Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 77

Table 5: Typical hub height turbulence intensities for different land covers.

Land cover Typical I u (%)

Offshore 8

Open grassland 10

Farming land with wind breaks 13

Forests 20 or more

0.14

8 m
48 m

0.12

0.1

0.08

0.06

0 5 10 15 20 25 30

Windspeed (m/s)

Figure 19: Turbulence versus wind speed (offshore) [ 23 ].

term in the momentum equations, parameterised as a function of the tree height
and leaf density. The turbulence model might also be changed to simulate the
increased turbulence.
As a rule of thumb measured data indicates that the turbulence intensity created
by the forest is signifi cant within a range of fi ve times the forest height vertically
and 500 m downstream from the forest edge in a horizontal direction. Outside
these boundaries the ambient turbulence intensity is rapidly approaching normal
values [ 22 ].
In or near forest, the mechanically generated turbulence intensity increases in
high wind speeds due to the increasing movement of the canopy. A similar phe-
nomenon can be observed in offshore conditions where increasing waves lead to
increased turbulence in high wind speeds (Fig. 19). The increasing turbulence with
increasing wind speed is of great importance when calculating the extreme gusts
for a site.