[Onze]

For a turbulent flow in tube with circular section, how can i estimate the value of Y+ to ensure the mesh and adoption of wall functions. Especially, how to determine the friction velocity, or the wall shear stress?

p.s. The estimation of Y+ for boundary layer is clear now, but i don't know whether it is ok to use the same approach or relation in internal flow.

[oozcan]

y+ is dimensionless, it changes flow Reynolds number according to flow medium type (like internal flow in pipes and external flows around cylinder)

First you should define local reynolds number and then that boundary layer is into laminar or turbulent flow boundary layer.

then you should specify it if your model is in laminar boundary layer y+=1 or max 5

or otherwise you might as well define it as y+=30 ( if you are working on compressible flow, y+>30 or 60 for sure.)

then some formulation is available in websites by calculating y+ (you have defined) you will find out y (first layer thickness) then give it in your mesh model ,

[Onze]

Originally Posted by oozcan

y+ is dimensionless, it changes flow Reynolds number according to flow medium type (like internal flow in pipes and external flows around cylinder)

First you should define local reynolds number and then that boundary layer is into laminar or turbulent flow boundary layer.

then you should specify it if your model is in laminar boundary layer y+=1 or max 5

or otherwise you might as well define it as y+=30 ( if you are working on compressible flow, y+>30 or 60 for sure.)

then some formulation is available in websites by calculating y+ (you have defined) you will find out y (first layer thickness) then give it in your mesh model ,

Thanks for your reply. But i wonder if the formula for the calculation of friction factor Cf in external flow works well in internal flow too. One more question about your answer is the determination of local reynolds number, is it defined by friction velocity or average velocity for internal flow in pipes.

[oozcan]

cf is piece of cake (http://www.cfd-online.com/Wiki/Skin_...n_coefficient) you could use formula in top page of website)

https://en.wikipedia.org/wiki/Law_of_the_wall, you need to calculate manually y ( first layer thickness).That link help you out with.( you can choose any cf in first link)

You could combine all formulas ( second link and first link-top pag of website)

that is, mean velocity is u , u(subscript t) = friction velocity both terms are used in your calculation.

Kind Regards,