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| Titel |
The New Year Wave: Generation, Propagation, Kinematics and Dynamics - Registered in a Seakeeping Basin |
| VerfasserIn |
Günther Clauss, Marco Klein |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250032482
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| Zusammenfassung |
| In the past years the existence of freak waves has been affirmed by observations, registrations,
and severe accidents. One of the famous real world registrations is the so called “New Year
wave,” recorded in the North Sea at the Draupner jacket platform on January 1st, 1995. Since
there is only a single point registration available, it is not possible to draw conclusions on
the spatial development in front of and behind the point of registration, which is
indispensable for a complete understanding of this phenomenon. This paper presents the
temporal and spatial development of the New Year Wave generated in a model
basin.
To simulate the recorded New Year wave in the wave tank, an optimization approach for
the experimental generation of wave sequences with predefined characteristics is used. The
method is applied to generate scenarios with a single high wave superimposed to irregular
seas. During the experimental optimization special emphasis is laid on the exact reproduction
of the wave height, crest height, wave period, as well as the vertical and horizontal
asymmetries of the New Year Wave. The fully automated optimization process is carried out
in a small wave tank. At the beginning of the optimization process, the scaled real-sea
measured sea state is transformed back to the position of the piston type wave generator by
means of linear wave theory and by multiplication with the electrical and hydrodynamic
transfer functions in the frequency domain. As a result a preliminary control signal
for the wave generator is obtained. Due to nonlinear effects in the wave tank, the
registration of the freak wave at the target position generated by this preliminary
control signal deviates from the predefined target parameters. To improve the target
wave in the tank only a short section of the control signal in time domain has to
be adapted. For these temporally limited local changes in the control signal, the
discrete wavelet transformation is introduced into the optimization process which
samples the signal into several decomposition levels where each resulting coefficient
describes the control signal in a specific time range and frequency bandwidth. To
improve the control signal, the experimental optimization routine iterates until the
target parameters are satisfied by applying the subplex optimization method. The
resulting control signal in the small wave tank is then transferred to a large wave
tank considering the electrical and hydrodynamic RAOs of the respective wave
generator.
The extreme sea state with the embedded New Year Wave obtained with this method is
measured at different locations in the tank, in a range from 2163 m (full scale) ahead of to
1470 m behind the target position—520 registrations altogether. The focus lies on the detailed
description of a possible evolution of the New Year Wave over a large area and time
interval.
The analysis of the registrations reveals freak waves occurring at three different positions
in the wave tank and the observed freak waves are developing from a wave group of three
waves, which travels with constant speed along the wave tank up to the target position. The
group velocity, wave propagation, and the energy flux of this wave group are analyzed within
this paper. |
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