Aerodynamics of wind turbines

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).

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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.

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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).

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

4.1 Aerofoils and harnessing aerodynamic forces

There are two main types of aerofoil section:

1. Asymmetrical aerofoils – optimised to produce most lift when the underside of the aerofoil is closest to the direction from which the air is flowing.

2. Symmetrical aerofoils – able to induce lift equally well (although in opposite directions) when the air flow is approaching from either side of the ‘chord line’ (shown in Figure 7).

The angle which an aerofoil (or flat or cambered plate profile) makes with the direction of an airflow, measured against a reference line (usually the chord line), is called the angle of attack α (alpha). When airflow is directed towards the underside of the aerofoil, the angle of attack is positive.

The lift and drag characteristics of many different aerofoil shapes have been determined by measurements in wind tunnels, and catalogued (e.g. in Abbott and von Doenhoff, 1958 ). The lift and drag characteristics measured at each angle of attack can be described using non-dimensional lift and drag coefficients (C_L and C_D) or as lift to drag ratios (CL/CD). Knowledge of these coefficients is essential when selecting appropriate aerofoil sections in wind turbine blade design. Lift and drag forces are both proportional to the energy in the wind.

Harnessing aerodynamic forces

Modern horizontal and vertical axis wind turbines harness aerodynamic forces in a different ways.

In a HAWT with fixed-pitch blades, with its rotor axis in constant alignment with the wind direction, for a given wind speed and constant rotation speed the angle of attack at a given position on the rotor blade stays constant throughout its rotation cycle.

In most horizontal axis wind turbines the rotation axis is maintained in line with wind direction by a ‘yawing’ mechanism, which constantly realigns the turbine.

In addition to its swept area and rotor diameter, the performance (power output, torque and rotation speed) of a HAWT rotor depends on other factors, including the number and shape of the blades, the choice of aerofoil section, the length of the blade chord,, the blade pitch angle, the angle of attack at positions along the blade, and the amount of blade twist between the hub and tip.

By contrast, in a VAWT with fixed pitch blades, under the same conditions the angle of attack at a given position on the rotor blade constantly varies throughout its rotation cycle. This means that the ‘suction’ side reverses during each cycle, so a symmetrical aerofoil is needed to ensure that power can be produced irrespective of whether the angle of attack is positive or negative.