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# 4 Aerodynamics of wind turbines

An object in an air stream experiences a force (F) imparted from the air stream equivalent to two component forces acting in perpendicular directions, known as the drag force (D), and the lift force (L) (Figure 6).

Figure 6 An object in an air stream is subjected to a force F, from the air stream. This is composed of two component forces: the drag force, D, acting in line with the direction of air flow, and the lift force, L, acting at 90° to the direction of air flow.

The magnitude of these forces depends on the shape of the object, its orientation to the air stream, and the air stream velocity. Objects designed to minimize drag forces are described as ‘streamlined’, because the lines of flow around them follow smooth, stream-like lines, as in the aerofoil section shown in Figure 7.

Figure 7 Streamlined flow around an aerofoil section

At small angles relative to the direction of the air stream – that is, when the ‘angle of attack’ is small – a low pressure region is created on the ‘downstream’ side of the aerofoil section as a result of an increase in the air velocity on that side (Figure 8).

Figure 8 Zones of low and high pressure around an aerofoil section in an air stream

In this situation, the faster the airflow, the lower the pressure (this is known as the ‘Bernoulli’ effect). The lift force thus acts as a ‘suction’ or ‘pulling’ force on the object, in a direction at right angles to the airflow.

You’ll look a little bit more at aerofoils in the next section.

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