Twenty-Fourth Symposium on Naval Hydrodynamics (2003) / Chapter Skim
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Nonlinear Green Water Effects on Ship Motions and Structural Loads
Nonlinear Green Water Effects on Ship Motions and Structural Loads
Pages 413-427
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From page 413... ...
INTRODUCTION The objective of the work presented in this paper was to develop a sophisticated green-water-on-deck model that could be integrated directly into a timedomain ship motion calculation, thus allowing greenwater effects to be included in the calculation of nonlinear ship motions and loads. In order to attain this, an approach was selected in which the ship motions and green-water-on-deck calculations run concurrently in the time domain.
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From page 414... ...
The outer domain is solved with transient Green functions distributed over an arbitrarily shaped matching surface, while the inner domain is solved using Rankine sources. The advantage of this formulation is that Rankine sources behave much better than transient Green functions near the body and free surface juncture, and that the matching surface can be selected to guarantee good numerical behavior of the transient Green functions.
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From page 415... ...
1998~. Interface to Green-water Calculation At each time step, LAMP uses the ship rigid body motion, the incident wave definition, and the hull pressure distribution to compute the relative motion of the edge of the deck to the wave surface.
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From page 416... ...
s hs ds ~ —O (3) s=1 e where the bars account for average values between time step k and k -1, and where do are the four sides s corresponding to the horizontal surface Ce (see Figure 3.b)
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From page 417... ...
The body accelerations represented by b are the horizontal accelerations (normal to the gravity vector) induced by the motion of the platform combined with the effect of the local slope of the platform ~nz (nX;ny)
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From page 418... ...
Therefore, to render the calculation possible while at the same time stable, a minimum value of fluid level hmin must be defined such that when the fluid elevation of an element e drops below this value, that element should be considered dry. This minimum value will determine both the speed and the precision of the calculation Equation (29)
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From page 419... ...
The two horizontal components of V and the elevation H are computed by interpolation from the flow velocity and water elevation on the boundary of the computational grid. The relationship between H and the water level on the grid boundary is obtained from statistical data.
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From page 420... ...
Figure 4: Relation of water height on deck boundary and freeboard exceedance (Zhou et al., 1999~. VALIDATION Part of the present effort has been directed toward establishing the accuracy of the green-water model in relation to both theoretical and experimental data.
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From page 421... ...
The approach is fully integrated in the LAMP System, and several studies have been carried out in which green-water calculations for different ships in a varied range of sea conditions have yielded satisfactory results. At every time step, LAMP calculates the relative motion at the deck edge and passes it, along with the rigid-body ship motions, to the green-water calculation supervisor.
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From page 422... ...
, it can also be seen that the boundary conditions defining the superstructure effectively isolate the interior of the superstructure from the incoming water.
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From page 423... ...
Figure 11 part a depicts the ship motion, incident wave surface, and computed green-water elevations for the ship in large, regular head seas. The wave height is 11.0 meters, the wavelength is 300 meters, and the ship speed is 20 knots.
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From page 424... ...
Figure 12 shows the instantaneous deck pressure at one time step of the regular head sea case shown in part a above. The calculation of the deck pressure allows the approach to be used for evaluating deck and hatch cover loads directly or by using the pressure to create load data sets for detailed structural analysis.
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From page 425... ...
Lin, W.M., and Yue, D.K.P., "Time-Domain Analysis for Floating Bodies in Mild-Slope Waves of Large Amplitude," in Proceedings of the Eighth International Workshop on Water Waves and Floating Bodies, Newfoundland, Canada, 1993. Lin, W.M., Meinhold, M., Salvesen, N., and Yue, D.K.P., "Large-Amplitude Ship Motions and Wave Loads for Ship Design," in Proceedings of the Twentieth Symposium of Naval Hydrodynamics The University of California, Santa Barbara, U.S.A., 1994.
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From page 426... ...
The authors use a 3D body-nonlinear method for the global ship motion problem, while the quasi3D shallow water formulation is used for the local green water domain. However, the paper does not precisely describe the interaction between the local and global domains along their common border, namely the deck edge.
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From page 427... ...
Nevertheless, the green-water model implementation can work in a standalone mode, in which case, through an appropriate input file, the user can specify any arbitrary motion of the grid and boundary characteristics, both in terms of obstacles and water elevations, and regarding incoming or exiting water flow.
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