Environmental Engineering Reference
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fluctuations are indicative of cyclic wind forces imposed on a turbine blade as it rotates
through a quasi-static but non-uniform wind field.
In Figure 8-28(b) the time-series of wind speed measured by the central or “hub”
anemometer is compared to the array-average time series. The figure shows that the array-
average is much smoother than that of the single wind speed measurement at the center of
the array, because array-averaging acts as a low-pass spatial filter. The fluctuations in the
array-averaged wind are the average of the instantaneous fluctuations at each anemometer.
An examination of 10-min averages demonstrates that the steady wind speed at the hub is
representative of the array-average steady wind speed.
Figure 8-28. Examples of rotationally-sampled wind speed data. Rotational rate is 7.8
rad/s. (a) Synthesized wind speed compared to speed at the center of the array (b) Array-aver-
age wind speed compared to speed at the center of the array [Verholek 1978]
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