Environmental Engineering Reference
In-Depth Information
The variation in this ratio is caused by a large natural variability, but also to
some extent because it is sensitive to the averaging time and the frequency response
of the sensor used.
Two natural sources of turbulence can be identifi ed: thermal and mechanical.
Mechanical turbulence is caused by vertical wind shear and depends on the surface
roughness
z
0
. The mechanically caused turbulence intensity at a height
h
in neutral
conditions, in fl at terrain and infi nite uniform roughness
z
0
can be described as
1
=
I
(11)
u
ln(
hz
/
)
0
This equation shows an expected decrease of turbulence intensity with increas-
ing height above ground level.
Thermal turbulence is caused by convection and depends mainly on the tem-
perature difference between ground and air. In unstable conditions, with strong
heating of the ground, the turbulence intensity can reach very large values. In sta-
ble conditions, with very little vertical exchange of momentum, the turbulence is
generally very low. The impact of atmospheric stability is considerable in low to
moderate wind speeds.
The turbulence intensity varies with wind speed. It is highest at low wind speeds
and shows an asymptotic behaviour towards a constant value at higher wind speeds
(Fig. 18). Typical values of
I
u
for neutral conditions in different terrains at typical
hub heights of around 80 m are listed in Table 5.
Forests cause a particularly high ambient turbulence and require special atten-
tion (Table 5). While special precautions allow estimating the mean wind speed in
or near forest with standard fl ow models, the turbulence variations can only be mod-
elled with more advanced models. Different concepts are available for more advanced
CFD codes. One method to model forest is the simulation via an aerodynamic drag
Figure 18: Turbulence versus wind speed (onshore).
Search WWH ::
Custom Search