Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 113

Fig. 5.19 Mean (full circles), maximum (upper open circles), minimum (lower open circles),
median (full line with squares), 10th percentiles (lower crosses), 25th percentiles (lower dashes),
75th percentiles (upper dashes) and 90th percentiles (upper crosses) of turbulence intensity I u as
function of wind speed at 90 m height at FINO1 in the German Bight for the period September
2003 to August 2007 together with the number of values per wind speed bin (crosses connected
by a thin line) (from Türk and Emeis 2010 )
5.4.1 Turbulence Intensity
Offshore turbulence intensity I u [see definition (A.6) in the appendix] depends on
roughness length according to Eq. ( 3.10 ) and is therefore a function of wind speed
(Hedde and Durand 1994 ; Vickers and Mahrt 1997 ). The knowledge of the tur-
bulence intensity over the open sea is relevant for a better general understanding of
the marine boundary layer as well as for the construction and operation of offshore
wind turbines. Loads on the structure of the turbines and power output both
increase with increasing turbulence intensity.
The dependence of median, arithmetic mean, minimum, maximum, the 10th,
25th, 75th, and 90th percentiles of turbulence intensity on wind speed—for the
measuring period from September 2003 to August 2007 and a measuring height of
90 m—is shown in Fig. 5.19 . For low wind speeds the mean of turbulence
intensity rapidly decreases with increasing wind speed to a minimum value of
about 4.5 % at 12 m/s wind speed. Above this minimum, turbulence intensity
increases nearly linear with increasing wind speed. The high turbulence intensity
values and the strong decrease up to wind speeds of about 12 m/s originated from
the dominance of thermal induced turbulence at low wind speeds during unstable
atmospheric conditions with water surface temperatures significantly above the air
temperature. With furthermore increasing wind speed and so increasing roughness
length, z 0 the mechanically part of the turbulence intensity begins to dominate over