Twenty-Third Symposium on Naval Hydrodynamics (2001) / Chapter Skim
Currently Skimming:
Investigation of Global and Local Flow Details by a Fully Three-Dimensional Seakeeping Method
Investigation of Global and Local Flow Details by a Fully Three-Dimensional Seakeeping Method
Pages 355-367
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From page 355... ...
Abstract A fully three dimensional P nkine panel method in the frequency domain is validated for local pressures, motions, and added resistance Previous formulae for added -- -i Lance contained errors result ing in large differences to e cperiments This has n w been remedied The method is linearized with respect to wave height The steady fl w contribution is cap tured completely by solving the fully nonlinear w we resistance problem first and linearizing the seakeep ing problem around this solution The same grids on the hull are taken for both steady and se keeping computation On the free surface different grids are used, either following quasi streamlined grids or re t angular grids with cut out for the hull The results from the steady solution are interpolated on the new free surface grid The method is applied to various test cases Motions are in good agreement with em periment, but this is also the case for trip method rmults Local pressurm, especially for sho ter wa:vm, are much better predicted than by strip method The added resi Lance is sensitive to higher derivatives of the potential and a numerical differentiation of these terms may be preferable to using highe~order panels 1. Introduction The most commonly used tools to determine seakeeping properties are based on strip theory The strip method approach is cheap, fast, and for most cases also quite accurate H wever, strip methods do not perform so w 11 for high speed ships, full hull form (tankers)
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From page 356... ...
7 Wwes created by the ship should les:ve zrtifi cial boundaries of the computational domain without reflection They may not reach the ship gain (Open boundary condition) 8 Forces on the ship result in motions (Average longitudinal forcm are subsumed to be counter acted -- corresponding propulsive forces, i e the werage speed U remains constant )
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From page 357... ...
Elements use mirror im ges at y = 0 For the symmetrical cases, all mirror im ges have ~ same strength For the anti ymmetrical case, the ir mirror images on the negative y se tor have negative element strength of s me absolute magnitude f =~m\p m\ is the strength of the fth element, p the potential of an element of unit strength including all mirror im ages p is real for the Pnnkine element and complex (8) Or the Thinrt elements The same grid on the hull is used as for the tends problem The grid on the free surface is :-e at d new The quantities on the new grid are linearly interpolated within the new grid from the valum on the old grid Out ide the old grid in the far field, all quantities are set to uniform flow on the new grid The interpolation of results introduces only mall dif Structured grids on the free su face ar crated -- one of the foil wing techniques: I The longitudinal grid lines follow quasi stream lines am :- i the hull The transverse grid lines are equidistantly spaced on lines y =const A maximum entrance angle of 30° is kept which results in zones not covered -- the grid near the b w and stern of blunt ships 2 A rectangular grid is created consi ting of lines x =const and y =const Panels within the war terline are deleted The first technique is well suited for slender ships, the second technique better for blunt ships The second technique, called 'cut out'technique was proposed for the steady wave resistance problem by [6]
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From page 358... ...
= m(~g x 7~22) + 1cr22 I= o~ O o~z 0 o9 0 o~z O oz u Msss di tribution sJmmstrical in y is sssumed ~g stc zrs ths moments of inertiz znd ths centrifugal moments with rssps t to ths origin of ths bodJ fixed Oxyz sJstsm: 0~ = J(y + z )
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From page 359... ...
ordinats sJstsm Ons coordinats foil ws rather longi tudinal linss from stsrn to b w, ths other coordinats foil ws ths hull contour from ths frss su facs d wn to ths kesl Ons of ths longitudinal coordinats linss fol low ths contour of ths stsadJ ws:vs profils and this is ths 'zero' lins for ths other 'ssction' coordinats This modifisd watsrlins contour C accounts also for tsadJ trim and sink gs and difisrs usuallJ ths still watsr lins contour Ths contour lins C splits at ths stsrn and both sidss run from stsrn to b w Ths'ssction' coordinats runs from ths actual frss surfacs Z to ths kml K Then we can rs writs anJ integral over ths w tted surfacs ss: K I 1 (42)
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From page 360... ...
rical/zntisJmmetrical prmsure normal combinations onlJ (AntisJ mmetri cal /SJ mmetrical combi nati ons Jioid zero contributions ) The decomposition into
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From page 361... ...
