Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 115

Deployable conventional flaps (see Figure 3.76a) are trailing edge flaps (TE flaps), leading edge flaps being very unusual, and are similar to conventional aircraft control surfaces such as ailerons, hinged at the rear of the blade section and operated by a mechanical actuator. They increase or decrease section lift by increasing or decreasing the effective camber of the blade section. They have been proposed for wind turbine blades but are rarely ever used because of the additional mechanical complexity, weight near the blade tips where they would be most useful, cost, and maintenance issues. Their main advantage is that they can be sited where changes in lift are most useful, typically outboard regions. As turbine blade lengths increase and the blades become ever more flexible, this localisation offers the possibility of distributed control with advantages over pitch control at the blade root, and it is possible that more use will be made of them in future. This is particularly relevant for active control to mitigate the effect of turbulence and gusts because small flaps can be actuated very rapidly and act locally.

Morphing blade sections (Figure 3.76b) are a recent development that is really a variation of the conventional trailing edge flap. The actuator is within the rear section of the blade, which is fabricated from a flexible composite. The result when actuated is to generate a flap effect but with a more smoothly curved camber and with all mechanical parts protected by being internal and hence presumed to be less vulnerable to dirt and corrosion. The continuous curvature of the camber can be tailored for maximum aerodynamic efficiency. The technology is considered to be a promising method of providing distributed control with some mechanical benefits over conventional TE flaps.

Fixed (Gurney) flaps (Figure 3.76c, named after Dan Gurney, who invented this flap for down‐force wings used in motor racing) are small fixed flaps in the form of a length of thin, right‐angle bar section fixed to the trailing edge of a blade on the pressure side. A Gurney flap is thus like a small trailing edge flap deployed at 90° in the direction to increase lift. Such flaps usually have a flap chord (i.e. ‘height’ from the blade) equal to only 1% or 2% of the blade section chord. With that small length but large deployment angle, a useful increase in lift coefficient (0.1 to 0.25) can be obtained at the expense of a small increase in drag coefficient. The result with a well‐designed flap is that the lift coefficient can be increased while the lift/drag ratio remains constant or may even increase slightly. See, for example, Giguere et al. (1997).

Figure 3.76 Flaps and similar acting devices: (a) conventional trailing edge flap, (b) morphing rear blade section, (c) Gurney flap, (d) leading edge slat, (e) jet flap, and (f) circulation control.

Slats (Figure 3.76d), as deployed at the leading edges of aircraft wings to prevent separation during high angle of attack operation (during take‐off and landing), have also been tried on wind turbine blades for the same reason.