COM­PU­TA­TIONAL FLUID DY­NAM­ICS VS. MODEL TEST­ING

Yachts International - - Sternlines -

A deep-rooted—and healthy—skep­ti­cism guides en­gi­neers in their quest to bal­ance hu­man safety with trends in equip­ment and ma­te­ri­als. to that end, naval ar­chi­tects have cau­tiously em­braced com­pu­ta­tional fluid dy­nam­ics (CFD) as a means to re­duce de­sign-cy­cle time, es­pe­cially in the early stages of the de­sign process, while ask­ing whether the re­sults are as ac­cu­rate as those achieved by us­ing model test­ing.

Don­ald Blount, whose com­pany de­signed the 220-foot (67-me­ter) record-speed-set­ting Fin­cantieri Destriero, had a client so in­trigued by the sub­ject that, prior to mak­ing a build de­ci­sion for a 262-foot (80-me­ter) high-speed mo­to­ry­acht, funded an ex­haus­tive study to com­pare the pre­dictabil­ity of the two meth­ods.

Em­ploy­ing the test­ing fa­cil­i­ties at SSPA in Göte­borg, Swe­den, the study ex­am­ined three hull­forms: dou­ble chine, round bot­tom with spray rail, and sin­gle chine. Each would be tested at two dis­place­ments and as many as three lon­gi­tu­di­nal cen­ter of grav­ity po­si­tions. this pro­gram en­sured that a wide range of data would be col­lected to com­pare the hulls. “the ob­jec­tive of this in­ves­ti­ga­tion,” the study states, “was to iden­tify a hull­form that achieved suit­ably low re­sis­tance…to sat­isfy an en­durance re­quire­ment while re­main­ing dy­nam­i­cally sta­ble…to sat­isfy a top speed re­quire­ment.”

After run­ning the mod­els in each of its var­i­ous per­mu­ta­tions, some of the same test con­di­tions were com­pared to re­sults from a CFD ap­pli­ca­tion—which, the study stated, “demon­strated that it could sat­is­fac­to­rily dif­fer­en­ti­ate the bare hull re­sis­tance be­tween sev­eral can­di­date hull­forms,” and not­ing, “the ab­so­lute mag­ni­tude of the re­sis­tance pre­dic­tions is gen­er­ally within three per­cent [of] the ex­per­i­men­tal data.”

Blount says the re­sults of the two pre­dic­tion meth­ods in­di­cated some dif­fer­ence in speed when dy­namic in­sta­bil­ity was likely to oc­cur.

“i per­son­ally pre­fer ex­per­i­men­tal test­ing, since ex­per­i­men­tal re­sults are im­me­di­ate, con­firm­ing you have met de­sign ex­pec­ta­tions or not,” he said. “if ex­pec­ta­tions are not met, one can read­ily al­ter a model to eval­u­ate al­ter­na­tive so­lu­tions.

“An an­a­lyt­i­cal model rep­re­sent­ing hull ge­om­e­try re­quires a very ex­pe­ri­enced hy­dro­dy­nam­i­cist for in­put to CFD soft­ware to ob­tain re­al­is­tic re­sults,” he added. “in­ex­pe­ri­enced CFD users are dan­ger­ous, as they might not rec­og­nize the va­lid­ity or re­al­ity of out­put pre­dic­tions, which can vary due to poorly dis­trib­uted el­e­ments for hull ge­om­e­try.” —M.M.

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