simulation roughness
 

hi friends
I am a doctoral student at the university. currently I imply CFD modeling in fluent. I spent a year in the mesh , could you help me to choose the best mesh for (y plus) and (dy first cell no mesh) for a turbulent simulation with number of Re = 6000 to 100000 to calculate the constant (B) of the log law logaritmic profile.
Vx / (V *) = 2.5Ln (y / k) + B.
with roughness effect artificial
1. Please explain to me how chosen y +, how chosen turbulence model (k epsilon , k-w, scalable or enhanced RNG or standart ....)
2. Can i use the same mesh to calculate the shear stress and the logarithmic profile and constant B.
I work on an annulare geometry two coaxial tubes with artificial roughness (if attached the diagram)
I found in the bibliography that to realize the logarithmic profile must y +> 30 I found (dy = 0.65 mm for y + = 30) but I could not have realized the mesh because the diameter of the experience is very small 8.235mm. and also that there are roughness teeth.
and on the other hand there is a contradiction.
To calculate the shear stress et u* friction velocity it is necessary to refine the wall but must also calculate the logarithmic profile y +> 30 (contradiction).
here is the diagram.
tell me in detail what I can do for y + and turbulence model.Attachment 74778