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The boundary layer
When air flows past any part of an aircraft it appears to try to stick to the surface. Right next to the surface, there is no measurable relative motion. The relative velocity of the air flow increases rapidly with distance away from the surface, as illustrated in Fig. 3.1, so that only a thin ‘boundary’ layer is slowed down by the presence of the surface. Note, that individual air molecules do not actually physically stick to the surface, but fly around randomly, at a speed that is related to the temperature.
In reality, there is no precise edge to the boundary layer, the influence just fades. For the purposes of calculations, however, it is necessary to arbitrarily define an edge. In the simple case of the flow over a flat plate with no stream – wise variation in pressure, shown in Fig. 3.1, it is customary to define the edge of the layer as being the position where the flow speed reaches 99 per cent of the free-stream value.
From an aeronautical point of view, it is the wing boundary layer that is of greatest importance, and in Fig. 3.2 we show a typical example of how the boundary layer develops on an aerofoil. It will be seen that the thickness of this layer grows with distance from the front or leading edge.
There are two distinct types of boundary layer flow. Near the leading edge, the air flows smoothly in a streamlined manner, and appears to behave rather like a stack of flat sheets or laminae sliding over each other with friction. This type of flow is, therefore, called laminar flow. Further along, as indicated in
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Fig. 3.1 Variation of velocity within the boundary layer on a flat surface with no streamwise pressure variation In reality there is no precise edge to the boundary layer, but for this simple case, it is customary to define a nominal edge as being the position where the velocity reaches 99 per cent of the free-stream value (vo) |
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Fig. 3.2, there is a change or transition to a turbulent type in which a random motion is superimposed on the average flow velocity.
The two types of flow have important differences in properties that we can exploit. In simple terms, the main practical effects are that the laminar layer produces less drag, but the turbulent one is less liable to separate from the surface, as described later. To understand why these differences occur we need to look at the two types in a little more detail.
