# Natural convection (Convergence)

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 July 4, 2012, 07:04 Natural convection (Convergence) #1 New Member   Join Date: May 2012 Posts: 11 Rep Power: 7 Hi! I am studying Natural convection between a steel cylinder to -12.5 ° C (boundary condition:wall) and the atmosphere at standard conditions (pressure outlet boundary condition). The mesh is in 2D. I am doing the simulation in Steady and using air as Boussinesq with a thermal expansion coefficient value of 0.00333. As the Rayleigh number is greater than 10^8 I lowered the gravity and I used the first order scheme. This way I get a convergent solution. I am slowly going to approach the gravity value of 9.81, but when the gravity is 6.5 , I donīt obtain converged solution. I donīt Know where is the mistake. Any idea what is wrong? As I can solve it? Thank you very much!

 July 4, 2012, 08:32 #2 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,534 Rep Power: 25 Could you describe the computational domain (a picture would be nice)? Do you use the pressure outlet boundary condition for all exterior boundaries? What does your mesh look like and in which range are the Y+ values? Last edited by flotus1; July 4, 2012 at 08:51.

July 4, 2012, 09:11
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 Originally Posted by flotus1 Could you describe the computational domain (a picture would be nice)? Do you use the pressure outlet boundary condition for all exterior boundaries? What does your mesh look like and in which range are the Y+ values?
Hi,

I send a picture where you can see the boundary conditions.

Thank you very much!
Attached Files
 dominio.docx (76.7 KB, 36 views)

 July 4, 2012, 10:21 #4 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,534 Rep Power: 25 Did you use any prism layers for the mesh around the solid walls? These are crucial to achieve convergence especially when heat transfer is involved.

July 4, 2012, 13:43
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 Originally Posted by flotus1 Did you use any prism layers for the mesh around the solid walls? These are crucial to achieve convergence especially when heat transfer is involved.

Iīm sorry, but I donīt Know what prism layers are. Iīm learning to use the FLUENT and ICEM with a tutorial.

I created a regular mesh and after I used the command Ogrid in parts close to the walls.

You can see it in the next file:
Attached Files
 Dok2.docx (54.5 KB, 18 views) Dok3.docx (53.7 KB, 15 views) Dok4.docx (73.6 KB, 13 views)

 July 4, 2012, 14:01 #6 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,534 Rep Power: 25 The Ogrid creates the prism layers, the mesh looks ok. Although it could be refined close to the solid wall and coarsened further away, but this is not likely cause the convergence problems. If you want you can upload your file so I can have a closer look at it.

July 5, 2012, 09:06
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 Originally Posted by flotus1 The Ogrid creates the prism layers, the mesh looks ok. Although it could be refined close to the solid wall and coarsened further away, but this is not likely cause the convergence problems. If you want you can upload your file so I can have a closer look at it.
Iīve tryed to send the file, but I canīt send this type of file.

I donīt Know if the problem is to use "Pressure outlet" like boundary condition in the mesh to represent the atmosphere.

Thank you for all.

Last edited by Javieryrazusta; July 5, 2012 at 11:16.

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