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-   -   Compute Skin friction coef by hand (http://www.cfd-online.com/Forums/fluent/39548-compute-skin-friction-coef-hand.html)

 Francois February 7, 2006 11:56

Compute Skin friction coef by hand

Hi all,

I'm looking at the detachment point of a boundary layer over a rotating cylinder with the distribution of the Skin Friction Coefficient on the cylinder.

I've just discover that the Skin Friction Coefficient Cf in Fluent is given in absolute value (always >= 0).

So I manage to compute my own skin friction coefficient

Cf= tau_w / (1/2 * rho * U0^2) with tau_w= mu*du/dy |wall

using Custom field fubnction with:

viscosity-lam * dx-velocity-dy / (0.5 * density* U0^2)

where U0 is my freestream velocity.

But when I plot the distribution (on the cylinder) of the skin friction coef given by Fluent and computed by the custom field function ther are differences. I'm surprised that the results are sometimes different.

The definition of the derivatives is not the same when using Custom Field function ? How to explain those differnces ?

 Francois February 10, 2006 07:21

Re: Compute Skin friction coef by hand

Well done François !

I think It will be beter with this custom field function: viscosity-lam * dtangential-velocity-dradial-coordinate / (0.5 * density* U0^2)

SO if there is any guru here ... how to compute this derivative: d Utheta / dr <-> dtangential-velocity-dradial-coordinate

Thanks for you help

François

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