Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 116

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(U_J/U_∞)².t/c, U_J 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.