Aerofólio S809 - 3 - s2 0 - b9781856177931000109 - main

Aerofólio S809 - 3 - s2 0 - b9781856177931000109 - main

(Parte 6 de 6)

(1984), illustrating the variation of the chordwise force coefficient with the angle of attack, a,f or aileron percent chord of 20 and 30% for several aileron deflection angles. The general conclusions to be drawn from these results is that increasing the aileron chord length and the aileron deflection angle contribute to better aerodynamic braking performance.

10.14 BLADE TIP SHAPES

The blade geometry determined with various aerodynamic models gives no guidance of an efficient aerodynamic tip shape. From a basic view of fluid mechanics, a strong shed vortex occurs at the blade tip as a result of the termination of lift and this together with the highly three-dimensional nature of the flow at the blade tip causes a loss of lift. The effect is exacerbated with a blunt blade end as this increases the intensity of the vortex.

Many attempts have been made to improve the aerodynamic efficiency by the addition of various shapes of “winglet” at the blade ends. Details of field tests on a number of tip shapes intended to

FIGURE 10.28

Effect of Chord Length on Chordwise Force Coefficient, C, for a Range of Angles of Attack. (Adapted from Snyder et al., 1984, Unpublished)

10.14 Blade Tip Shapes 405 improve performance by controlling the shedding of the tip vortex are given by Gyatt and Lissaman (1985). According to Tangler (2000), test experience has shown that rounding the leading-edge corner, Figure 10.29, with a contoured, streamwise edge (a swept tip) yields good performance. Tip shapes of other geometries are widely used. The sword tip also shown is often chosen because of its low noise generation, but this is at the expense of a reduction in performance.

10.15 PERFORMANCE TESTING

Comparison and improvement of aerodynamic predictive methods for wind turbine performance and field measurements have many inherent limitations. The natural wind is capricious; it is unsteady, nonuniform, and variable in direction, making the task of interpreting performance measurements of questionable value. As well as the non-steadiness of the wind, non-uniformity is present at all elevations as a result of wind shear, the vertical velocity profile caused by ground friction. The problem of obtaining accurate, measured, steady state flow conditions for correlating with predictive methods was solved by testing a full-size HAWT in the world’s largest wind tunnel, the NASA Ames low speed wind tunnel6 with a test section of 24.4 m 36.6 m (80 120 ft).

10.16 PERFORMANCE PREDICTION CODES

Blade Element Theory

The BEM theory presented, because of its relative simplicity, has been the mainstay of the wind turbine industry for predicting wind turbine performance. Tangler (2002) has listed some of the many versions

Sword tip Swept tip

FIGURE 10.29 Blade Tip Geometries (Tangler, 2000; Courtesy NREL)

6Further details of this facility can be found at windtunnels.arc.nasa.gov/80ft1.html.

406 CHAPTER 10 Wind Turbines

(Parte 6 de 6)

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