first node in turbulence model
I'm a beginner with CFD (CFX4.3): in dealing with the kepsilon turbulence model I've to set the first node at y+>11.Since y+ is a function of the unknown friction velocity how can I estimate the first grid position? Are there any empirical relationships which allow a crude estimate of the friction velocity? Thank you in advance for anybody helping me.

Re: first node in turbulence model
Yes, you can estimate the walldistance you need to get a certain y+. There is even a nice little free online calculator which will do it for you  you can find a link to it in the Resources/Calcs/Misc section of CFD Online.

Re: first node in turbulence model
In the manual you will see that you can set a USRSCAL (user scalar) to return the y+ values and plot it.
Regarding u*, for open channel for example you can use the crude assumption that a mean u* = sqrt(g*R*So) where g is the gravitational acceleration, R the hydraulic radius and So the energy gradient. 
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