Skin friction drag is a type of aerodynamic or hydrodynamic drag, which is resistant force exerted on an object moving in a fluid.
[1] In this conceptualisation, lift-induced drag is an artificial abstraction, part of the horizontal component of the aerodynamic reaction force.
Laminar flow over a body occurs when layers of the fluid move smoothly past each other in parallel lines.
This results in a thinner laminar boundary layer which, relative to laminar flow, depreciates the magnitude of friction force as fluid flows over the object.
The skin friction coefficient is defined at any point of a surface that is subjected to the free stream.
[3] This immediately implies that laminar skin friction drag is smaller than turbulent skin friction drag, for the same inflow.
The skin friction coefficient is a strong function of the Reynolds number
[4] The above relation shows that the skin friction coefficient decreases as the Reynolds number (
CPM, suggested by Nitsche,[5] estimates the skin shear stress of transitional boundary layers by fitting the equation below to a velocity profile of a transitional boundary layer.
(skin shear stress) are determined numerically during the fitting process.
where: The above equation, which is derived from Prandtl's one-seventh-power law,[6] provided a reasonable approximation of the drag coefficient of low-Reynolds-number turbulent boundary layers.
In the point of view of engineering, calculating skin friction is useful in estimating not only total frictional drag exerted on an object but also convectional heat transfer rate on its surface.
[8] This relationship is well developed in the concept of Reynolds analogy, which links two dimensionless parameters: skin friction coefficient (Cf), which is a dimensionless frictional stress, and Nusselt number (Nu), which indicates the magnitude of convectional heat transfer.
Turbine blades, for example, require the analysis of heat transfer in their design process since they are imposed in high temperature gas, which can damage them with the heat.
[10][11] There are two main techniques for reducing skin friction drag: delaying the boundary layer transition, and modifying the turbulence structures in a turbulent boundary layer.
[13][14] Riblets are small grooves in the surface of the aircraft, aligned with the direction of flow.
[15] Tests on an Airbus A320 found riblets caused a drag reduction of almost 2%.
[13] Another method is the use of large eddy break-up (LEBU) devices.
[13] However, some research into LEBU devices has found a slight increase in drag.