Twenty-Fourth Symposium on Naval Hydrodynamics (2003) / Chapter Skim
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Genesis of Design Wave Groups in Extreme Seas for the Evaluation of Wave/Structure Interaction
Genesis of Design Wave Groups in Extreme Seas for the Evaluation of Wave/Structure Interaction
Pages 231-253
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From page 231... ...
The desired characteristics like wave height and period as well as crest height and steepness are defined by an appropriate objective function. The subsequent optimization of the initially random phase spectrum is solved by a Sequential Quadratic Programming method (SQP)
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From page 232... ...
As an illustration of this technique a reported rogue wave - the Draupner 'mew Year Wave" is simulated and generated in a physical wave tank. Also a "Three Sisters" wave sequence with succeeding wave heights HS...2Hs...Hs, embedded in an extreme sea, is synthesized.
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From page 233... ...
Starting with a linear approximation of the desired wave train by optimizing an initially random phase spectrum for a given variance spectrum we obtain an initial guess for the wave board motion. This control signal is systematically improved to fit the wave train to the predetermined wave characteristics at target location.
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From page 234... ...
Assuming linear wave theory, the synthesis and up-stream transformation of wave packets is developed from this concentration point. At this position all waves are superimposed without phase shift resulting in a single high wave peak.
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From page 235... ...
The associated wave board motion which has been determined by the above semi-empirical procedure is the key input for the nonlinear analysis of wave propagation. As has been generally observed - at wave groups as well as at irregular seas with embedded rogue wave sequences - we register substantial differences between the measured time series and the specified design wave train at target location if a linearly synthesized control signal is used for the generation of higher and steeper waves.
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From page 236... ...
rrad/s Fig. 4 Registrations, Fourier spectra, and transfer functions of a typical seakeeping test with a high-speed catamaran in transient wave trains (model scale 1:7, Vm=4~0 m/s; full scale: V=20.5 kn; Fn=0.56~.
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From page 237... ...
As a prerequisite, however, the computer controlled loop in the experimental generation process should imply nonlinear wave theory and develop the wave evolution by using a numerical time-stepping method. The two dimensional fully nonlinear free surface flow problem is analysed in time domain using potential flow theory.
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From page 238... ...
A finite element method developed by Wu and Eatock Taylor (1994, 1995) is used to determine the velocity potential, which satisfies the Laplace equation for Neumann and Dirichlet boundary conditions.
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From page 239... ...
If viscous effects are also considered and approach of transient viscous free surface flow computation with (RANSE/VOF) solver is used.
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From page 240... ...
- velocities due to wave/filter interaction for submerged wall and 1 1% filter. rNTEGRATION OF DESIGN WAVE GROUPS IN IRREGULAR SEAS - LINEAR APPROACH 350 In general, extremely high 'rogue' waves or critical wave groups are rare events embedded in a random seaway.
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From page 241... ...
For the selected spectrum- significant wave height Hs = 0.7m, peak period Tp = 4.43s, water depth d = 5.5m - a high transient design wave within a tailored group of three successive waves in random sea is optimized. The target zero-upcrossing wave height of the design wave is Hd = 2Hs with a maximum crest height (~(Xtargett~arget)
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From page 242... ...
15) all target features regarding global and local wave characteristics, including the rogue wave specification HmaX = 2Hs and '7c = 0 6 Hmax are met Of course, this result is only a first initial guess as linear wave theory used is not appropriate for describing extreme waves since nonlinear free surface effects significantly influence the wave evolution.
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From page 243... ...
Wave board motion resulting from SQP optimization From the velocity potential which has been determined as a function of time and space all kinematic and dynamic characteristics of the wave sequence are evaluated.
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10 i1 1 1~, 1 1 10.1 In ns 14 ~16 18 20 ~;:2 ~ 26 xJh Fig. 22 Kinematic and dynamic characteristics of rogue wave sequence HS ...
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Based on deviations between the measured wave sequence and the design wave group at target location the control signal for generating the seaway is iteratively optimised in a fully automatic computer-controlled model test procedure (Fig.23)
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From page 246... ...
at GM=1.36m v=151m, z-manoeuvre with ,u = ~ 10°. wave sequences consisting of random seas or regular wave trains with an embedded deterministic high transient wave - are systematically varied to investigate the ship model response with regard to metacentric height, model velocity, and course angle for each of both ship types.
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From page 247... ...
Using the proposed wave generation technique, the wave board signal is calculated from the target wave sequence at the selected wave tank location.
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From page 248... ...
Based on the wave focussing technique for the generation of task-related wave packets a new technique is proposed for the synthetization of tailored design wave sequences in extreme seas. The physical wave field is fitted to predetermined global and local target characteristics designed in terms of significant wave height, peak period as well as wave height, crest height and period of individual waves.
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From page 249... ...
(2001~: Generation and Numerical Simulation of Predetermined Nonlinear Wave Sequences in Random Seaways, 20th OMAE Symposium, Rio de Janeiro, Brazil Clauss, G., Schmittner, C and Stutz, K
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From page 250... ...
So the discusser wants to express his congratulations to the authors. My discussion is about the importance of 3dimensional effects of design waves or extreme waves interacting with floating structures.
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From page 251... ...
In conclusion, the rogue wave story and the development of wave focussing procedures as well as the deterministic generation of extremely high waves and wave sequences embedded in realistic seas must continue to reveal the secrets of wave-structure interaction and to improve the design of safer ships and offshore structures.
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From page 252... ...
Next, dedicated computercontrolled capsizing tests with deterministic wave trains are carried out at model scale embedding rogue wave sequences in severe seas. Based on these results, we finally developed a non-linear numerical method for simulating ship motions in extreme seas.
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From page 253... ...
d) Finally, non-linear numerical methods can be validated by dedicated seakeeping model tests in deterministic wave sequences.
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