Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 116

Fig. 5.22 Observed 90th percentiles (red curves) of turbulence intensity I u at four different
heights as function of wind speed at these heights (from Türk and Emeis 2010 ) compared to the
results of Eq. ( 5.18 )(dashed lines)
15 m/s wind speed. The first term on the right-hand side of ( 5.18 ) gives the mean
standard deviation of the wind speed for thermally neutral stratification making the
assumption of a logarithmic vertical wind profile ( 3.6 ) and setting 1/j equal to 2.5
[see ( 3.9 )]. For the second term it is assumed that the values for the standard
deviation of the wind speed follow a Gaussian distribution around its mean, so that
the 90th percentile of the standard deviation of the wind speed, r u90 is 1.28 times
the standard deviation of the standard deviation of the wind speed, the latter
represented by 1.44 m/s times I 15 . We will suggest an update to this formula below
in Eq. ( 5.19 ).
Figure 5.22 shows the 90th percentiles of measured turbulence intensity
depending on wind speed at different heights for the period September 2003-
August 2007 (solid lines) compared to turbulence intensity given by IEC 61400-3
(Eq. 5.18 , dashed lines). Similar to the median of turbulence intensity the values of
the 90th percentiles of turbulence intensity also decrease with increasing wind
speed till a minimum of about 7-8.5 % at 10-12 m/s wind speed and then increase
again with furthermore increasing wind speed. 90th percentiles of turbulence
intensity also decrease with height. Compared to turbulence intensities given by
the IEC standard, we can detect three sectors: Below wind speeds of 8-10 m/s—at
wind speeds that are not really load-relevant—measured 90th percentiles of the
turbulence intensities are covered by the IEC-curves not very well. At wind speeds
between 10 and 22 m/s the 90th percentiles of measured turbulence intensities lie
below the values given by the IEC standard except for two values at 30 m height.
In this sector the slopes of the curves of measured turbulence intensity are steeper
than the slopes of the curves calculated according to IEC 61400-3. Above wind
speeds of about 22 m/s the slopes of measured and calculated curves of the 90th
percentile of the turbulence intensity become nearly identical. The discrepancy
between the measured data and the computed values from ( 5.18 ) for lower wind