Wall function's solving procedure.
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Hi guys,
In this bounch of wall functions, we have this equation: see the image. just like it said: Provided U_p and Y_p is know, my problem arises, How can I know U_p? Y_p is desided by the mesh, but U_p? And I try to calculate this manually, via U_p, I can have tau, k, epsilon all of them, But all of them's value is up to U_p. so anyideas? Thanks in advance. 
It is the value in the first cell near the wall, taken at the previous iteration.

wall function calculating procedure
i am sorry to disturb.
i have a question ,is it wall function only adopteded to the first cell layer ?i mean from the second layer ,begin to solve the ke equation ,am i right ?i do really appreciate if you can give me some light . 
The goal of wall functions is to provide approximate boundary conditions which would enable the resolution of viscous wall bounded flows without having to resolve the thin viscous layers at the wall.
As such, this boundary conditions are obviously applied only to the first cell near the wall. This, at least, in general. There are specific models where a different set of equations is solved over an extended region over the wall. Hence, in a generic sense, the wall model is applied to more than one cell layer. But the philosophy, i think, is also different 
At last, I think I know how it deal with the solving precedure. At first, you should solve Ueqn using the initial k,u,p. Then you get the velocity field. Solving the pressure equation, you can get the pressure field.
Next, solving the k and epsilon equation. In this step, you have already the U_p; solve these equation and using the wallfuntions. You can get the k and epsilon. If I was wrong correct me. Thanks. 
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