The Aerodynamics Of The Wind Turbine – Part 3

THE CHANGE OF FORCES ALONG THE BLADE

The drawings previously studied, mainly illustrate the air flow situation near the blade tip. In principle these same conditions apply all over the blade, however the size of the forces and their direction change according to their distance to the tip. If we once again look at a 450 kW blade in a wind speed of 10 m/s, but this time study the situation near the blade root, we will obtain

slightly different results as shown in the drawing above.

In the stationary situation (A) in the left hand drawing, wind pressure is still

80 N/m2 . The force ÒFÓ becomes slightly larger than the force at the tip, as the blade is wider at the root. The pressure is once again roughly at a right angle to the flat side of the blade profile, and as the blade is more twisted at the root, more of the force will be directed in the direction of rotation, than was the case at the tip.

On the other hand the force at the root has not so great a torque-arm effect in relation to the rotor axis and therefore it will contribute about the same force to the starting torque as the force at the tip.

During the operational situation as shown in the center drawing (B), the wind approaching the profile is once again the sum of the free wind ÒvÓ of 10 m/s and the head wind ÒuÓ from the blade rotational movement through the air.

The head wind near the blade root of a 450 kW wind turbine is about 15 m/s and this produces a resulting wind ÒwÓ over the profile of 19 m/s. This resulting wind will act on the blade section with a force of about 500 N/m2.

In the drawing on the right (C) force is broken down into wind pressure against the tower ÒFaÓ, and the blade driving force ÒFdÓ in the direction of

rotation.

In comparison with the blade tip the root section produces less aerodynamic forces during operation, however more of these forces are aligned in the correct direction, that is, in the direction of rotation. The change of the size and direction of these forces from the tip in towards the root, determine the form and shape of the blade.

Head wind is not so strong at the blade root, so therefore the pressure is likewise not so high and the blade must be made wider in order that the forces should be large enough. The resulting wind has a greater angle in relation to the plane of rotation at the root, so the blade must likewise have a greater angle of twist at the root.

It is important that the sections of the blade near the hub are able to resist forces and stresses from the rest of the blade. Therefore the root profile is both thick and wide, partly because the thick broad profile gives a strong and

rigid blade and partly because greater width, as previously mentioned, is necessary on account of the resulting lower wind speed across the blade. On

the other hand, the aerodynamic behavior of a thick profile is not so effective.

Further out along the blade, the profile must be made thinner in order to produce acceptable aerodynamic properties, and therefore the shape of the profile at any given place on the blade is a compromise between the desire for strength (the thick wide profile) and the desire for good aerodynamic properties (the thin profile) with the need to avoid high aerodynamic

stresses (the narrow profile).

As previously mentioned, the blade is twisted so that it may follow the change in direction of the resulting wind. The angle between the plane of rotation and the profile chord, an imaginary line drawn between the leading edge and the trailing edge, is called the setting angle, sometimes referred to as ÒPitchÓ.