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.
3.18.3 Circulation control (jet flaps)
The circulation and hence the lift around an aerofoil section can be controlled very rapidly by the action of a jet applied at the trailing edge. The jet may be directed over the suction surface of the blade at the trailing edge. This may simply have a suitably oriented exhaust nozzle (Figure 3.76e) or may use the Coanda effect running over a short length of curved surface (Figure 3.76f [lower]) to generate a jet sheet deflected so as to increase the effective camber of the section and hence the lift. It acts in a manner very similar to a conventional structural flap, hence is known as a jet flap, and is as shown in Figure 3.76e. Alternatively, jets may be emitted from slots either side of a rounded trailing edge to produce a highly deflected jet in either direction to control the circulation and hence the lift on the blade section, as shown in Figure 3.76f (upper). Figure 3.77 shows a plot of lift coefficient vs jet momentum coefficient for a device of this type where the jet momentum coefficient Cμ = 2(UJ/U∞)2.t/c, UJ is the jet velocity and t it should be noted is the thickness of the jet exit slot (not to be confused here with the maximum thickness of the aerofoil section). Very high values of lift coefficient are possible if sufficient jet momentum is applied with a large deflection angle because the jet momentum removes the separation limit of a conventional flap.
Figure 3.77 Lift coefficient vs jet momentum coefficient for jet circulation control.
These devices mimic the effect of a conventional (solid) flap but have the advantage that large rates of change of lift can be achieved very quickly by sudden changes in the jet pressure and hence momentum. The effectiveness of both in controlling circulation is due to the Coanda effect, whereby an exiting ‘wall‐jet’ sticks to a highly curved surface. In the jet flap case, the effectively ‘active’ length and curvature of the jet sheet depends on the jet momentum. In the rounded trailing edge case, the jet sticks to the highly rounded surface to a greater or lesser extent according to the jet momentum, thus exhausting from the aerofoil trailing edge at a greater or lesser angle. The resulting free jet in both cases simulates a deployed structural flap but without the need to overcome significant inertia in rapid activation. Such devices deployed along the outboard trailing edge of the blade give two advantages. They allow high lift coefficients to be obtained (not limited in the same way by separation as a solid flap) so that the blade chord may be substantially reduced to achieve the same power production. This reduces the weight of the blade and also blade loads when the parked blade is impacted by high wind gusts at large angles to the blade. Second, very rapid control is possible. However, the system has obvious disadvantages of complexity, maintenance, and cost (although less prone to problems of dirt ingress, as it is an overpressure device), the aerofoil section with the jet turned off generates higher drag than a typical ‘sharp’ trailing edge section, and there is a power requirement to provide the jet momentum. Johnson et al. (2008) give an extensive review of many of the above types of devices for blade load control.
3.19 Aerodynamic noise
3.19.1 Noise sources
Since deployment of wind turbines became widespread onshore from the1990s onwards, growing public resistance to the siting of turbines in areas close to dwellings has become a major planning issue. The two most important points of objection are normally visibility and noise. Efforts to minimise the first of these focus on detailed siting and surface appearance of the turbine, noting that there is generally a conflict between siting to reduce visibility and siting to maximise wind energy capture. The issue of noise, however, is closely related to turbine operation because the two main sources of wind turbine noise are the machinery and the blade aerodynamics. Radiated noise from wind turbine machinery (generator, gearbox, etc.) has been greatly reduced in modern wind turbine designs over the last two or three decades. Considerable attention has been paid with successful results to reducing the intensity of mechanical noise by identifying and suppressing sources of noise within the machinery and providing noise insulation. As a result, mechanical noise is now regarded as much less important than aerodynamic noise for large modern wind turbines.
This section deals with aerodynamic noise generated by the blades and methods of reducing it in the form of modifications to blade geometry and section profile. A fuller description of wind turbine noise and its measurement, prediction and assessment of environmental impact is given in Section 10.3.
Aerodynamic noise arises mainly from two sources: (i) self‐noise, which is generated by the air flow over the blades interacting locally with the blades, an effect that would occur even if the incident wind flow were to be smooth, and (ii) noise induced by the turbulence in the inflow (mainly atmospheric turbulence but also on occasion wake turbulence from upstream turbines, interacting with blades and inducing fluctuating blade loading). Aero‐acoustic noise may be either broadband or tonal. The latter is less common but more irritating if significant. There are also a number of other specific noise sources. One important example is the cyclic interaction between the blades and the tower. Although the blade passing frequency is below the audible range, the frequency content from the quasi‐impulsive interaction as a blade passes through the quite narrow influence field of the tower can in some circumstances contribute audible sound at a significant level (in addition to the unsteady loading).The effect can be made small to negligible for an upwind rotor because the aerodynamic interaction between blade and tower reduces as the square of the separation distance between the blade and the tower axis. The interaction is usually therefore minimal unless the blade passes very close to the tower. The main effect can be effectively removed by providing adequate clearance (> 1 tower diameter) through nacelle overhang and rotor tilt. It then only becomes appreciable in high wind operation when blades tend to bend towards the tower, reducing the clearance, but in this case blade noise is less of an issue because of background wind noise. Blade–tower interaction noise can be significant for downwind rotors with solid towers of non‐negligible diameter. This type of design is very uncommon, and hence this noise source has not been studied greatly. When it does occur, it can be difficult to remedy because the noise originates from the interaction of the blades passing through the tower wake, and such wakes only diffuse gradually over large distances. For the general aero‐acoustic sources present on wind turbine rotors, a very good review is given by Wagner et al. (1996).
Those noise sources that arise from unsteady incident flows inducing fluctuating forces on the turbine blades scale as the sixth power of the local inflow wind speed relative to the