Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
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Radiation and Diffraction Waves of a Ship at Forward Speed
Radiation and Diffraction Waves of a Ship at Forward Speed
Pages 29-44
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From page 29... ...
With this technique we are able to view the diffraction and/or radiation waves which are usually invisible in tank tests due to the existence of other effects such as incident waves and steady Kelvin waves. Measured wave pattern especially with blunt bow and shallow draft ships is used to investigate validity of an analytical method to predict the diffraction and radiation waves and added resistance of a ship.
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From page 30... ...
We test the accuracy of our analytical approach by comparing the diffraction wave field predicted with the measured with our improved technique We presume that if the prediction of the diffraction wave elevation around the bow is accurate, then the prediction of wave pressure on the bow part will be accurate and the added resistance, on which the wave loads at the bow is the most influential, will be predicted accurately. 2 Measurement of Radiation and Diffraction Waves Neither radiation wave nor diffraction wave field generated by a ship advancing in waves is visible at tank test because of the coexistence of other waves such as steady wave generated by the forward speed of the ship on otherwise a calm water and incident waves.
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From page 31... ...
This transverse transform is possible when the measured wave data as shown in Fig.3 and 4 are available. The wave pattern in Fig.3 and 4 exemplifies that diffraction waves vanish rapidly as y become larger; the transverse transform is more convenient for one may terminate the transform at a finite y.
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From page 32... ...
Ballast Condition, En = 0.17, A/L = 0.5 ; Figure 4: Diffraction Wave of a Series 60 (CB = 0.8) at wt = 0, at Full Load(left)
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From page 33... ...
This agreement suggests that our linear model of the wave given by equation (2) is correct in describing diffraction waves around a ship.
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From page 34... ...
3 Computation of Diffraction and Radiation Waves Full nonlinear treatment of radiation or diffraction waves generated by a running ship is not straightforward. Even linear or qusi-linear theory is not so easy to be numerically implemented (Sclavounos (1996)
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From page 35... ...
This free surface condition, though it is linear with respect to ¢, apparently produces an intricate situation for the diffraction wave; the incident waves coming far upstream which is usually assumed linear will be diffracted on the displaced free surface and the ship surface. Numerical implem~entation of this formulation has not been successful for the diffraction waves.
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From page 36... ...
satisfies linear free surface conditions without the terms including Us and Us: U ,~ ° _ -iw¢o - gCo on z = 0 0(o . Who U—=—twCo + ax Liz on z = 0 Nevertheless ~ + no must satisfy the free surface condition retaining the nonuniform steady flow and wave elevation represented by Us and Us As a consequence the diffraction wave o involves a part to correct the difference of the actual free surface condition from (13)
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From page 37... ...
This is rather contradicting however because if the steady flow velocity in x direction is large enough to make U + loos/8x small, then the diffraction wave propagates upstream and the present method assuming no effect from downstream to upstream is invalidated. 37 Series-60 (Ballast)
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From page 38... ...
0.5 _.... (/`A O -0.5 0.4 0.5 0.6 Present method -- -- - With uniform steady flow o Measured .
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From page 39... ...
~ With uniform steady flown a 3D pane! method 0 Measured .
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From page 40... ...
~ _.... 0.5 ;/(A O -n Present method With uniform steady flow ~ 3D panel method 0 Measured .
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From page 41... ...
Consequently one can ascribe the larger added resitance to the diffraction wave supposed to be larger for the blunt bow hull forms. If it is true, the present method which we confirmed is able to compute the diffraction waves more correctly than other approaches may possibly predict added resitance of those hull forms more correctly.
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From page 42... ...
Solid line represents added resitance due to the diffraction wave which was computed by substituting the velocity potential and wave elevation obtained by the present theory into equation (16~. Two circles in this figure are the value obtained by equation (17)
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From page 43... ...
18th Symp. on Naval Hydrodynamics 1990 Ohkusu,M:"Hydrodynamics of Ships in Waves", Advances in Marine Hydrodynamics, Computational Mechanics Publications 1996 Ogilvie,T.F.: "Singular Perturbation Problems in Ship Hydrodynamics", Advances in Applied Mechanics 17 Academic Press 1977 Sclavounos,P.:"Computation of Wave Ship Interaction" Advances in Marine Hydrodynamics, Computational Mechanics Publication 1996 SR211:"Study on Improvement of Middle Size Bulk Carriers Performance", Report of Japan Ship Research Association 1993 Stoker,J.J: " Water Waves ",Interscier~ce Publishers 1 957 43
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From page 44... ...
In the process of validating analytical methods, it is very important to check how accurately we are predicting the individual components of waves generated by a ship moving in waves. In this regard, this paper shows the proper direction in which we have to focus our investigations.
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