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-   -   Skin Friction Coefficient (http://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.

Thanks in advance

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...

thanks in advance.

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??

Thanks in advance!

LuckyTran March 14, 2016 00:14

Quote:

Originally Posted by ArmanCFD (Post 589480)
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??

Thanks in advance!

The reference velocity and density are determined from the definition of the coefficient. Use the same velocity and density as the one from your definition of skin friction coefficient. Different people use different references, so use the values that is consistent with your definition.


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