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CFD Analysis with DO Radiation

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Old   April 29, 2013, 13:15
Default CFD Analysis with DO Radiation
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Hi All,

I'm a new member of the forum and an engineering student working with CFD for the first time. I'm new to this so please be patient with me. I work with ANSYS 14.0.

My task is to model a lamp in a duct/box and establish a radiation field with some flow going through this contained space. I've already setup my geometry with inlets and outlets with the lamp in the middle and solved the velocity profiles. My problem is then working with Discrete Ordinate Radiation (DO) emanating from the lamp.

I've tried looking at some manual for basic DO radiation application and followed it quite closely. I found though that my final radiation field looks like a flower (bare with me..). This is very strange as the lamp shape I used is circular so I expected the radiation field to be circular as well. I should probably post some pics.. ** I will do that soon..**

Is there something wrong that I did? Is this a common problem with DO model?

Thanks!
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Old   April 29, 2013, 13:38
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Hi. I think this phenomenon is usually referred to in the literature as "ray effect". Try increasing the order of the angular discretization. Could you post a picture of the radiation field and the mesh?

Cheers,
Michujo.
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Old   April 29, 2013, 15:18
Default Pictures of Situation
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Hi. I should have posted pictures with my post. Appologies. Here are some screenshots. Thanks for any help!

Edit: I will look up "ray effect" and try adjusting the angles. Thanks.
Attached Images
File Type: jpg Velocity Profile Vector.jpg (24.7 KB, 27 views)
File Type: jpg Boundary Condition.jpg (14.4 KB, 24 views)
File Type: jpg Boundary Condition 2.jpg (13.9 KB, 24 views)
File Type: jpg Radiation Contour.jpg (13.6 KB, 27 views)
File Type: jpg Radiation Contour 2.jpg (15.5 KB, 25 views)
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Old   April 29, 2013, 15:29
Default mesh and geometry
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Hi I've also got screenshot of my mesh and geometry. Spent a long time on this. I think its good .

Basically, I have a face mapped projected from one side to another with some inflation near edges where things might get tricky. Let me know if you think it can be improved!

Thanks
Attached Images
File Type: jpg Geometry.jpg (14.9 KB, 17 views)
File Type: jpg Mesh1.jpg (35.3 KB, 24 views)
File Type: jpg Mesh2.jpg (43.2 KB, 23 views)
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Old   April 29, 2013, 16:36
Default Increased Angle of Discretization
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Hi, Sorry for the many posts. Last post of the day I promise.

So, I increased the angle of discretization to 8 by 8 (Theta and Phi) and the problem still persists. **defaults were 2 by 2** I tried increasing the pixel as well to 3x3 ** defaults were 1x1** and it made the solution look a bit better but still really off.. (as in its not circular) I am running out of ideas of what to try next

I also can't really find any good information on "ray effect". Could anybody kindly point me in the right direction? Thanks!
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Old   April 30, 2013, 09:01
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Hi, I am not an expert on radiation modelling but I think that the ray effect can be mitigated if you come up with a mesh that is somehow aligned with the radiation field. From a radiating cylinder I would expect radiation to propagate along lines of theta=constant.

Could you try building an axisymmetric mesh around your cylinder? Your geometry is not cylindrical so you might not be able to extend it all the way to the walls of your rectangle, but you could do the following:

1) Cut a cylinder off your cube, concentric with your cylindrical lamp. Try getting the cylinder boundary to be as close as possible to the rectangle walls.

2) Put the cylinder and the rest of the cube in the same part in Design Modeler so that you get a conformal mesh.

3) Build an axisymmetric mesh on the cylinder. you can define "Size edges" at the inner and outer boundaries of the cylinder with the same number of elements so that you get mesh lines with theta=constant.

4) Mesh the rest of the rectangle. Hopefully the mesher will build a smooth transition from the cylinder to the rectangle walls, if not try reducing the size of the cylinder.

5) Sweep the face mesh to get the 3D mesh.

I drew a picture of the mesh I'm talking about (it's awful I know).


Also here are some references you might want to have a look at about the ray effect:

http://www.tandfonline.com/doi/abs/1...9#.UX-7kaJFDdU

http://sfera.sollab.eu/downloads/Sch...OM_FVM_MCM.pdf

http://uwspace.uwaterloo.ca/bitstrea...assanzadeh.pdf


Cheers,
Michujo.

P.S: If there's a radiation expert out there please help.
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File Type: jpg cylmesh.jpg (39.3 KB, 18 views)
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Old   April 30, 2013, 11:44
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Thanks Michujo. I really appreciate all this help.

I'll go through these papers thoroughly. I had a sinking suspicious feeling inside that the mesh was the problem. I will confirm if that is the case today. Will post update of new mesh if problem was solved. Thanks!

Update:
So, I've taken your advise and created an asymmetric mesh around the cylinder, sweep it, everything's looking good. Solved for velocities no problem. Tried the radiation and this time I got a different shaped "flower" if you will.. **I've worked on many iterations of modifications to try and fix it to no avail** .. ..

I also have some trouble understanding the behavior I'm observing when I change the radiation intensities.. Read the help files and still confused :S

Sometimes FLUENT refuses to solve for incident radiation.. I checked all the absorption coeff and its 0. so.. It isn't being absorbed by anything.. where did all that radiation go??

If there are experts out there, please help! Thanks!
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File Type: jpg Square Radiation Contour.jpg (22.7 KB, 14 views)

Last edited by ChaChaLaLa; April 30, 2013 at 16:02.
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Old   May 1, 2013, 12:07
Default Success
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Playing around with the mesh a little more. I managed to get just the profile I was looking for. Hooray! Here is a pic.

Thanks for all the input.
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File Type: jpg Success.jpg (40.2 KB, 20 views)
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