# Compute Skin friction coef by hand

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 February 7, 2006, 11:56 Compute Skin friction coef by hand #1 Francois Guest   Posts: n/a Sponsored Links 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 ? Thank's for your suggestions. Francois

 February 10, 2006, 07:21 Re: Compute Skin friction coef by hand #2 Francois Guest   Posts: n/a 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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