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-   -   Natural convection (Convergence) (http://www.cfd-online.com/Forums/fluent/104179-natural-convection-convergence.html)

 Javieryrazusta July 4, 2012 07:04

Natural convection (Convergence)

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!

 flotus1 July 4, 2012 08:32

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?

 Javieryrazusta July 4, 2012 09:11

1 Attachment(s)
Quote:
 Originally Posted by flotus1 (Post 369746) 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!

 flotus1 July 4, 2012 10:21

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.

 Javieryrazusta July 4, 2012 13:43

3 Attachment(s)
Quote:
 Originally Posted by flotus1 (Post 369762) 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:

 flotus1 July 4, 2012 14:01

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.

 Javieryrazusta July 5, 2012 09:06

Quote:
 Originally Posted by flotus1 (Post 369804) 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.

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