[medguedem]

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

[LuckyTran]

Just go for the finest mesh you can practically afford to do in the time you have to carry out your study.



You are confusing the inner coordinate system y+ with the y+ of the wall-adjacent cell that is needed for wall boundary conditions and wall modeling. Unfortunately, this second y+ (which is your dy of first cell) is commonly misunderstood as being y+.


Forget what turbulence model you are using. Let's say you make a mesh and pretend you did a DNS. Whatever your mesh, you get a velocity field. Each cell will have a y+ and you will have a velocity profile. Whatever the velocity profile you can calculate the wall shear stress. Whatever the profile, you may have a certain number of points in each range of y+. For coarse meshes you may have no cells in the logarithmic layer. For medium meshes you may have some cells in the logarithmic layer and none in the linear layer. For finer meshes you'll have cells in the linear and logarithmic layer.


You can mechanically calculate B for any mesh. Check out Nikuradse's work if you want detailed examples. Of course, better mesh resolution generally means your solution is more accurate.


So how small of cells do you want? That depends on your desired modeling approach. Certain modeling approaches requires different wall y+. But nearly all of them benefit from having very small cells such that the y+ of the wall adjacent cell is ~1. Certain (obsolete) high y+ wall function modeling approaches are only valid in the logarithmic region and require meshes with no cells in the lower y+ range. They are obsolete because nowadays all the approaches are blended low y+ and high y+ wall functions (i.e. all y+ wall functions) valid for any y+.

[medguedem]

thank you so much LuckyTran
I want advice
the purpose of my work is to calculate the friction constant B of logarithmic profile
Vx / (V *) = 2.5 Ln (y / k) + B.
1.for a turbulent simulation with number of Re = 6000 to 100000 i need calculate the constant (B) of the logarithmic profile of the law of log.
the problem is that after Re = 50000 the simulation diverges.
the channel diameter is 16.23mm and the reynolds numbers are high.
2.I want exactly the best model (k epsilon , enhanced scalable, omega sst )
and how much do I divide the mech 2d ou 3d.