[peterhess]

Well, that will take a while

Are the dimenions of the case such small?

You have 0.0005 X 0.0005 X 0.0005 m³

Each side has just 0.5 mm length!

Is it realy such small?

Are you using OF7.0?

Regards

Peter

[Raza Javed]

Well, that will take a while

Originally Posted by peterhess

Are the dimenions of the case such small?

You have 0.0005 X 0.0005 X 0.0005 m³

Each side has just 0.5 mm length!

Is it realy such small?

I am sorry I was re-scaling it in Allrun, that's why it became so small. Now I removed that line from Allrun script.

Originally Posted by peterhess

Are you using OF7.0?

I am using OpenFOAM 4.1.

[peterhess]

Here is your case in OF7.0

Regards

Peter

[Raza Javed]

Originally Posted by peterhess

Here is your case in OF7.0

Regards

Peter

Thank you so much for your help.


I have one general question here:


If I have two regions in my geometry, just like I have Now, (one is solid and one is air).


And I am putting radiations on Solid region using viewFactor radiation model. and those radations will radiate from solid to air.


Do I need to put radiation model also on air region to accept the radiations coming from the solid region?



Thank you

[peterhess]

Originally Posted by Raza Javed

If I have two regions in my geometry, just like I have Now, (one is solid and one is air).


And I am putting radiations on Solid region using viewFactor radiation model. and those radations will radiate from solid to air.

No!

Activating the radiation in Solid calculates the radiation in solid. i.e. inside the solid!

Anyway you can try it in the last model I uploaded by activating the radiation in Solid and deactivating it in Fluid ans see what happens...

Originally Posted by Raza Javed

Do I need to put radiation model also on air region to accept the radiations coming from the solid region?

There is no radiation comming from Solid to Fluid.

There is a radiation coming from the Fluid-Side of the interface between the Solid and Fluid.

If you delete the solid region and defined the same temperature on the Fluid-Side of the interface, you will get the identical results!

Activating the radiation in a region lets Radiation in this region calculated.

Viewfactor method does not calculates the interaction between two (or more) regions. The only method for my poor knowledge that is able doing that is FVDOM!

Example: The radiation from a LED in a car head lamp through the polycarbonate front cover.

Anyway, I dont know if OF is able to do that for "Semi transparent medium".

Regards

Peter

[Raza Javed]

Originally Posted by peterhess

No!

Activating the radiation in Solid calculates the radiation in solid. i.e. inside the solid!

Anyway you can try it in the last model I uploaded by activating the radiation in Solid and deactivating it in Fluid ans see what happens...



There is no radiation comming from Solid to Fluid.

There is a radiation coming from the Fluid-Side of the interface between the Solid and Fluid.

If you delete the solid region and defined the same temperature on the Fluid-Side of the interface, you will get the identical results!

Activating the radiation in a region lets Radiation in this region calculated.

Viewfactor method does not calculates the interaction between two (or more) regions. The only method for my poor knowledge that is able doing that is FVDOM!

Example: The radiation from a LED in a car head lamp through the polycarbonate front cover.

Anyway, I dont know if OF is able to do that for "Semi transparent medium".

Regards

Peter

I think I am completely out of track.


what I am doing is the following.


I put viewFactor radiation model in the Air region. and OpaqueSolid model in the solid region. And I am putting the temperature of 500K at the solid region.


Now as we know the temperature on the solid is 500K, and the temperature of the air is 300K, and I put the velocity (U) of the air equals to zero, so I am expecting to see the heat transfer through radiation from solid to air. I think this is wrong approach?

And what happened is that I saw the heat transfer in paraview, but when I removed the radiations from the simulations, the results remained the same. It means that the radiations are not working.


My only focus is, to simulate the radiations coming out of solid(because it is HOT), into the air. Just like we have heaters in our home.


And I am unable to decide, which radiation model will work in this case.


Thank you

[peterhess]

For sure the simulations with and without radiation are not identical!

They seams to be identical, but the wall heat flux shows the difference...

Because you have constant walls temperatures:

- The temperature fields will be identical

- The very weak velocity field will be also identical because the very weak free convection fields is getting its buoyancy from identical temperature fields

- The density is also identical, cause everywhere an identical temperature fields govers

- The pressure is also identical because the temperature and density are identical (perfect gas low)

Calculate the heat flux from the solid to the fluid (or at the walls of the fluid) with and without the radiation and you will find the difference.

Alternativly you could change the constant temperature of the solid to constant heat generation.

In this case the temperature of the solid/fluid interface will be much higher without radiation in comparing with radiation!

