Radiation Target Body

[Luve]

Hello

I am looking for an explanation for the entry
Radiation Target Body = -1.
What does "-1" mean in the example :
"Heat Equation - 2D - Axi Symmetric Steady State Radiation". I did not find
any explanation in the tutorial as well as in ElmerModelsManual. What is the
influence of another number in "Radiation Target Body" ?

With kind regards

Luve

translate with deeple

[raback]

Hi Luve,

I updated the documentation to clarify this:

"This flag may be used to set the direction of the outward pointing normal. This is used when computing viewfactors. If the value of emissivity is given in the Material section then the boundary then the normal is assumed to point from the material having the property outwards. If the value of emissivity is given in the Boundary Condition section then the direction of normal is ambiguous. This keyword may then be used to give the direction of the normal by specifying the material to which the normal points to. Value -1 means that the normal is pointing outwards to non-existing material (this is also the default). Hence, this keyword should be given on internal ambiguous boundaries or on external boundaries where we are radiating into the domain."

-Peter

[Luve]

Thanks Peter

that helps me.
May I ask one additional question? Is the data in the ViewFactors.dat file just the Gebhardt factors or a multiplication Gebhardt factors times area ?

With kind regards

Luve

translate with deepl

[raback]

Hi Luve,

No. The view factors present the area fraction seen by each element while the Gebhardt factors present the fraction of light from each element that reaches the other elements considering the infinite (diffuse grey) reflections. Only when emissivity is exactly one everywhere are these the same.

Nowadays the Gebhardt factors are computed internally since if the emissivity is temperature-dependent the computation of Gebhardt factors must be updated with the solution. Otherwise Gebhardt factors could be computed as a preprocessing step. The solution of Gebhardt factors is rather costly since NxN system requires N solutions of linear systems. Unfortunately both the View factor and Gebhardt factor computations scale poorly with problem size.

-Peter

[Luve]

Thank you Peter

that was a wonderful answer.
Good luck for the future!

With kind regards

Luve