Twenty-Third Symposium on Naval Hydrodynamics (2001) / Chapter Skim
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Prediction of Wave Pressure and Loads on Actual Ships by the Enhanced Unified Theory
Prediction of Wave Pressure and Loads on Actual Ships by the Enhanced Unified Theory
Pages 368-384
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From page 368... ...
Fukushim a (Mitsubishi Heavy Industries, Japan) ABSTRACT To establish n new practical calculation method in place of the conventional strip method, perfor mance of the enEnnced unified theory is investigated through the comparison of computed rmults with n large number of experiments conducted with VLCC tanker and container ship models in this paper, compared are the ship motions the pressure dis tribution, and the wave loads The enhanced uni fled theory is msentinllJ based on 2 D computations but takes account of 3 D and forw rd speed effects Furthermore the e' ts of wave diffraction from the b w part near the waterline are taken into account in n rational WAN Despite these theoretical improve meats, the results of comparison for the w we loads are not so good as expected Since the pressure and wave loads are strongiJ influenced -- the nccuracJ of ship motions, more improvement is needed for precise prediction of the ship motions particularly near the resonance of he we, roll, and pitch INTRODUCTION In the design stage of actual ships, the strip the ore is still in routine use for computing the ship mo tions, added resistance in warm, pressure distribu ti on, and so on Recentl J
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From page 369... ...
of a container ship. We can see some noticeable improvements over the strip method particularly in the pressure distribution and wave loads, but predictions of the enhanced unified theory are still not perfect in some cases when compared closely with experiments.
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From page 370... ...
from (11) , thereby completing the inner solution, which will be used for computing the pressure, the added mass and damping coefficients, and the wave loads Diffraction problem Unlike the mdiation problem, we assume that the rapidly varying part with respect to x is of the same form as fo; that is, em Thus the scattering potential may be sought in the form of f7 = D7(X; y, e)
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From page 371... ...
of the transverse section at station x are evaluated using the toll wing 2 D results: p / of f Is = () JC~ The w we exciting force in the f th direction can be computed -- integrating PD multiplied by by over the ship hull Using (22)
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From page 372... ...
ME z Fig. 2 Positive directions of the wave loads Since the force is the sum of the pressure force and the inertia force due to the body acceleration, Here, to be consistent with the pitch restoring moment, C55, in the motion equation, the contribution of rll-term is included in computing the hydrostatic pressure force.
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From page 373... ...
is satisfied; that is, the wa:venumber in the free surface condition is not K but ko The experimental data of ship motions and the pressure distribution used for comparison in this par per are for z VLCC tanker model The principal particulars of this tanker model are shown in Ia ble I The experiments w re carried out at Ship Pesearch institute and their results were reported by Tzniz we he cl (1993) Although many experimental data exist, only the amplitudes of heave and pitch are shown in Fig 3 for various angles of the wave incidence, together with corresponding result by EUT and STFM The F oude number was set equal to Fr = 0 131 EUT tzLes account of 3 D and forward speed effects in the mdiation and difiraction forces
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From page 374... ...
Premure distribution Comparison of the pressure distribution is shown from Fig 4 to Fig 6 for n VLCC tanker model
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From page 375... ...
The nbsci~sn of the roll motion nround X = 30° Therefore the ench figure is the position nlong the contour and pressure distribution in Fig 4 mnJ be regarded as f = 90°, 0°, nnd 90° correspond to the wenther the pressure induced bJ the w e difiraction onlJ side, the centerline, nnd the lee side, respectiveiJ The overnll ngreement between computed results bJ
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From page 376... ...
.. 90 90 ~o o 30 90 90 9fd~0 Fig 5 Pressure distribution nt S S 4 22 of n VLCC EUT nnd the measurements is sati factorJ Com pared with the results of STFM, EUT gives n no ticenble improvement nenr the ship bottom nt X = 180° H wever, EUT tends to overestimate the pressure on the wenther side in oblique wa:ves Figure5 sh ws the results of N/L~, = 0 75, nt ~ .
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From page 377... ...
~ |r2 = co d~g|4 I I ~ I I ~ X~. Of d~0 r- r" 7 | |, = 90 deg | _ L _ J _ L 6 ~ I ~_1___~ 90 90 ~o o 30 90 90 ~fd~0 Fig 6 Pressure distribution nt S S 4 22 of n VLCC We cnn see that the w we pressure nt N/L~ 1 25 sh wn in Fig 6 is relativelJ smnll in nmplitude except for X = 90° in the present case, the roll mo tion becomes Intge nround X = 90° due to the roll resonnnce in fact, the chnnge in the hJdrostntic - r- 7 ~ ' ' r~ ~|, - 180 degl_ - Cul by kL7T Cul by STFk ffi kxperime=t Pre88ure Di8tributio= A/Lpp = 1.25.