Regards

Peter

[Raza Javed]

Originally Posted by peterhess

No!

Activating the radiation in Solid calculates the radiation in solid. i.e. inside the solid!

Anyway you can try it in the last model I uploaded by activating the radiation in Solid and deactivating it in Fluid ans see what happens...



There is no radiation comming from Solid to Fluid.

There is a radiation coming from the Fluid-Side of the interface between the Solid and Fluid.

If you delete the solid region and defined the same temperature on the Fluid-Side of the interface, you will get the identical results!

Activating the radiation in a region lets Radiation in this region calculated.

Viewfactor method does not calculates the interaction between two (or more) regions. The only method for my poor knowledge that is able doing that is FVDOM!

Example: The radiation from a LED in a car head lamp through the polycarbonate front cover.

Anyway, I dont know if OF is able to do that for "Semi transparent medium".

Regards

Peter

Thank you so much for your reply.

From your this post, I understood the following:

1. If I am putting radiation (for example: viewFactor model) in one region, then it just calculates radiation inside this region, not outside this.

2. If I want to see radiations from one region to the other, then I should use fvDOM radiation model.

Please let me know, If I understood wrong.

Thank you

[peterhess]

Originally Posted by Raza Javed

1. If I am putting radiation (for example: viewFactor model) in one region, then it just calculates radiation inside this region, not outside this.

Yes!

Every region has its own viewFactorDict that just calculates the viewFactor inside it.

There is no viewFactorDict for regions exchange.

If Exchange between the regions to be recognized, then fvDoM must be used.

By the way, you can not calculate the radiation in one region and turn it off in the other!

The "other" region, in our case Solid MUST also turn the radiation on, but it as opaqueRegion to be defined!

That is why opaqueRadtiation is at all exists as a radiation model...

Like that the region with viewFactor calculate the radiation in it and the other region with opaque namely does not..

Originally Posted by Raza Javed

2. If I want to see radiations from one region to the other, then I should use fvDOM radiation model.

Yes!

Regards

Peter

[Raza Javed]

Originally Posted by peterhess

Yes!

Every region has its own viewFactorDict that just calculates the viewFactor inside it.

There is no viewFactorDict for regions exchange.

If Exchange between the regions to be recognized, then fvDoM must be used.

By the way, you can not calculate the radiation in one region and turn it off in the other!

The "other" region, in our case Solid MUST also turn the radiation on, but it as opaqueRegion to be defined!

That is why opaqueRadtiation is at all exists as a radiation model...

Like that the region with viewFactor calculate the radiation in it and the other region with opaque namely does not..



Yes!

Regards

Peter

Thank you so much for your reply.

One more question please, can we also use P1 radiation model in our case?

because I checked one paper and in that, the geometry was closely related to our geometry, and he is using P1 model.

In case if you want to have a look, the link is given below:

http://www.tfd.chalmers.se/~hani/kur...Foam_final.pdf

Thank you

[peterhess]

Originally Posted by Raza Javed

can we also use P1 radiation model in our case?

I can not comment that cause I do not have sufficient experience using P1.

Regards

Peter

[Raza Javed]

Originally Posted by peterhess

I can not comment that cause I do not have sufficient experience using P1.

Regards

Peter

Hi.


I have one general question, how can we see that our radiation is working?


because I have run the same case, with radiation and without radiation, but it looks exactly the same when I see temperature in Paraview.


I don't know how I can check that my radiation is working or NOT?


And one more question, Is it necessary to use topoSet to make cellSets when using view Factor radiation model?


Thank you

[peterhess]

Originally Posted by Raza Javed

because I have run the same case, with radiation and without radiation, but it looks exactly the same when I see temperature in Paraview.

I don't know how I can check that my radiation is working or NOT?

Because the temperature at walls in both simulations are identical/fixed, you will not recognize a difference in Termperature field.


To see the difference with and without radiation, you need to calculate the heat flux leaving the solid and resieved by the outer walls of fluid.

I told this way before...

To calculate the wallHeatFlux leaving the interface (Fluid/Solid) and recieved by the outer boundaries of the fluid you need to use the utility:

WallHeatFlux

https://www.openfoam.com/documentati...lHeatFlux.html

Originally Posted by Raza Javed

And one more question, Is it necessary to use topoSet to make cellSets when using view Factor radiation model?

The topoSet has nothing to do with the solving or calculation of the viewFactors.

topoSet just tells which cells belongs to which region and which boundaries belongs to which region.

Regards

Peter

[Raza Javed]

Originally Posted by peterhess

Because the temperature at walls in both simulations are identical/fixed, you will not recognize a difference in Termperature field.


