Page 48 - Wind Energy Handbook
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22 THE WIND RESOURCE
where I 15 ¼ 0:18 for ‘higher turbulence sites’ and 0.16 for ‘lower turbulence sites’,
with corresponding values of a of 2 and 3 respectively. For the lateral and vertical
components, a choice is allowed: either I v ¼ 0:8I u and I w ¼ 0:5I u , or an isotropic
model with I u ¼ I v ¼ I w . The Germanischer Lloyd rules (GL, 1993) simply specify
20 percent turbulence intensity. Figure 2.4 shows example longitudinal turbulence
intensities for the GL, IEC and Danish standards. The values for the Danish
standard are given for 50 m height with roughness lengths of 0.3 and 0.03 m
respectively.
30
25
20
Turbulence intensity 15
GL
10
IEC-high
IEC-low
5 DS472-low @ 50m
DS472-high @ 50m
0
0 5 10 15 20 25 30 35
Mean wind speed (m/s)
Figure 2.4 Turbulence Intensities According to Various Standards
2.6.4 Turbulence spectra
The spectrum of turbulence describes the frequency content of wind-speed varia-
tions. According to the Kolmogorov law, the spectrum must approach an asympto-
tic limit proportional to n 5=3 at high frequency (here n denotes the frequency, in
Hz). This relationship is based on the decay of turbulent eddies to higher and
higher frequencies as turbulent energy is dissipated as heat.
Two alternative expressions for the spectrum of the longitudinal component of
turbulence are commonly used, both tending to this asymptotic limit. These are the
Kaimal and the von Karman spectra, which take the following forms:
