Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 81

Figure 23: Vertical wind profi le in front and behind a wind turbine [16].

Figure 24: Horizontal gradient due to part wake operation .

It can clearly be seen that some areas of the deformed profi le show very large
gradients. If the wind turbines operate in part wake situations (Fig. 24), the wind
turbines behind the front row will not only be exposed to vertical gradients but
also to horizontal gradients which add signifi cantly to the loads.

• Atmospheric stability . As explained above, the vertical wind speed profi le de-

pends on the vertical temperature profi le. With increasing heating of the ground
(unstable conditions) the turbulence increases and therefore the profi le is being
“smoothed” out. This leads to a steep wind speed profi le characterised by very
little increase of wind speed with height. Stable conditions in contrast are
characterised by a very fl at profi le with signifi cant wind speed gradients (see
Fig. 11). Depending on the prevailing atmospheric stability of the site the
gradient of 0.2 assumed for most load cases can be exceeded. Additionally in
stable conditions signifi cant wind direction shears are common.