To see the difference with and without radiation, you need to calculate the heat flux leaving the solid and resieved by the outer walls of fluid.

I told this way before...

To calculate the wallHeatFlux leaving the interface (Fluid/Solid) and recieved by the outer boundaries of the fluid you need to use the utility:

WallHeatFlux

https://www.openfoam.com/documentati...lHeatFlux.html



The topoSet has nothing to do with the solving or calculation of the viewFactors.

topoSet just tells which cells belongs to which region and which boundaries belongs to which region.

Regards

Peter

Thank you so much for your reply.


I am sorry I am asking you stupid question, but I didn't get how to use wallHeatFlux in my simulation?


Where do I need to mention it?


Thank you

[peterhess]

Wall heat flux in OF 4.1

Regards

Peter

[MatteoQ]

Originally Posted by peterhess

To reduce calculation time of radiation you are able to reduce the number of faces that calculate the radiation.

Suppuse you have a surface with 120 cells. i.e. 120 faces.

Instead of recognize every face as a sparate face, you can say:

nFacesInCoarsestLevel 10;

In this case the viewFactor-Generator will collect every 12 faces (120/10) as one face (or called patch). The temperature of the patch is the avarage temperature of the 12 faces included and the viewFactor is calculated and applied one time for one patch instead 12 times.

For sure the 10 patches in this case are collecting faces that are bounded or neighboured to each other as a patch.

If You increase to:

nFacesInCoarsestLevel 120;

Then every surface is one patch!

As higher the number as more accurate the results. But also as more calculation time needed.

By the way it is a good idea to apply renumberMesh utility before calculating the viewFactor.


And now to:

featureAngle 10;

During viewFactor calculation every patch is emitting rays in all directions and see which target patch is recieving more.

If patch-1 sends 10 rays and patch-3 recieve 6 of the 10 then the view factor between patch-1 and patch-3 is 0.6!

As smaller the featureAngle as more rays will be emitted. Or as higher the resolution of the emitted rays.

In this case:

featureAngle 10;

Means every 10 degrees emitte one ray!

As smaller the value as exacter the calculation.

Peter

Thanks a lot this is very useful to know! Can you please tell me where did you find those information? I searched for keywords like "angle" and "feat" in the shootRays.H, viewFacotrsGen.C and faceAgglomerate.C files but I couldn't find anything relevant. My C++ skills are not the best so I might have missed something.

Furthermore, when I change the featAngle the rays seems that they do not change, I attached two figures, the first one with featAngle 1 and the second with 180, they are the same, I don't understand why.

The explanation you gave about the faces is confirmed by the last picture, there is a cube with 36 faces, I used nFacesInCoarsestLevel 20, so 36/20=1.8 and indeed the faces are gatherd 2 by 2.



Still I cannot visualize in paraview what concerns the rays.


Regards

Matteo

[peterhess]

Hello MatteoQ!

And sorry for the late answer.

Originally Posted by MatteoQ

Can you please tell me where did you find those information? I searched for keywords like "angle" and "feat" in the shootRays.H, viewFacotrsGen.C and faceAgglomerate.C files but I couldn't find anything relevant. My C++ skills are not the best so I might have missed something.

Well the source of my informations is actualy the help of Ansys-Fluent...

The S2S (surface to surface) Radiation method is very good described there.

See Clustering:

http://www.afs.enea.it/project/neptu...th/node116.htm

Originally Posted by MatteoQ

Furthermore, when I change the featAngle the rays seems that they do not change, I attached two figures, the first one with featAngle 1 and the second with 180, they are the same, I don't understand why.

Well it depends how you validate the difference!

If you compared the size of the F file in the region, where the view factor is calculated, between different values for the angle, you will find that the size is identical.

That is because the number of patches is identical.

Anyway, if you compared the values inside the F file for different angles, you will notice that they are slightly different!

Originally Posted by MatteoQ

Still I cannot visualize in paraview what concerns the rays.

I can not comment that because I did not made any visualisation for the rays bevore

[MatteoQ]

Thanks again for you answer ,

I will have a look inside the F files to observe the changes you mentioned and I will investigate deeper in the openfoam codes to see if I find something.


Your answers have been very usefull to me.


Regards


MatteoQ

[Raza Javed]

Originally Posted by peterhess

Here is your case in OF7.0

Regards

Peter

Here are the images...

[peterhess]

Well, you have vacuum!!!

The values on the scale are absolute values.

Take a look to rho and you will see that rho in paraFoam is very low!

rho has an avarage value of 6e-5.

Regards

Peter