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_ref²) 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 (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.

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 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!

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.

Hi.
I am working on a model with periodic variables; one of them is skin friction coefficient. And I cannot clearly figure out what the mean skin friction coefficient means. Does that simply reflect the movement of fluid relative to the surface? Does lower mean skin friction coefficient means smaller friction between the two objects? If not, would you please help figure it out?
Many thanks!