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-   -   Skin Friction Coefficient (https://www.cfd-online.com/Forums/fluent/36011-skin-friction-coefficient.html)

 FrenchyTim March 14, 2005 06:05

Skin Friction Coefficient

Hi everybody,

I'm a new fluent user and i'm now studying a turbulent flow along a skewed ramp in 2D ( kind of diffuser). I've found on fluent how to plot skin friction coefficient but i really don't know how fluent calculate this coefficient and, what does this coefficient depend on? Is there any references that i have to enter in fluent to calculate it? I don't know if i' ve been clear enough but don't hesitate to let me know wether you need more information.

TIM

 Karl March 14, 2005 07:37

Re: Skin Friction Coefficient

Skin Friction Coefficient is a nondimensional parameter defined as the ratio of the wall shear stress and the reference dynamic pressure Cf = tau_w/(0.5*rho_ref*vel_ref˛) where tau_w is the wall shear stress and rho_ref and vel_ref are the reference density and reference velocity defined in Reference Value Panel.

I hope this helps.

 FrenchyTim March 14, 2005 09:09

Re: Skin Friction Coefficient

Thanks a lot,

i can now see on what the skin friction coefficient depend. However, Does the wall shear stress depend on walls materials i defined in boundary condition? Is there a better model which takes in account or not this wall shear stress? Anyway i thankyou again and hope you ll enjoy your next fluent simulations..

TIM

 FrenchyTim March 14, 2005 09:38

Re: Skin Friction Coefficient

Hi, it s Tim again,

just to know, if it s possible... When you define your boundary conditions, and you select your wall, you have then to choose a shear condition between "No Slip", "Specified Shear" and "Marangoni Stress", so when you choose the "Specified Shear" one, you have to enter a X component and a Y component and that s where the probleme is, i don't know where i could find those components...so if yu've got an idea, don't hesitate to let me know. Thanks in advance...

TIM.

 dirk March 14, 2005 09:48

Re: Skin Friction Coefficient

The wall shear stress does not depend from the wall material, it's just proportional to the velocity gradient at the wall du/dy. You will have to use the "no slip" boundary condition.

Dirk

 FrenchyTim March 14, 2005 10:07

Re: Skin Friction Coefficient

Ok thanks,

so do i have to understand that fluent calculates this wall shear stress (tau-w,cf second message from Karl) in comparison to the inlet velocity?

Plus, for a turbulent flow, is there a rule to dertermine the velocity reference? Cause, when you fix at a coordinate X ref, it s easy to know reference pressure but what about the reference velocity for a turbulent flow...

 Dr. Chattopadhyay March 16, 2005 00:42

Re: Skin Friction Coefficient

You can set the values of reference length or velocity at a panel. So its upto you to define the reference value.

Pl. do not run CFD unless you are trained in basic Fluid Mechanics. It is a must to know the subject otherwise you miss almost everything. A CFD package, unlike other tools say excel or word, is not for everybody.

 FrenchyTim March 16, 2005 04:13

Re: Skin Friction Coefficient

THanks, i know but i have to...

 ArmanCFD March 13, 2016 15:12

skin friction coefficient-reference value

Hi.
I am working on a channel with periodic variable cross-section and i want to calculate the mean skin friction coefficient on this channel. I tried Area-Weighted Average from Fluent option but i got a wrong skin friction coefficient.

I guess this problem will be solved if i change the reference values (as i know this parameter depends on ref_velocity and ref_density) but i do not know what are the ref_velocity and ref_density??