Design of support structures for offshore wind turbines - Wind Power Generation and Wind Turbine Design - page 575

4.3 Wind

Figure 22 shows a wind speed profi le for a certain point in time. From this fi gure
a number of characteristics can be deduced. First, that the mean wind is stronger
at higher altitudes than near the surface of the earth. This is caused by friction of
the moving air with the terrain. The effect becomes less pronounced as the altitude
increases. The resulting difference in wind speed over altitude is called wind shear.
Secondly, it is evident that the actual wind profi le is very irregular. The actual wind
speed deviates from the mean wind speed and direction as a result of turbulence.
These two phenomena will be discussed briefl y.
There are two commonly used models to describe wind shear: the logarithmic
profi le and the power law. The logarithmic profi le is given by eqn (10), while eqn
(11) describes the power law [4]:

⎛⎞
⎜ ⎝⎠

z
z

ln

0

Vz () = V

r

⎛⎞
⎜ ⎝⎠

z
z

( 10) r

ln

0

a

⎛⎞

z

( 11)

Vz () = V

⎜ ⎝⎠

r

r

In Fig. 23 both the logarithmic profi le and the power law are shown. It clearly
shows the difference between both models.
While the above gives a description for the mean wind speed, in reality the wind
is never a steady fl ow of air that can be described with only one parameter. Local
disturbances in the airfl ow called eddies cause the instantaneous wind speed to
fl uctuate around a mean value. This phenomenon is called turbulence. A measure

z

Mean profile

Actual wind
speed profile

x

Figure 22: 3D turbulent wind velocity profi le.