How the boundary layers form

In a laminar boundary layer, molecules from the slow-moving air near the surface mix and collide with those further out, tending to slow more of the flow. The slowing effect produced by the surface thus spreads outwards, and the region affected, the boundary layer, becomes progressively thicker along the direction of the flow. The way in which the boundary layers grow is illustrated in Fig. 3.2.

At the position called transition, an instability develops, and the flow in the layer becomes turbulent. In the turbulent boundary layer, eddies form that are relatively large compared to molecules, and the slowing down process involves a rapid mixing of fast and slow-moving masses of air. The turbulent eddies extend the influence outwards from the surface, so the boundary layer effectively be­comes thicker. Very close to the surface, there is a thin sub-layer of laminar flow.

Surface friction drag

Just as the surface slows the relative motion of the air, the air will try to drag the surface along with the flow. The whole process appears rather similar to the
friction between solid surfaces and is known as viscous friction. It is the pro­cess by which surface friction drag is produced.

The surface friction drag force depends on the rate at which the air adjacent to the surface is trying to slide relative to it. In the case of the laminar bound­ary layer, the relative air speed decreases steadily through the layer. In the tur­bulent layer, however, air from the outer edge of the layer is continually being mixed in with the slower-moving air, so that the average air speed close to the surface is relatively high. Thus, the turbulent layer produces the greater amount of drag for a given thickness of layer.