Environmental Engineering Reference
In-Depth Information
boundary layer expands. This causes the wind to slow down. If layering is stable
at night, the top air flow is decoupled from the layer close to the ground and thus
often reaches very high wind speeds /2-15/.
In the segment between 50 and 100 m above ground, the mean diurnal course
of the wind velocity is the so-called "transition type". At this altitude, a double
wave with two velocity maxima occurs at mid-day and at mid-night. Two minima
can be observed in the morning and in the evening, but the amplitudes are rela-
tively small in that case /2-15/.
Frequency distribution.
In spite of the diurnal and the annual course observed on
average, measured wind velocities can vary significantly at different times, in
different locations, and at different heights above ground. This is why it is very
difficult to compare measured time series. Therefore time series of measured wind
velocity, with varying time dissipation, are characterised by their distribution
functions. On this basis they can easily and reliably be compared among each
other. For that purpose, the measured wind speed values are classified to different
wind speed classes. For each class the probability of occurrence of the measured
wind speed values, allocated to this class with regard to the overall amount of
measured wind velocity values, is calculated. This frequency distribution always
shows a typical course.
28
28
Si
te A
Site B
Site C
Site D
W
eibu
ll
Rayleigh
24
24
20
20
16
16
12
12
8
8
4
4
0
0
0
2
4
6
8
10
12
14
16
18
20
02468 0 2 4 6 8 0
Wind velocity in m/s
Wind velocity in m/s
Fig. 2.35
Frequency distribution of wind velocity time series for different sites (left) and a
corresponding mathematical approximation for site B (right) (see /2-7/)
Fig. 2.35, left, shows distributions of hourly mean wind velocity values meas-
ured at different sites. According to these findings, in the region of the annual
mean wind velocity the respective highest occurrence probability is given. If the
mean wind speed is relatively low, these probabilities show comparatively high
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