Twenty-Third Symposium on Naval Hydrodynamics (2001) / Chapter Skim
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Wave Resistance Computations - A Comparison of Different Approaches
Wave Resistance Computations - A Comparison of Different Approaches
Pages 792-804
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From page 792... ...
mproach to describe file fiee su face Although it still neglects viscous effects m the flow held, details like breaking waves in file vicinity of file free su face are predicted Test cases indicate signiEc mt improvements in file wave held computed md m the numerical prediction of file wave resi t mce The currant version of the code is implemented in a multiblock mode it is pamllelised using the message passmg prodigm md runs on shared md distributed memo y computers 3 Additionally a commercial RANSE-solver Comet is applied This code also allow to compute the fiee water smface The method is simile to the one described under 2, but tw phases are considered 4 b mother Unite volume solve' a level set technique inhoduced by Osher & Sethim, is used to cmt re file free smface between tw distinguished phases, water md air b file formulation used here, the goven ing equations are solved in bodh file water md file air domains md the two phases are considered as one A level set function, ˘, deEmed in both phases is mitialised as file did mce, wish sign, to file undi turbed fiee su face instead of solving the tw cur nit tiw equations for density md viscosity, which w uld c mse numerical difficulties, the scalar function ˘, which is continuous even under ch mges of topology, is con ected by the velocity held Dependmg on file sign of ˘, the density md the viscosity are given the values of water md air, respectively Compared to fiont hackmg medhods to compute free su face flow, no regadding is necessary md no boundary conditions are mplied at file free su face Further, file position of the fiee so face need not be explicitly evaluated during file computation, but is canted out m the po t-processing The steep dersih gradients at file tree su face necessitate the mtemolation of file physical prope ties owr a few cells around file tree su face However, the inte face itse f remams sham Unfortunately, no stable computational results for file sele ted test cases were available at the submission deadline for flus paper Hence, the method had to be excluded Impor mt details of the fEree different Pph ache., mcludmg the mathematical formulation of the fiee so face conditions used are show in the following
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From page 793... ...
mitially at the mmdist bed water so face Thus, hi- md quxLilateral pmels are distributed owr the wetted part of file body only To avoid p mels wish m unfavourable Am= =~ | Cut p mel me h at wet- | I tedbomdxy l | Generate grid tm w ter | 1 ~U.I~ O.UU..U..U.. 1 | Inte polate flow fiom | I Devious sten I | Co efficients for body | | Coeffclents for free | | boundary condition | | surface boundary | | Solve linear equations | I forsourceshength I I Compute pressure I I Compute velocites md I | md forces on body | | wave heights l | Estimate chmge in | | Calcuiate enorinnon- | | tam mdsmkage | | linear conditions | aspect ratio near file watts line, some comer points are rifler moved or omitted A point solace is located near the contra of each p mel md shi ted mside file body depending on si e md shape of the p mel There are no p mels on the h .Itrll; they are simply led open A mesh of collocation points is distributed on file fiee su face around the hull The total length md width of the grid, as well as the spacing, are deters mined automatically based on file Fronds Number Also, file lateral did mce of file innemmost row of collocation points is pre-set, based on the FroudeNumber For submerged h msoms or ve y blunt water lines at the stem, additional row of collocate
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From page 794... ...
Thus, ~ tone Deem of An why ~ ~~! ~c~t mix =d AL ~~ Durham} a; de~ ~e unknown mume sag 8, solver comb~n~'ng elms - ~= and -~on steps ~s urn.
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t~ mew ger~era - .
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2.~. H~ - ~n mn~= of ~e previous exerted ~LC - F mew shows sty higher wave elusions' Ails ~n tum Ding mom - chic the ~e - at.
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From page 797... ...
ii3dS = 0 (3 4) Below file tree su face, this leads to a mass conservation equation The cell All ratio F is file ratio of file cell volume hlled by file liquid to the total cell volume |fdQ F= 3 (35)
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T~ ~~= Integral ~ ~r the EY ~~:~ IS - EXIT USES ~e p0.~t of ~ nuX =d the o~n Ce~:]
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Em: 3~- M~ gem =d ~~ (~)
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From page 800... ...
The u" ~ ~e UD scheme cams wry Bong snowing of ~ Id. The CO p~= ~e shad of He ~~e ~t ~ ~ same time ~}~trMu=s non-phys~:t -ens ~~nd ~e inter ~~e =d p - ~~s vows of the Ague -on which am - - ~ prosily meanings Adds F~ 4.
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From page 801... ...
a,5 AD ~ -5 ~ ? 0 o-5 -~.5 _~.O _~.5 H~ 5 ~ ~~partsQn of ~~e porn- ~ C - ~t - ~~ solution ( Imp d - Imp' Whop PATTERNS fear Me KRISŁ} mn~'n=~p ~sŁ ~ crimp=-: tom wem perched ~r ~e mMe]
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From page 802... ...
, lastmg apple ox 25 hour for 12000 iterations K = 120 A) The results presented in fLis article are promising This holds less for the accuracy of global results as for the resist mce of a ship but con ainly for the flow held phenomena such as the wave patted or pressure dishibutions it is ob ious Hat for practical applications potential flow computations are not yet so passed Newnheless, the held type methods solving either Euler or R NS equations are more promising for file futme Today's design process relies heavily on file use of potential flow codes for design optimization The present mvestigation how that more reEmed results especially in teams of details predicted m the flow held cm be expected fiom ~ OF or lewl set methods As computation time is still a major concern for these predictions, considerable effort is presently pend on pamllelising codes md running fLem on PC clus8.
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From page 803... ...
: A calculation procedure for t rbulent flow m comple geomehies, Computers & Fluids, 15, E Schreck, M Peric: Computation of fluid flow wfh a pamllel multigad solved' Int J Numer Medhods m Fluids, 16, 303-327 (1993)
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From page 804... ...
ji University of Tokyo, Jcp m This papery Rmkine Source Method used uniform flow et first value es base flow of fiee su face But recently, double model flow or exact flow is used to R Mine Source Method es base flow How cutout do you thi k for base flow? AUTHOR'S REPLY According to our experience you save et most one iteration tep ff you use c double model solution es the start up This does not seem to make the extra complexity of She code worthwhile I do not underst Ed what you me m with the term "exact flow" in the coot :xt of the start up of c panel method DISCUSSION L Rahejc Indi m Petit te of Techmology, b din You have mentioned that in your first code, you used desmgulari ed panel medhod in pk e of con- emmn~l panel method I would like to k ow, how much cd mtage you gain by using this method over the con- emigre one bee mse sometimes you require to make adjustments in the did mce fi om the wall where the source is placed AUTHOR'S REPLY G Jensen In using desmgulari ed panel medhod w used the boundary condition in terms of no-flux across the panel Ed w did not find my problem with this Ed we have been using it for lo t ten years We even use it for fiee surface boundary The computation is very fast bee mse of no singularity being present
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