) beha:vior Ths second derivativm of the harmonic potentials are ne glected in the exprmsion for Vp(~)
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From page 362... ...
One point on the port side was then plotted on its corresponding position on the starboard side Pressures computed by thePSM greewell with mean sured pressures for N/L < I 25 for ~ = 180° and N/L < I 0 for ~ = 150° These limits correspond for the investigated I w F oude number to r values around 0 35 0 4 For sho t w wes, the computations underpredict the pressures at the bottom of the ship compared to measurements However, as the pres sores should decay exponentially with depth like all w e effect, for short waves the near zero values of the computation appear to be more plausible and we assume that they reflect in this case reality better than the measured values For waves of moderate length 0 5 < N/L < 0 75, measured and computed pressures at the ship bottom agree well The trip method results for pressures are worse for short w wes N/L = 0 2, 0 3 where diffraction effects are stronger than radiation effect In summary, the PSM predicted pressures and motions well, the trip method predicted pres sores in short waver badly, but motions well The PSM is currently limited in practice to approximately r > 0 4 Unless techniques are developed to e tend it to smaller r Values, the PSM will remain a research tool of limited functionality We see hybrid methods matching an inner PSM solution to an outer Green function method or Fourier Kochin solution as most promising approach to extend the method to I w r valu m, but at present no such research is planned due to lack of funds Table 1: Test case VLCC L99 307 00 m 29 4 333 m B 54 00 m he 19 193 m T 19 50 m k 73 987 m CB 0 813 .: 76 750 m KG 15 17 m ken 0 m xg 10 045 m 3.2. Added resistance We show hers applications to the ITTC tan hard test case S 175 containership in head seas Com put tions are compared to experiment of Mitsubishi Heavy Industries Fig 5 shows results for Fin = 0 25 and Fig 6 for Fin = 0 3 For all three motions the agreement is good in fact, the computational result for hes:vs for long waves appear to he more plan sibls, as they tend as expected monotonously to I The discrepancies between experiments and compu tations for the higher F ouds number Fin = 0 3 are most likely due to nonlinear damping enacts in the 0 275, [2]
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From page 363... ...
"Exciting forces on n high speed contniner ship in regular oblique w es F equencJ sele tions for calculating exciting forces bJ the strip method ~', J Knnsni Soc Nw Arch Jnpan 187, 1982, pp 71 83
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From page 364... ...
1 u3/h 10 05 10 05 _ I no -t ~ t o - to 05 10 g 1: Motions fi ut/h 02^^oo 0 ~ 05 .
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From page 365... ...
P A/L=02 p A/L=03 Pt ' A/L=04 10- ~ 10- dl..
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From page 366... ...
O . 3 o 3 t I :: ^ t 05 10 15 20 >/: 05 05 Fig 5: AOs for motions and added resistance for S175, Fin = 025, 3~ Row/pgLh2 nl05 03~ n a_ U ~ 0 1 ~3/h .
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From page 367... ...
AU7 HORS'RRPLY We used'cut-out'method for the computation of LCC he reason is that qmtsi-ttretmlined grids either give ve y distorted grids near the bow md stern or do not locate collocation points on She water surface near the ship ends Distorted cells lead to problems with She radiation condition he tut-out'medhod is robuster m this respect for f 11 waterline forms We recommend the cut-out method for ships with block~oefficients clove 0 7 DISCUSSION M Ohkusu Ky shu University, Jcp m Appmently, c pressure t Reducer is located on th water line m your experiment This t msducer is mtturally out of water some duration during one period of the motion Then the time histo y of the pressme measured will be of not c sinusoidal but c truncated sinusoidal So I wonder how you treated with the truncated curve to derive your value of pressure if you take She first harm onic components of f is curve, you will obtain much smaller amplitude of the pressure Nevertheless, your pressme It the water line looks consistent with the pressure It other locations AU7 HORS'RRPLY As you pointed out, w observedth time history of c rmnctred simmsoidal curve m the pressure measurement m She vicinity of he water Ime From the time hi tory data, cmplit de was defmed Is variation betw en zero-level md the positive peck value[l2] So She experimental data plotted m Figs 3 md 4 is not th first harm mic components of She curve in the computations, w do not take She truncated effect mto account The reason why the calculated accuracy is imufhcient in She vicinity of water Ime may be due to treatment of the truncated effect
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