Which formula defines skin-friction coefficient Cf?

Which formula defines skin-friction coefficient Cf?

• A. Cf = τ_w /(0.5 ρ V^2)
• B. Cf = ρ V^2
• C. Cf = q / S
• D. Cf = drag / lift

Answer: (A)

Skin-friction coefficient shows how strong the wall shear stress is compared to the flow’s dynamic pressure. It is defined as Cf = τ_w / (0.5 ρ V^2), where τ_w is the wall shear stress from viscous forces in the boundary layer, ρ is the fluid density, and V is the free-stream speed. The dynamic pressure 0.5 ρ V^2 is the pressure head of the moving fluid, so dividing τ_w by this quantity gives a dimensionless measure of surface friction. This form is the correct representation because it directly relates the viscous friction at the surface to the energetic content of the incoming flow, independent of surface area. Other expressions either omit τ_w, use the wrong reference quantity, or mix quantities that don’t produce a dimensionless coefficient, so they don’t define skin-friction coefficient.

Skin-friction coefficient shows how strong the wall shear stress is compared to the flow’s dynamic pressure. It is defined as Cf = τ_w / (0.5 ρ V^2), where τ_w is the wall shear stress from viscous forces in the boundary layer, ρ is the fluid density, and V is the free-stream speed. The dynamic pressure 0.5 ρ V^2 is the pressure head of the moving fluid, so dividing τ_w by this quantity gives a dimensionless measure of surface friction.

This form is the correct representation because it directly relates the viscous friction at the surface to the energetic content of the incoming flow, independent of surface area. Other expressions either omit τ_w, use the wrong reference quantity, or mix quantities that don’t produce a dimensionless coefficient, so they don’t define skin-friction coefficient.