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STANDARD WALL FUNCTIONS vs ENHANCED WALL TREATMENT

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Old   February 22, 2013, 13:09
Default STANDARD WALL FUNCTIONS vs ENHANCED WALL TREATMENT
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Hi everybody,
I'm simulating with fluent the natural ventilation in a building, so I've realized a mesh of a cubic building with a window, which is surrounded by a parallelepiped representing the external air. I've had to refine the mesh in some zones, as those close to the building walls and to the parallelepiped basis. The mesh is that of the attached image.
I've a doubt about the near-wall treatment. First I've set k-epsilon RNG with standard wall function. Setting the Yplus of the building walls and the basis of the parallelepiped (which is considered as a rough wall) I've obtained Y+ include between about zero and a 700. Now, I don't know how to proceed, becouse I should coarsen the mesh in those zones where Y+ are less than 30 and refine the mesh whare Y+ are more than 300. Is it right?
I know that Fluent can just refine the mesh and not coarsen it. So, what can I do?
Thanks a lot.
Bests

Last edited by Roby_1986; February 24, 2013 at 11:52.
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Old   February 22, 2013, 18:31
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Hi,
I think you should refine the y plus in the near wall region. The manual in fluent says that too fine a y plus value for a standard wall function can deteriorate your results. They suggested a y plus value of 11.13 ( i think). That should fall in the log region in the law of the wall.
The way i would adapt my meshes in fluent is thru this way ( in the near wall region). Adapt> Yplus> specify value. Run simulation and check again.
Then i would adapt regions of high velocity gradients ( or shear layer)
Hope this helps!
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Old   February 23, 2013, 02:33
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Quote:
Originally Posted by Roby_1986 View Post
Hi everybody,
I'm simulating with fluent the natural ventilation in a building, so I've realized a mesh of a cubic building with a window, which is surrounded by a parallelepiped representing the external air. I've had to refine the mesh in some zones, as those close to the building walls and to the parallelepiped basis. The mesh is that of the attached image.
I've a doubt about the near-wall treatment. First I've set k-epsilon RNG with standard wall function. Setting the Yplus of the building walls and the basis of the parallelepiped (which is considered as a rough wall) I've obtained Y+ include between about zero and a 700. Now, I don't know how to proceed, becouse I should coarsen the mesh in those zones where Y+ are less than 30 and refine the mesh whare Y+ are more than 300. Is it right?
I know that Fluent can just refine the mesh and not coarsen it. So, what can I do?
Thanks a lot.
Bests
Use Scaleable Wall Funcitions aproach. Do a quick google search or consult Fluent's theory guide, you will learn how to activate them. Actually they prevent wall y+ to go below 30 in areas of high refinement.
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Old   February 23, 2013, 03:29
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Thanks for the replies.
Sincerely, I have a doubt. I'm studying the natural ventilation in a cubic building, through the presence of a huge parallelepiped of air surrounding it.
Now, where I've to do the control of the Y+ ?
As I said, I set the basis of my parallelepiped as a rough wall (representing the ground) and the walls of the cubic building (both internal and external) as smooth walls.
The Y+ control has to be done just for confined flow, so just for the internal walls of my building, or for all the walls of the model?
Thanks a lot
Bests
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Old   February 23, 2013, 14:30
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Fluent can't coarsen mesh more than its initial size. Suppose you start with a mesh, refine it in Fluent (Adapt => ...), then you can coarsen it back to its initial size, not bigger.

That said, you could start with a coarse mesh, this way you should get rid of the low y+ regions. Then you can refine the mesh in the regions where the y+ values are large. The disadvantage is that mesh refinement is costly: A hexahedron is split into 8 hexahedra and a tetrahedron is split into 8 tetrahedra.
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Old   February 25, 2013, 12:15
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Just to remind that caution is needed when modeling surface roughness with wall functions(no matter standard or enhanced). You may have mesh restrictions. Consult the chapter of boundary condition in the user's guide for some information.
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