# Urgent! 2*Roughness Height >grid height

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 January 10, 2008, 09:33 Urgent! 2*Roughness Height >grid height #1 rodi Guest   Posts: n/a 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

 January 14, 2008, 18:37 Re: Urgent! 2*Roughness Height >grid height #2 red lemon Guest   Posts: n/a 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.

 January 15, 2008, 08:56 Re: Urgent! 2*Roughness Height >grid height #3 rodi Guest   Posts: n/a 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！rodi

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