Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 120

Fig. 5.27 Forward inclination of turbulence elements in degrees between 60 and 80 m height at
FINO1 in the year 2005. Negative values indicate backward inclination
determined from maxima in the spectra of the three wind components. This gives
values in the same order of magnitude but also shows considerable differences for
the three stability classes with the largest values for the horizontal components
with neutral stratification and largest values for the vertical component with
unstable stratification. Details can be found in Table 5.2 .
Another parameter characterizing the turbulence is the inclination of the tur-
bulence elements with respect to the vertical. A forward inclination of the tur-
bulence elements is expected, because the wind speed decreases towards the
surface. Forwardly inclined elements show up earlier at greater heights than at
lower heights, i.e. the upper tip of a rotor is impacted slightly earlier than the lower
tip. This can cause differential loads on the rotor. Figure 5.27 shows that this
forward inclination occurs in the majority of all 10 min intervals. An inclination
angle of 30 between 60 and 80 m is most frequent while an angle of about 40 is
most frequent between 40 and 60 m (not shown) due to the larger vertical shear in
that height interval. The inclination increases with wind speed. For wind speeds
higher than about 20 m/s nearly no backward inclinations are found any more (see
Fig. 5.28 ). Figure 5.28 also demonstrates that large inclination angles usually
coincide with non-neutral thermal stratification.
5.4.4 Gust Events
So far, bulk statistical parameters characterizing atmospheric turbulence have been
discussed. But the actual wind speed variation during a gust event can be decisive
as well for load calculations. The IEC standard 61400-3 (2006) defines as a worst