The air flow around an aerofoil section

In Fig. 1.13 we show the streamline patterns around an aerofoil section at a small angle of attack. Streamlines indicate the instantaneous direction of flow, and if the flow is steady, they also show the path that a particle would follow. Streamlines are defined as imaginary lines across which there is no flow. Therefore, the closeness of the lines gives an indication of flow speed. If the streamlines converge, the air is funnelled through at an increased speed, just as it does in the narrowing part of a duct, as described earlier (Fig. 1.10). Notice how the streamlines converge over the front of the upper surface of the aero­foil in Fig. 1.13, indicating an increase in speed, and diverge underneath, show­ing a decrease. A similar effect may be seen in the flow around the rotating cylinder in Fig. 1.12.

Some important features of the flow around the aerofoil may be seen by looking at the dividing streamline; the streamline which effectively marks the

division between the air that goes over the wing, and that which flows under it. We have already mentioned that the flow divides not on the nose, but at a point under it, even on a flat plate. Notice also, how the air is drawn up towards the aerofoil at the front, as well as being deflected downwards from the trailing edge. This is also true for the spinning cylinder. Behind the wing of an aircraft, there is an overall downward flow of air, or downwash, but it should be noted, that this is predominantly a three-dimensional effect, as described in Chapter 2. The downwash seen in Fig. 1.12 would not be nearly so pronounced if the cylinder completely spanned the tunnel from wall to wall.