Skin Friction Coefficient

Skin friction coefficient, abbreviated as \(C_f\), a dimensionless number, is used in fluid dynamics and aerodynamics to describe the amount of drag or resistance experienced by a fluid as it flows over a solid surface.  It is a measure of the frictional resistance created by the fluid as it moves in contact with the surface.  The skin friction coefficient is an important parameter in the study of boundary layers, which are the thin layers of fluid adjacent to a solid surface where the velocity of the fluid changes from zero at the surface to the free stream velocity.

The skin friction coefficient quantifies the relative importance of viscous effects near the surface compared to inertial effects in the fluid flow.  A higher \(C_f\) indicates greater frictional drag and a thicker boundary layer, which can increase the overall drag force on an object moving through the fluid.  Engineers and scientists use this coefficient to analyze and design aerodynamic or hydrodynamic components, such as aircraft wings, ship hulls, or automotive bodies, to minimize drag and improve efficiency.

 

Skin Friction Coefficient formula
\( C_f \;=\; \tau \;/\; \frac{1}{2} \; \rho \; v^2 \)
Symbol	English	Metric
\( C_f \)  = skin friction coefficient	\( dimensionless \)	\( dimensionless \)
\( \tau \) (Greek symbol tau) = shear stress of fluid	\(lbf \;/\; in^2\)	\(Pa\)
\( \rho \)  = density of fluid	\(lbm \;/\; ft^3\)	\(kg \;/\; m^3\)
\( v \)  = velocity of fluid	\(ft \;/\; sec\)	\(m \;/\; s\)