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-   -   Urgent! 2*Roughness Height >grid height (http://www.cfd-online.com/Forums/fluent/46987-urgent-2-roughness-height-grid-height.html)

rodi January 10, 2008 09:33

Urgent! 2*Roughness Height >grid height
 
I am modeling open channel flow(2D). The wall roughness height Ks=0.07m and the flow depth H=0.1m.

Frist question is:

According to Fluent User Guide, the distance from the wall to the centroid of the wall-adjacent cell should be greater than Ks, implying my first cell height should be 2*Ks=0.14m,which is larger than the flow depth H=0.1m!It's impossible to mesh it!

Second:

For i use the K-e turbulence model.If i choose the Enhanced Wall Treatment,which require the Y+ <5.The first cell height will be almost 0.0001m.It's inconsistent with the first question,too.

What should I base my cell height on? Please help!

Thanks in advance!rodi

red lemon January 14, 2008 18:37

Re: Urgent! 2*Roughness Height >grid height
 
roughness height larger than 1st cell height is meaningless. Use standard wall functions and Y+=30-100 range. A wall roughness of 0.7m is not valid for the empirical data (nikuradse) where k/Dh is massive. You need to model explicitly.

rodi January 15, 2008 08:56

Re: Urgent! 2*Roughness Height >grid height
 
Thanks for your reply~!

The roughness height(Ks) is computed from the constant(n) in Manning's formula. In my case, n=0.025 and there is a empirical formula : 1/n=17.72*lg(11.89/Ks), then Ks=0.066m

As the User's Guide says,make sure that the distance from the wall to the centroid of the wall-adjacent cell is greater than Ks. 1st cell height should >0.132m (2Ks), at the same time, Y+ will out the range of (30,100).It looks like incompatible!

Could you help me?

Best regards´╝ürodi



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