Analysis 01: OWF Effect on the Atmosphere - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 46

4.2.2.4 Analysis of the Consistency of METRAS OWF Effect
on the Wind Field

Wind turbines only rotate in a limited window of wind speeds. In the case of
METRAS, technical data of the wind turbine type NORDEX N80/2500 are con-
sidered in the wind turbine parameterization. Hence, the wind turbine parameter-
ization only acts between 2.5 and 17.0 m/s (personal correspondence with
M. Linde). This is leading to the question on how the wind wake changes in matters
of different OWF operation modes. Thereby, METRAS provides another advan-
tage, compared to using the Brostr ¨ m approach—the possible time analysis of wake
development of a wind farm.
Figure 4.7 illustrates the 10-m horizontal wind field, simulated by METRAS
(run M_T012ug08*onoff), for different time steps and OWF operation cases. The
relevant step is, on one hand, the time when the OWF is switched off and the
periods of switching on and switching off of the OWF. A nonoperating OWF in
METRAS is still seen in the wind field because frictional resistance of rotor disc is
considered. That leads to an increase within the OWF of 1 m/s. Therefore, a
nonoperating OWF in METRAS is treated like a building, which ends in an effect

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being comparable with a flow around a building.
Due to the dynamic pressure, the wind speed increases. In front of the OWF, the
wind speed is reduced; pressure increases based on transformation of kinetic
energy. At top and borders, a wind flow separation occurs with an increased flow
due to depression. Behind the wind farm, a lag curl with a depression is expected;
that is why even in the case of nonoperating wind farm, a wind wake is simulated
behind the wind farm.
After turning on the wind turbines, which means using the rotor disc approach,
the wind is suddenly reduced within the OWF district and at the flank the wind is
increased due to depression (Fig. 4.7 ). With time, the wake grows and affects an
area, which is significantly greater than the wind farm itself. With distance to the
wind farm wind reduction slowly migrates to the wind speed of the surrounding.
The produced wind wake and its flanks by OWF do not suddenly disappear after
turning off the wind turbine operation; the main wind field advects wake and flanks.
With time, the effect of the OWF on the wind field can disappear by switching
off the OWF. It can be said that in the ocean, the OWF effect is more dominant and
is not erased within a few hours after turning off the OWF. However, it is important
to understand and conceive the OWF effect on the ocean. These signals of the OWF
on the wind field under different cases of OWF operation will be used as forcing for
the ocean in the analysis of the OWF effect on the ocean.
To summarize, the OWF dominantly changes the wind field depending on wind
speed and OWF conditions. The wind wake occurs within minutes of simulation
time and becomes more intense in case of greater wind speeds and higher number of
turbines. The herein presented 10-m wind fields were used as forcing for simula-
tions of the OWF effect on the ocean, which is performed in Chap. 5 .