Environmental Engineering Reference
In-Depth Information
z D displacement height
Problem areas
WasP solution
5 z D
Figure 15: Modelling forest in WAsP: displacement height [ 19 ] .
As explained above the vertical profi le also depends on the atmospheric stability.
Even at moderate wind speeds, deviations from the logarithmic profi le occur when
the height exceeds a few tens of meters. Deviations are caused by the effect of
buoyancy forces in the turbulence dynamics; the surface roughness is no longer the
only relevant surface characteristic, but has to be supplemented by parameters
describing the surface heat fl ux. With cooling at night, turbulence is lessened,
causing the wind profi le to increase more rapidly with height; conversely, daytime
heating causes increased turbulence and a wind profi le more constant with height
(see Section 4.2.3).
In order to take into account the effects of the varying surface heat fl ux without
the need to model each individual wind profi le, a simplifi ed procedure was adopted
in WAsP which only requires the climatological average and root mean square of
the annual and daily variations of surface heat fl ux. This procedure introduces the
degree of 'contamination' by stability effects to the logarithmic wind profi le when
conditions at different heights and surfaces are calculated [16].
It is important to appreciate that as the distance of the turbines from the meteo-
rological mast increases, the uncertainty in the prediction also increases. This
increase in uncertainty is typically more rapid in complex terrain than in simple
terrain. When developing a site the increased uncertainty should be refl ected in
the number of measurement masts on site and the measurement height. As a
rule of thumb the measurement height should be minimum 2/3 of the planned
hub height.
A great challenge is the modelling of forests using WAsP. To model the wind
speed correctly in WAsP a so-called displacement height must be introduced
together with a very high roughness [19]. The displacement height is an artifi cial
increase of terrain height for the area covered by forest. It should be around 2/3
of the tree height depending on the tree's density and the shape of the canopy.
At the edge of the forest the displacement height should taper off linearly out to
a distance of fi ve times the displacement height (Fig. 15). The displacement height
shall correct for the speed up of the wind as the forest to some extent acts like an
artifi cial hill leading to accelerated fl ow across the forest. The roughness length to
be applied for forested areas should be in the order of 0.4 to more than 1 m. The
increased roughness will lead to a wind profi le exhibiting a higher shear when
modelling the forest [19]. This approximation is only valid in simple terrain.
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