[Trixalian]

Hello,


I am a new user of fireFoam (for 3-4 month) and I am trying to verify some results with basic cases of radiation.


The case is the following :
- An infinite (obtained usig symmetryPlane as boundary condition foar the side of the domain) hot plate at 2000K considered as a blackbody()
- The rest of the space is at 200K(for limitation consideration according to JANAF tables)
- The medium is non-participating (emission and absorption = 0)


In this case the analytical result of G, the incident radiation intensity, is a constant and equal to 907.2kW/m² =
Using the fvDOM radiation model on fireFoam solver to do the radiation simulation, the result obtained is a constant but equal to 1814.8kW/m². So twice the result we should have. I Dont know where this difference comes from...

Anyone have any idea?
Thanks !

[harshin]

Hi Did you figure this out? If yes could you explain what happened?

[Trixalian]

Hi,

Yes I found the answer. I just misunderstood the meaning of G. It is not the same as the heat flux recieved by a cell or emmited by a plate
It is analytically : and discretised in fireFOAM :. In that peculiar case its value is .

Hope it helps !

[harshin]

Hello,

Thank you very much for the explanation.I found the magnitude G much higher that qr on solar radiation tutorial examples in openfoam. qr is about 1000 W/m^2 which makes sense but G is about 4 times that value. The surfaces involved in the simulation is mostly soil ground which is not very reflective.

[harshin]

I will just leave this thread for anyone ending up reading this and need more insights.

Difference between IDefault and G (radiation)

[Trixalian]

First are you using FVDOM model or P1 model ?

Originally Posted by harshin

Hello,

Thank you very much for the explanation.I found the magnitude G much higher that qr on solar radiation tutorial examples in openfoam. qr is about 1000 W/m^2 which makes sense but G is about 4 times that value. The surfaces involved in the simulation is mostly soil ground which is not very reflective.

I don't know this tutorial case ... but if you are using FVDOM model, you might want to check the value of the solid angle in the main directions of radiation. The values of these might be non constant which might lead to errors... I have the same problem in a case of a cube of 1m x 1m x 1m with one heated side. An error is introduced due to the value of the solid angles. But i did not find solutions to that problem yet...

[dlahaye]

An error is introduced due to the value of the solid angles.

What is meant here? What kind of error? What solid angles?

[harshin]

Originally Posted by dlahaye

What is meant here? What kind of error? What solid angles?

Solid angle discretization used for fvDOM radiation model. It is specfied in radiationProperties file. I am not sure how they are non-constant as they said.

[Trixalian]

Originally Posted by harshin

Solid angle discretization used for fvDOM radiation model. It is specfied in radiationProperties file. I am not sure how they are non-constant as they said.

The solid angles are assimilated to the directions in space and proportion of space. It has a value that is non constant in certain case. For exemple in a cube of 1x1x1 with a mesh uniform on every direction, with the fvDOM parameters and , an extract of the log of the comptutation shows that the angles are :
fvDOM : Allocated 128 rays with average orientation:
...
I30 : dAve : (-0.0266904 0.039945 0.06942) : omega : 0.0851265 : d : (-0.308658 0.46194 0.83147)
I31 : dAve : (-0.00937243 0.0471184 0.06942) : omega : 0.0851265 : d : (-0.108386 0.544895 0.83147)
I32 : dAve : (0.02052 0.103161 0.06942) : omega : 0.127401 : d : (0.162212 0.815493 0.55557)
I33 : dAve : (0.058436 0.0874557 0.06942) : omega : 0.127401 : d : (0.46194 0.691342 0.55557)
I34 : dAve : (0.0874557 0.058436 0.06942) : omega : 0.127401 : d : (0.691342 0.46194 0.55557)
I35 : dAve : (0.103161 0.02052 0.06942) : omega : 0.127401 : d : (0.815493 0.162212 0.55557)
I36 : dAve : (0.103161 -0.02052 0.06942) : omega : 0.127401 : d : (0.815493 -0.162212 0.55557)
I37 : dAve : (0.0874557 -0.058436 0.06942) : omega : 0.127401 : d : (0.691342 -0.46194 0.55557)
I38 : dAve : (0.058436 -0.0874557 0.06942) : omega : 0.127401 : d : (0.46194 -0.691342 0.55557)
I39 : dAve : (0.02052 -0.103161 0.06942) : omega : 0.127401 : d : (0.162212 -0.815493 0.55557)
I40 : dAve : (-0.02052 -0.103161 0.06942) : omega : 0.127401 : d : (-0.162212 -0.815493 0.55557)
I41 : dAve : (-0.058436 -0.0874557 0.06942) : omega : 0.127401 : d : (-0.46194 -0.691342 0.55557)
I42 : dAve : (-0.0874557 -0.058436 0.06942) : omega : 0.127401 : d : (-0.691342 -0.46194 0.55557)
I43 : dAve : (-0.103161 -0.02052 0.06942) : omega : 0.127401 : d : (-0.815493 -0.162212 0.55557)
I44 : dAve : (-0.103161 0.02052 0.06942) : omega : 0.127401 : d : (-0.815493 0.162212 0.55557)
I45 : dAve : (-0.0874557 0.058436 0.06942) : omega : 0.127401 : d : (-0.691342 0.46194 0.55557)
I46 : dAve : (-0.058436 0.0874557 0.06942) : omega : 0.127401 : d : (-0.46194 0.691342 0.55557)
I47 : dAve : (-0.02052 0.103161 0.06942) : omega : 0.127401 : d : (-0.162212 0.815493 0.55557)
I48 : dAve : (0.0284025 0.142789 0.0287547) : omega : 0.150279 : d : (0.191342 0.96194 0.19509)
I49 : dAve : (0.0808836 0.121051 0.0287547) : omega : 0.150279 : d : (0.544895 0.815493 0.19509)
I50 : dAve : (0.121051 0.0808836 0.0287547) : omega : 0.150279 : d : (0.815493 0.544895 0.19509)
I51 : dAve : (0.142789 0.0284025 0.0287547) : omega : 0.150279 : d : (0.96194 0.191342 0.19509)
...
Total omega : 12.5664


These represent the directions (d) and the values (omega) of the solid angles. If you look at the values of the direction I40 and I50 they don't have the same value and this introduce an error on the computation results between the theoretical value and the computed one.