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 
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. 
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 
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. 
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 
Re: Skin Friction Coefficient
Ok thanks,
so do i have to understand that fluent calculates this wall shear stress (tauw,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. 
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. 
Re: Skin Friction Coefficient
THanks, i know but i have to...

skin friction coefficientreference value
Hi.
I am working on a channel with periodic variable crosssection and i want to calculate the mean skin friction coefficient on this channel. I tried AreaWeighted 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! 
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