Twenty-Fourth Symposium on Naval Hydrodynamics (2003) / Chapter Skim
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Wash Waves Generated by Ships Moving on Fairways of Varying Topography
Wash Waves Generated by Ships Moving on Fairways of Varying Topography
Pages 441-457
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From page 441... ...
Strong wash waves can be generated by a fast ship at high speeds or by a large ship at moderate speeds, operating on a near-shore fairway or on an inland waterway. The resulting wash-wave system is basically nonlinear due to the wave characteristics in shallow water and usually unsteady due to the nonuniform seabed or inland waterway topography.
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From page 442... ...
a computer program BEShiWa, standing for Boussinesq's Equations for Ship Waves, has been developed with the following features: - extension of the shallow-water equations of Boussinesq type to longer and shorter waves over an uneven bottom, — inclusion of the near-ship flow into the shallowwater equations either through the law of conservation of mass or through a free-surface pressure distribution equal to the hydrostatic pressure on the hull bottom or through a unified shallow-water theory, — implementation of suitable boundary conditions, and — application of numerically efficient and robust methods. In the present study, we focus on the washwave systems generated by a Panmax containership and a fast inland passenger-ferry.
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From page 443... ...
Table 1: Main dimensions of investigated ships Inland Passenger Ferry Panmax Container Vessel EWL 39.3 m 280 m B 8.8 m 32.2 m T 1.2m 11 m Representative Waves in a Large Shallow-Water Region To demonstrate the capability of the computer program BEShiWa to predict ship waves over a huge computational domain, Figure 1 shows three representative wave systems generated by the subject inland passenger-ferry moving in an unbounded shallow-water region of uniform depth. The computational domain was of 17.5 ship lengths long and 7.5 ship lengths wide, taking advantage of transverse symmetry.
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From page 444... ...
At a supercritical speed, Fnh= 1.5 in graph (c) , the final wave system comprises only divergent waves, no initial transverse waves generated during the acceleration phase could keep up with the ship.
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From page 445... ...
However, other computations have shown that shape and speed of the wave ahead of the ship do depend on the initial acceleration pattern. Influence of Channel Section Shape on Wash Waves Figure 3 shows wave patterns generated by the inland passenger-ferry moving at critical speed (referred to the water depth along the channel centerline)
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From page 446... ...
Within the main wave system, acceleration phase with those later generated by ship the initial acceleration influences strongly the so- at constant speed can be noticed in the transition called primary wave but only weakly the trailing between the deepened and shallow regions, see graph waves. This phenomenon can be clearly observed in 7(a)
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From page 447... ...
and shallow banks (h = 3 m) Figure 3: Influence of transversally varying bottom topography on the wave pattern generated by the subject inland passenger-ferry moving at constant speed As = 7 m/s (corresponding to local Fnh = 1 on the channel centerline)
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From page 448... ...
High acceleration Figure 6: Contour plots of wave patterns generated by the subject containership at a speed V=8.35 m/s in a nearshore fairway (Note: Here the direction of motion is from right to left)
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From page 449... ...
~ I 1 1 1 0 50 100 150 200 250 300 t [s] Figure 7: Wave records showing the influence of initial ship acceleration ~ fast, -- -- slow acceleration)
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From page 450... ...
As the harmonic primary waves = /~///////~/////~) Lit Cal milcantly depends on bottom topography, ship speed and motion history, any measures for reducing wash waves deduced from computational predictions need to be validated by experiments.
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From page 451... ...
evolution of the wave pattern while the ferry moves beyond the ramp (e) wave pattern of the ferry at subcritical speed in the deeper region Figure 9: Evolution of the wave pattern generated by the subject inland passenger-ferry moving at a constant speed of 8 m/s over a fairway with a ramp as shown in Figure 8.
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From page 452... ...
~ it, it' An 25 30 35 40 45 50 -30.44 m 30 35 40 45 50 10 15 20 25 30 35 40 45 t [s] 50 Figure 10: Records of wave elevation at 6 different wave-probe locations for an initially harmonic wave of period 2.02 s and amplitude 0.02m propagating over an uneven bottom (see Figure 12)
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From page 453... ...
.:~ ~ V ~ r ~ it' ~ . 15 it [~ As on ' 1 0,04 E 0,02 0,00 -0,02 -0,04 -0,06 0 5 10 0,08 0,06 0,04 0,02 E BOO -O,02 -O,04 -0,06 -0,08 o 15 20 25 30 35 40 x = 37.04m Figure 11: Records of wave elevation at 6 different wave-probe locations for an initially harmonic wave of period 2.525 s and amplitude 0.029 m propagating over an uneven bottom (see Figure 12)
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From page 454... ...
and Garner, J.: "Seabed modifications to prevent wake wash from fast ferries," Proceedings of the RINA International Conference on Coastal Ships and Inland Waterways, London, 1999. Henn, R., Sharma, S
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From page 455... ...
and Grollius, W.: "Fast vessels on inland waterways," Proceedings of the RINA International Conference on Coastal Ships and Inland Waterways, London, 1999.
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From page 456... ...
However, we would emphasize again our statement that due to the nonlinear and unsteady nature of ship waves in shallow water the linear theory remains to be a restricted approximation. Furthermore, it should be clarified that the vertical distribution of the transversal velocity components is explicitly described as an analytical function of the averaged horizontal velocity in the Boussinesq's shallow-water theory.
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From page 457... ...
In the present work, a 'slender-body' type condition is used: the passage of the ship imposes a lateral velocity distribution, which is averaged over the entire water depth. This is consistent with Boussinesq theory; but intuitively one would expect that this is less accurate for higher water depth / draught ratio's.
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