Skin Friction

The shear stress generated in the flow is also known as skin friction. This is a natural consequence of real flow (viscosity non zero) and the no slip boundary condition at the surface. This causes a velocity profile to develop as the fluid flows over the surface.

For flow over the flat plate

From the figure it is clear that the fluid adjacent to the surface (usually called wall) is being slowed down. This happens through a shear mechanism. A principal element in determining the value of the shear force/stress required for maintaining the flow is the viscosity of the fluid. There are several models to represent this calculation. In the case of the common fluids we interact with, called Newtonian fluids, the relationship between shear stress and viscosity can be written as

If you bend the above plate you have the surface of an airfoil. In that case there will also be a pressure gradient that will affect the profile.

The shear stress above can be expressed in nondimensional form as skin friction coefficient or just skin friction at any location on the plate

Since this varies along the plate, for design we would like the average value of skin friction over the entire plate