How is skin-friction drag formed?
Turbulent flows are the predominant flows both in nature and engineering applications. The presence of a solid boundary or wall along the direction of a uniform viscous fluid flow introduces a new set of characteristic length and time scales into the shear free flow. Shear emerges both in the small and large scales and plays a vital role in the dynamics of the resultant flow. Consequently, the flow is no longer considered as shear-free, and at a sufficiently high Reynolds number it is known as a turbulent wall-bounded flow. These boundary layers lead to skin friction drag. In this region, the wall-normal gradient of the majority of the flow variables are the largest due to the no-slip boundary condition. As the boundary layer transforms from laminar to turbulent, the associated skin-friction drag increases. Turbulent skin-friction drag constitutes approximately 50% of the total drag on airliners (Gad-el Hak, 1994).
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