[romant]

When you take a look at the link that is posted with this explanation, you can see that the linear interpolation (which is the only one available in a CFD code) is used to insert the logarithmic form for the wall.

[kimyusik]

Originally Posted by EnricoA

Hello romant,

I wanted to ask you why the velocity derivative used to compute the wall shear stress is approximated with a linear function, when the velocity distribution is known to be logaritmic. Don't you think that this lowers too much the accuracy of the simulation?

Regards,
Enrico

Dear Enrico,
Thanks for raising this question. I had the same question. In my opinion, the linear relation,

(mut+mu)du/dy = (mut+mu)Up/yP

is not valid in the log-region (y+>30). This relations is used in openfoam, the wiki (http://www.cfd-online.com/Wiki/Two_e...wall_functions) and also Bredberg, J. (2000), On the Wall Boundary Condition for Turbulence Models, Chalmers University of Technology Goteborg, Sweden.


The purpose of the wall modelling is to provide an appropriate skin friction where the velocity gradient cannot be calculated using a linear relation.

So I think

nu_t = nu*( kappa*yplus -1)

would be more reasonable instead of

nu_t = nu (yplus * kappa/ln(E*yplus) -1 )

for the standard wall function.

Please correct me if I am wrong.

Yusik

[romant]

Originally Posted by kimyusik

Dear Enrico,
Thanks for raising this question. I had the same question. In my opinion, the linear relation,

(mut+mu)du/dy = (mut+mu)Up/yP

is not valid in the log-region (y+>30). This relations is used in openfoam, the wiki (http://www.cfd-online.com/Wiki/Two_e...wall_functions) and also Bredberg, J. (2000), On the Wall Boundary Condition for Turbulence Models, Chalmers University of Technology Goteborg, Sweden.


The purpose of the wall modelling is to provide an appropriate skin friction where the velocity gradient cannot be calculated using a linear relation.

So I think

nu_t = nu*( kappa*yplus -1)

would be more reasonable instead of

nu_t = nu (yplus * kappa/ln(E*yplus) -1 )

for the standard wall function.

Please correct me if I am wrong.

Yusik

It is true that in the log region, linearity cannot be used, however, you don't have any other values. Please take a look at Pope, Turbulent Flow, 2000, pp. 443, where the method on how to obtain this can be found.

Otherwise, there are some more threads in this forum (http://www.cfd-online.com/Forums/ope...tml#post484290) where you can find more about the implementation and why this is correct.

[Jalil786]

Originally Posted by kurne

Dear All
Got the answer thanks to all.

Can you please share your experience how you solved the issue. I have the same problem. I don't know which wall function to use, what would be the value of Ks and Cs in the nut folder.







Thank you in advance!