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y plus and standard wall functions

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Old   August 14, 2012, 18:38
Default y plus and standard wall functions
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spk
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Hi guys,

i use k-ε with standard wall functions. The values of y+ are between 25 and 308. I tried different grids but i can't achieve the range that fluent suggests 30<y+<300. It will be better if i have a coarser mesh, in order to have a y+ range from 30 to 320, approximately?

Thanks a lot,
spk
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Old   August 14, 2012, 23:14
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According to FLUENT user Guide 14.0
"When wall functions are used, it is essential to avoid meshes with y+ values lower than ~30 as the wall shear stress and the wall heat transfer can and will seriously deteriorate under such conditions. For this reason, the usage of Enhanced Wall Treatments (to be selected for \epsilon-equation and default for \omega-equation based models) is recommended."

You don't have to worry if you use Enhanced Wall Treatments.
The interval 30<y+<300 suggested must be for the wall cells and not for all cells of your domain (I am not sure check for it).
If the interval 25 and 308 is for the wall cells of your domain and you really want to use k-ε with standard, you must redo your mesh.

In FLUENT you can use from the menu Adapt -> Yplus/Ystar and give the minimum and maximum allowed.
I hope it could help you instead of remeshing the entire domaine.

Last edited by Touré; August 15, 2012 at 00:47.
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Old   August 15, 2012, 02:39
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No matter which wall treatment is used or what the manual says, Yplus values of 300 are too high.
Even though the enhanced wall treatment should make the results independent of Yplus values in a certain range, it still works best for Yplus below 100.
The use of wall functions assumes a certain profile of velocities and turbulence quantities. This profile is quite the same in most turbulent flows for Yplus below 100. Further away from the wall, the profiles is no longer universal.
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Old   August 16, 2012, 23:26
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Hi spk,
with my experience, it's difficult to adapt y+ value to the range by remeshing. so you can solve it by using other type wall funciton equation and turbulent model, such as k-e turbulent model with scalable wall funciton, SST K-w turbulent model with standard wall function.
Hope that helps
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Old   August 17, 2012, 15:44
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For k-e model and enhanced wall function u need to have y+ below 10 near wall and rest of the mesh can have higher y+ it dont matter.
For that try to have minimum possible mesh spacing near the wall. It will help.
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Old   January 8, 2014, 04:48
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can someone please explain me what is exact definition of wall function and how it is applied in cfd
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Old   January 9, 2014, 07:27
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Hi anvesh,

Try this link for some basics.

http://www.computationalfluiddynamic...oundary-layer/

You can get more details in ANSYS CFD Turbulence modelling presentation if you have customer credentials.

Hope this helps
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Old   May 26, 2015, 17:10
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Quote:
Originally Posted by Guava Wang View Post
Hi spk,
with my experience, it's difficult to adapt y+ value to the range by remeshing. so you can solve it by using other type wall funciton equation and turbulent model, such as k-e turbulent model with scalable wall funciton, SST K-w turbulent model with standard wall function.
Hope that helps
Hi sir,

I have a equation:

Is there SST K-w turbulent model with standard wall function in Fluent?

I think that such a option is not in Fluent.

I am grateful that someone show me this option in fluent.

Thanks,

Best

Amir
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Old   May 26, 2015, 17:21
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Quote:
Originally Posted by Guava Wang View Post
Hi spk,
with my experience, it's difficult to adapt y+ value to the range by remeshing. so you can solve it by using other type wall funciton equation and turbulent model, such as k-e turbulent model with scalable wall funciton, SST K-w turbulent model with standard wall function.
Hope that helps
Hi sir,

Do you explain more about scalable wall function?

Where do this option in Fluent?

Thanks,

Best.

Amir
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Old   May 26, 2015, 18:36
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Hi everybody,

I am grateful that someone explain me about following question and If someone can, please place useful links about this equation.

What difference is between following options in Fluent?

-standard wall functions
-non-equilibrium wall functions
-Enhanced wall treatment
-User-defined wall functions

Thanks,

Best.

Amir
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