Wind Energy

90 J. Mann
60
y [m] 40
20
0
20
40
60
x = –Ut [m]
Fig. 15.2. u-turbulence simulation of velocity jump based on the spectral tensor
by Mann. Shown in gray scale is u in the rotor plane at several several times. The
velocity u at the centreline where the constraints are made, is also shown on the
front of the box. Its maximum is around 5 m s −1 and minimum −5ms −1 as seen
from the scale on the front right edge of the box
15.4.2 Scanning Laser Doppler Wake Measurements
To test under which conditions, if any, the wake deficit can be considered
as a passively advected quantity we are currently doing an experiment on a
small Tellus turbine, with a rotor diameter of 20 m. A wind lidar [9] has been
modified to be able to scan the wake downstream of the turbine by mounting
it on the rear side of the nacelle. The instrument can remotely measure the
component of the wind speed along the laser beam up to a distance of 200 m.
By mounting the laser head (see Fig. 15.4) on a movable platform we can make
horizontal cuts through the wake deficit.
An example of this is seen in Fig. 15.5. At each downwind distance the
lidar scans four times. At the closest distance the wake seems to cover almost
the scanning angle. At the other distances the wake appears to the left. As
we scan further downstream the wake deficit seems to widen and lessen in
amplitude. Scanning at each downstream distance takes of the order of 4 s.
Fig. 15.3. Modelling of wakes advected as passive objects by the large scale environmental
turbulence. (M. Nielsen, Risø)
80
100
−1
−3
−5
135
u [m/s]
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40
20
0
z [m]
wind [m/s]
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2
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−4