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Skin Friction Coefficient

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Old   March 14, 2005, 06:05
Default Skin Friction Coefficient
  #1
FrenchyTim
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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
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Old   March 14, 2005, 07:37
Default Re: Skin Friction Coefficient
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Karl
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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.

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Old   March 14, 2005, 09:09
Default Re: Skin Friction Coefficient
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FrenchyTim
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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
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Old   March 14, 2005, 09:38
Default Re: Skin Friction Coefficient
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FrenchyTim
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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.
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Old   March 14, 2005, 09:48
Default Re: Skin Friction Coefficient
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dirk
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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
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Old   March 14, 2005, 10:07
Default Re: Skin Friction Coefficient
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FrenchyTim
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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.
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Old   March 16, 2005, 00:42
Default Re: Skin Friction Coefficient
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Dr. Chattopadhyay
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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.
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Old   March 16, 2005, 04:13
Default Re: Skin Friction Coefficient
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FrenchyTim
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THanks, i know but i have to...
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