Analysis 02: OWF Effect on the Ocean - On the Effect of Offshore Wind Farms on the Atmosphere and Ocean Dynamics - page 81

During on, wake is developed and increased; at off, wake is advected with the

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main wind field; and at the wind field is set to the reference run.

Operation case 01 (opc01) starts with 4.2 h of operating wind turbine, followed
by 7.8 h

off_ref,

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' off, ' 4.2 h ' on, ' 4.2 h ' off, ' and finally 26 h ' off_ref. '

Operations case 02 (opc02) starts with 4.2 h

' on, ' 6.1 h ' off, ' and finally 36.5 h

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Operation case 03 (opc03) starts with 2.6 h of operating wind turbines and is
then switched off and uses 44.1 h

off_ref.

off_ref.

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The analysis concentrates on the cross section from south to north through the
OWF; see Fig. 5.19 .
Figure 5.20 /Figs. 5.21 and 5.22 summarize the development of the OWF effect
based on the three operation cases opc01, opc02, and opc03 for velocity field and
hydrographic conditions along cross-section S-N through the OWF.
Overall the Hovm¨ ller diagrams of the three operation cases strongly differ from
the previous long-time analysis (Sect. 5.1.2 ), which is based on a constant wind
forcing per time step. Due to the turning on and off of the OWF, an additional side
effect occurs-an inertial oscillation.
In all three cases, it was not possible to bring the ocean back to dynamical
conditions that are comparable with the reference run. Even in case of absolutely
ignoring the OWF for more than 44 h, that is, after 1.8 days, the ocean

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s response to
the OWF does not fully disappear. Comparing the three operation cases, it can be
said that the stronger and longer the OWF acts on the atmosphere and the ocean, the
longer and stronger the ocean is affected.
Whereas changes in hydrographic (Fig. 5.21 ) do not end up in surprising
physical differences, the velocity field (Fig. 5.20 ) leads to horizontal circulation,
which strongly affects the vertical component.
Foremost, the velocities at surface are analyzed. As expected, with the turning
on of the OWF and with it an increase of wind wake, the ocean velocity field is
affected by speed reduction in the wake area. Maximal decrease is reached till the
point of turning off the OWF. Although the first operation time is quite short, with
4.2 h for opc01 & opc02 and 2.6 h for opc03, the reduction counts around 0.1 m/s
for opc01 & opc02 and 0.07 m/s for opc03. As indicated in Sect. 5.1 , the v -
component of velocity encroaches into the dynamical system by strong changes
close to the OWF. The change of v -component, compared to the reference run, is
dominant at the end of the first OWF operation duration.
The v -component increases by 0.04 up to 0.07 m/s at the surface. It is a
horizontal effect to counteract against the wake in u -component and thus the
produced dipole of

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. At the point of turbine shut down, the u -component increases
again, while the v -component is reduced, additionally, w -component is changed to
keep the equation of motion.
During that time, the dipole structure of surface elevation is built on factual
connection—the stronger the wake is, the stronger the tilt of

ζ

ζ is. The formation of

extrema for is delayed, compared to the velocity wake, by more than 5 h for all

three cases. Furthermore, it is recognized that surface elevation

ζ

ζ does not conse-

quently decrease by time during the off phase of the OWF. It decreases by pulsing.

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