The development of the boundary layer
For the flow around a body with a sharp leading edge, the boundary layer on any surface will grow from zero thickness at the leading edge of the body. For a typical aerofoil shape, with a bluff nose, boundary layers will develop on top and bottom surfaces from the front stagnation point, but will not have zero thickness there (see Section 2.10.3).
On proceeding downstream along a surface, large shearing gradients and stresses will develop adjacent to the surface because of the relatively large velocities in the mainstream and the condition of no slip at the surface. This shearing action is greatest at the body surface and retards the layers of fluid immediately adjacent to the surface. These layers, since they are now moving more slowly than those above them, will then influence the latter and so retard them. In this way, as the fluid near the surface passes downstream, the retarding action penetrates farther and farther away from the surface and the boundary layer of retarded or ‘tired’ fluid grows in thickness.