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Difference between IDefault and G (radiation) 

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October 4, 2016, 12:22 
Difference between IDefault and G (radiation)

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Fabien Salmon
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Hi,
Sorry but I am a new OpenFOAM user so maybe it is an easy question. When the radiation is activated, we must have two files of boundary condition: G and IDefault. I understand that G gives the Marshak boundary condition to the walls (usually). Then, IDefault gives greydiffusivewalls (in tutorials). I do not understand that. It appears to be redundant to me, we already give a radiation boundary condition. So I just would like to know the meaning of this two files. I think there is something that I do not understand. Thanks. 

October 10, 2016, 04:56 

#2 
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Fabien Salmon
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Nobody knows ?


October 10, 2016, 08:55 

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G is used for P1 model, IDefault for fvDOM model and Qr for viewFactor model.


October 10, 2016, 09:07 

#4 
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Fabien Salmon
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Thank you for the response.


February 17, 2017, 13:30 

#5 
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nasir musa yakubu
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Hi, from my understanding, G is incident radiation and iDefault is radiation intensity ray which calculates radiation intensity.
G= Qin + Qem so if you think about it this way, G is the total heat that's been absorbed and emitted within a media and IDefault which i believe is used for calculating Qin, which stands for incident radiation heat flux. not too sure but you can easily find out using the search box here, its pretty effective. http://www.openfoam.com/documentatio...b34c63f9ffab05 That's IDefault https://github.com/OpenFOAM/OpenFOAM...IntensityRay.C 

March 7, 2017, 12:40 

#6 
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Fabien Salmon
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Hello,
Actually, Qr=QinQem where Qem is emissivity*Sigma*T⁴ I have checked it with an easy case. Qin is the total incident radiative heat flux field. G is used with the P1 model. But, even if I use the fvDOM method, OpenFoam calculates G. G is incident radiation. Actually, we have Qr=1/(3a)*grad(G) (if we do not have scattering). So I do not understand one thing. Why do we call G the incident radiation ? Qin and G are different but their name seem to be the same for me. Both are in W/mē and it is what the wall receives. If someone can explain what is the physical difference between G and Qin, I would be grateful. 

March 7, 2017, 14:32 

#7  
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nasir musa yakubu
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Hello Fabien,
hmm interesting: well looking at the source code, it shows that Qr, Qin and Qem are independently calculated: Quote:
to answer your question specifically G accounts for the black body radiation and is the summation of all radiation contribution from all rays and can be calculated my multiplying the intensity of the ray by its solid angle. so once G is updated, Qr, is calculated from G. and I as you clearly stated in your last reply, the Qem and Qin are then calculated. However, i would reconfirm this with an expert like Bruno santos if i were you Last edited by esujby; March 7, 2017 at 16:22. 

March 13, 2017, 06:12 

#8 
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Fabien Salmon
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Well I think you are right concerning the calculations of each parameter.
But my last question is what is the difference between Qin and G ? They have similar names but it is not the same thing. What is the power received by the wall for example, Qin or G ? Thank you for your responses. 

March 13, 2017, 15:18 

#9  
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nasir musa yakubu
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they are completely different!!!!
if you try to run a simple case, you will realize that Qin is only calculated at the boundary and G is calculated on the boundaries and throughout the region itself. Qin is the energy generated at the surface. for example, if you have a black body under the sun, Qin will be the heat flux on the intercepting surface, which will be conducted by the regions bellow the surface. in practical terms, Qin is calculated based on absorptivity value. G, like I said earlier, is: Quote:
I will confirm all these details with an expert like Bruno Santos and a few others. 

April 9, 2017, 22:09 

#10  
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nasir musa yakubu
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Hi,
i am wondering where you got this from? is it based on your results? Quote:


April 10, 2017, 04:07 

#11 
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Fabien Salmon
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Hello,
here: https://www.sharcnet.ca/Software/Flu...ug/node577.htm It is the user guide of fluent. We use the same boundary condition (Marshak) in OpenFOAM so I guess G is the same for both softwares. 

November 24, 2017, 10:30 

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In my opinion the calculation of G in the fvDOM model is wrong. At our institute we developed our own and improved view factor model and work with it quite a lot. We also calculate and write the Gfield there and so I compared both models in a test case. I found out that the Gfield differs exactly by a factor of 4. I digged in the source code and found the calculation of G:
Code:
G_ += IRay_[rayI].I()*IRay_[rayI].omega(); Do you agree with me and should I write a bug report? I never did this so far 

November 24, 2017, 11:02 

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Fabien Salmon
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November 24, 2017, 11:39 

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Hi,
thanks for your quick reply. So far, I just have seen G with the definition in the book, I have never seen G defined by I*4*Pi. When I look at the source term for the fluid, you seem to be right as the emission term is and not . This probably also confuses some other people, thanks for pointing out the difference. 

November 24, 2017, 11:51 

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Fabien Salmon
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November 24, 2017, 11:56 

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Exactly. It is due to the fact that G is used for the energy balance of a single ray and not of a physical surface. The programmer redefined it a little, but for me knowing G from my lectures and books, it was really confusing.


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