running, but I am not sure about the values.

Questions:

1. How we decide what should we write in viewFactorsDict? At this time, I randomly put 20 and 10, as you can see below, but I don't know how to decide these values.

My viewFactorsDict is given below:
Code:

writeViewFactorMatrix     true;
writeFacesAgglomeration   true;
writePatchViewFactors     false;

defaultFaces
{
nFacesInCoarsestLevel     20;
featureAngle              10;
}

heater_to_room
{
nFacesInCoarsestLevel     20;
featureAngle              10;
}
2. What should be check in Paraview to see if our model is working fine? because as I have temperature difference of 200K between two solids, so the heat will flow in any case from green to blue solid.

I shall be very thankful if someone can help me out in this.

Thank you
Attached Images

August 18, 2019, 12:42
#22
New Member

Matteo Quirino
Join Date: Feb 2019
Posts: 6
Rep Power: 7
Hello Rava Javez,

I assume that the green box is a solid region and the blue one a fluid region, thus the viewFactorField is generated only in the blue region.

1) nfacesInCoarsestLevel decides the number of agglomerated faces that will be used in the radiation exchange. For example, if you have a cube with 36 faces on each side and you set nfacesInCoarsestLevel to 6 then after the agglomerations the faces used by the view factor model shall be around six. Attached you can see that as you raise the number there are more agglomerated patches and they become smaller. Rember that the number you set is not exactly the number of agglomerated faces you will have.
The featureAngle value is still a mystery. By changing the value I observed that it does have an impact on the computation time but not on the results. Hopefully, someone will explain to us what it does exactly.

2) To check that the view factors are correct the first thing to do is to open the viewFactorField file and check that there are not too many faces with values way above one. If you have too many faces with a values above 1 or with very big values (this happens with small gaps due to the algorithm used in viewFactor.C) increase the number of nfacesInCoarsestLevel this will make the discretization finer and the values of the view factors shall move towards one.

To visualize the view factors in paraView see
convert viewFactorField to ascii and visualization
and just check that the values of all the patches are around 1. The corners will always have the biggest values as the distance between the faces will be small thus due to the algorithm used the value is bigger. See picture attached

You can also check the view factor from one patch to the another using the option
Code:
debug  1;
in the viewFactorDict after the write(...) options

To check the heat fluxes you can use paraView calculator to integrate the "qr" field over the patches or run the following command when the solver has reached convergence

Code:
buoyantSimpleFoam -postProcess -func wallHeatFlux -latestTime > log.HeatFlux
Attached Images
 nFaces6.png (30.1 KB, 53 views) nFaces20.png (31.7 KB, 47 views) viewFactorField.png (35.9 KB, 53 views)

 April 12, 2022, 06:05 #23 New Member   Nghiep Join Date: Nov 2021 Posts: 6 Rep Power: 4 Hi MatteoQ, I would like to view the result of the view factor, please help me with how to extract these results. Thank you in advance.

 April 12, 2022, 06:31 #24 New Member   Matteo Quirino Join Date: Feb 2019 Posts: 6 Rep Power: 7 Hi Nghiep, in my last post on this thread just after point "2)" there is a link to another thread that is dedicated in visualizing the viewFactorField. I leave you here the link to the thread. Keep in mind that what you will see in paraview is the sum of the view factors between a face agglomeration and all the others, thus for an enclosure such value must be close to one. If you see some faces with values ~1.2 is ok. If you want the view factor between one face agglomeration and another face agglomeration such information is written in the file named "F" in the constant folder (or /constat folder if the case is multiregion).

 April 12, 2022, 22:01 #25 New Member   Nghiep Join Date: Nov 2021 Posts: 6 Rep Power: 4 Hi MatteoQ, Thank you for your reply. I did it. I have another question, Is there a way to see the output of View Factor like F01 = .., F11 =... I want to compare them with theoretical results. Thank you in advance!!!