Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 70

Figure 12: Effect of topography on the vertical wind speed profi le gentle hill (top),
steep slope (bottom).
of the terrain, on the surface roughness and the stability. In very steep terrain the
fl ow across the terrain might become detached and form a zone of turbulent sepa-
ration. As a rule of thumb this phenomena is likely to happen in terrain steeper
than 30% corresponding to a 17° slope. The location and dimensions of the separa-
tion zone depend on the slope and its curvature as well as roughness and stability.
In cases of separation, the wind speed profi le might show areas with negative ver-
tical gradient, where the wind speed is decreasing with height.
4.2.5 Infl uence of obstacles
Sheltering of the anemometer by nearby obstacles such as buildings leads to a dis-
tortion of the vertical profi le. The effect of the obstacles depends on their dimen-
sions, position and porosity.
Figure 13 sketches out the reduction of wind speed behind an infi nite long two-
dimensional obstacle. The hatched area relates to the area around the obstacle
which is highly dependent on the actual geometry of the obstacle. The fl ow in this
area can only be described by more advanced numerical models such as computa-
tional fl uid dynamic (CFD) models.
4.3 Flow models
4.3.1 General
Due to the complexity of the vertical extrapolation described above, the predic-
tion of the variation of wind speed with height is usually calculated by a computer
model, which is specifi cally designed to facilitate accurate predictions of wind
farm energy. These models also estimate the energy variation over the site area