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From page 378... ...
Wa d loads In orderto make Thorough investigation on the wave loads, measurement of the vertical bending moment and the torsional moment h we been car Lied out using a container ship model at Nagasaki R&D Center of Mitsubishi Heavy industries The | _ Cal by BUT Cal by Stiff O Brperimen t Vertical Bending Yomenc In = 0.215 at S.S.5 of Container Ship a container ship (Fn = 0 215) experiments were conducted for various angles of the w we incidence and at five different F oude num hers F rthermore, the wave loads were measured at seven stations along the ship's length The length to breadth ratio, L/B, and the block coefficient, Cb, of the te ted ship model are 6 45, and 0 59, respectiveiJ The nondimensional met centric height in roll, GM/B, was set to 0 03 and the gyrational mdius in pitch, 9,/L, w 0 25 in the experimental setup Figurm 7 and 8 show the vertical bending mo ment and the torsional moment, respectively Thme
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From page 379... ...
, EUT is different from the meT lured results even in the variTtion tendencJ Th ie discrepTncies mTJ be Tttributed to imperfect Tgreement of ship motions, becTuse the WT e ioTds Tre strongiJ dependent on the r inlts of ship motions RegTrding the torsionTI moment Tt S S 5 (Fig 8) , the overTII Tgreement between computed Tnd meT lured r inlts is fT rTble, considering the vTIue itself is smTII compTred to the verticTI bend the variTtion tendencJ for oblique WT es (X = 60° ~nA 1211°N 0 003 0 002 _ _ _,~—~ _ _ _i_ ~3 o oo~ ~ ~ .
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From page 380... ...
drodJnamic forces in the same manner as for he we and pitch, and thus the longitudinal ship motions (surge, hewe and pitch) are computed from fully coupled motion equations among these three modes In the diffraction problem, EUT can also account for the wave diffraction from the b w part near the free su face, because contributions of the at term are retained in the body boundary condition We expected that these theoretical improvements over the strip method w uld result in good prediction of the di tribution of w we pressure and wave loads even for actual ships
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From page 381... ...
Ackn wledgmentz The authors would like to thank Mr H Sueck and Dr T KuroLwn of Mit ubishi Heavy Indu trim for their help in the course of the present study Mr Y Tozaki in assisting numerical computations is also greatly acknowledged REFERENCES Kashtwngi, M, "Prediction of Surge and its Effect on Added Pesistance by Means of the Enhanced Unified Theory," Tmrsactlors of West JcparSocl say of NCDCI Archdeces~ No 89, 1995, pp 77 89 Kashtwngi, M, 'Numerical Senkeeping Calculations Based on the Slender Ship Theory," Ship Tschrol ogy Rsssamh, Vol 4, No 4, 1997, pp 167 192 Kashtwngi, M, K wasoe, K and Inadn, M, "A Study on Ship Motion and Added P distance in Wwm (in Japanese) ," Jourrcl of Kcrscl Society of N A, Japan, No 234, 2000, in press
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From page 382... ...
D, 'The Difiraction of Free Surtace Wa:ves bJ n Slender Ship," Jourr21 of Ship Re search, Vol 28, No 1, 1984, pp 29 47 | - Cal by B7T Cal by STFM O Brperiment Vertical Bending Moment A' = l SO deg at S.S.5 O f Container Ship 5 of n contniner ship (X = 180°) Scl wounos, P
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From page 383... ...
E 3, cod A 3 The results of HighSpeed Slender-Ship Theory HSSST I are also shown m reference [A2] md HSSST gives obvious improvement in E3 This tendency coupling temms HSSST uses also She uniform flow assumption m the body boundary condition, but the forward-sped effects m She fiee-su face condition are fully taken mto account In fact, w have learned from Rctiorurl Ship Theo y RSTI of Of il ie & Tuck that the forward-speed effect in the fiee-su face boundary condition is importmt m the cross~oupling terms, which improves clearly over the strip theory HSSST encompasses RST, md thus it is natural to see good agreement with experiments Prom experiences mentioned clove, w suggest Nat She forward-speed effects in She freesurface condition may be more significmt f m the effects of disturbance potential m the
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From page 384... ...
md [A2] , EUT shows singularity et the frequency equal to ~114, md experimental date also show mpid variation near ~114 1 suppose She results of SWAN are impe feet near ~=1/4, md we carmot discuss this sensitive behavior with questionable mmmericcl reecho [Al]
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