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(Fig. 5.17 ). Although the most frequent turning angle over this height range is
around 0, there are much more positive turning angles (clockwise turning with
height) than negative turning angles. Negative values most probably occur with
low wind speeds and cold air advection (see last paragraph in Sect. 2.4 on thermal
winds above).
5.3 Extreme Wind Speeds
10 min-mean wind conditions were considered in the preceding subchapters. The
offshore IEC standard also gives limit values for extreme wind speeds in the
extreme wind speed model (EWM). The vertical profiles of 3 s-gusts with a return
period of 1 year, v e1 and of 50 years, v e50 are defined as follows:
v e1 ð z Þ¼ 0 : 8v e50 ð z Þ
ð 5 : 12 Þ
and:
0 : 11
z
z hub
v e50 ð z Þ¼ 1 : 4v ref
ð 5 : 13 Þ
The reference velocity, v ref is put to 50 m/s for class I offshore sites and to
42.5 m/s for class II sites. The vertical profile of 10 min-mean wind speeds with a
return period of 1 year, v 1 and of 50 years, v 50 are defined as follows:
v 1 ð z Þ¼ 0 : 8v 50 ð z Þ
ð 5 : 14 Þ
and:
0 : 11
z
z hub
v 50 ð z Þ¼ v ref
ð 5 : 15 Þ
The difference between Eqs. ( 5.13 ) and ( 5.15 ) is the gust factor 1.4 in ( 5.13 ).
Jensen and Kristensen ( 1989 ) find a gust factor (see Appendix A, Eq. A.33) at
70 m height on the little island of Sprogø in the Great Belt between the Danish
Isles of Fyn and Sjælland of:
G ð 3s ; 10 min ; 70 m ; 1 : 510 3 ; v Þ 1 : 15
ð 5 : 16 Þ
which is considerably lower than the factor 1.4 in Eq. ( 5.13 ). Abild and Nielsen
( 1991 ) give the following simpler relation for the offshore gust factor which does
not depend on wind speed:
k
ln
G ð z ; z 0 Þ 1 þ kI u ¼ 1 þ
ð 5 : 17 Þ
z
z 0
with k about 2.1. Equation ( 5.17 ) has obviously been derived from the relation
between the turbulence intensity (A.6) and the gust factor given in Eq. (A.35) in
the Appendix stipulating the validity of the logarithmic wind profile by using Eq.
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