Trends in CFD Ap­pli­ca­tions for the Mar­itime In­dus­try

SP's NavalForces - - TECHNOLOGY / HADR - VOLKER BER­TRAM, DNV GL MAR­ITIME AD­VI­SORY

hULL DE­SIGN IS ThE num­ber one fac­tor in fuel ef­fi­ciency. It im­pacts prof­itabil­ity, com­pet­i­tive­ness and ship value. Since the first com­mer­cial ship basin was com­mis­sioned in 1883, tow­ing tanks have pro­vided naval ar­chi­tects with a re­li­able method of pre­dict­ing the per­for­mance of a ship at sea. Tank test­ing is com­monly used for both re­sis­tance and propul­sion tests. how­ever, the cost and ef­fort of pro­duc­ing a model and test­ing it, means that this process is uti­lized late in the de­sign cy­cle. This method ver­i­fies and fine-tunes an es­tab­lished de­sign, rather than be­ing a tool to help drive and op­ti­mize the de­sign.

cFD has long been con­sid­ered a cred­i­ble al­ter­na­tive to tank test­ing. It pro­vides a nu­mer­i­cal model that can be im­ple­mented much ear­lier in the de­sign process. Naval ar­chi­tects can make use of en­gi­neer­ing data to in­flu­ence and im­prove the de­sign process. An­other ad­van­tage to cFD is the ac­cu­racy of re­sults, in­de­pen­dent of the scale of cal­cu­la­tion.

PRE­FERRED AP­PROACH

The im­prove­ments in com­put­ing power, have al­lowed ex­perts work­ing in ship­ping to use cFD cal­cu­la­tions to sim­u­late ves­sel hy­dro­dy­namic per­for­mance more ac­cu­rately and faster than ever be­fore. The in­dus­try’s abil­ity to han­dle com­plex ge­om­e­try with all rel­e­vant de­tails has also greatly im­proved. De­vel­op­ment in grid gen­er­a­tion has made it eas­ier to gen­er­ate high-qual­ity grids for ac­cu­rate cFD sim­u­la­tions.

Many as­pects have ad­vanced the wide ac­cep­tance of cFD as a de­sign and op­ti­miza­tion tool. The in­crease in hard­ware power com­bined with progress in var­i­ous as­pects of the flow solvers per­mit a wider scope of more so­phis­ti­cated ap­pli­ca­tions. Such analy­ses have be­come in­creas­ingly im­por­tant and have now re­sulted in cFD sur­pass­ing model tests as the pre­ferred ap­proach for many ap­pli­ca­tions in the mar­itime in­dus­try.

DE­VEL­OP­ING TECH­NIQUES

More so­phis­ti­cated cFD analy­ses for ships and off­shore plat­forms em­ploy a va­ri­ety of tech­niques that have be­come widely avail­able in re­cent years. One key as­pect for car­ry­ing out cal­cu­la­tions based on com­plex ge­ome­tries, such as anal­y­sis of off­shore plat­forms, is ge­om­e­try recog­ni­tion. In this case the pre-pro­cess­ing soft­ware uses pris­matic cells to rec­og­nize cylin­ders with ex­tru­sion along cen­ter­line and thin solids, or gaps, with pro­jec­tion from one side to an­other. The re­sult be­ing that to­day, cFD mod­els of­ten pro­vide a higher level of de­tail than achieved with model tests. cFD soft­ware can now han­dle mov­ing parts (pro­pel­lers or rud­ders), model com­plete sys­tems rather than sin­gle parts, and can re­place ge­om­e­try (if re­quired) to per­form anal­y­sis with and with­out spe­cific parts.

TUR­BU­LENCE MOD­EL­LING

In the 1980s and 1990s un­sat­is­fac­tory re­sults were of­ten blamed on the lim­i­ta­tions of tur­bu­lence mod­el­ling. This type of mod­el­ling is use­ful for an­a­lyz­ing the flow struc­tures and re­sult­ing re­sis­tance of bare hulls, as in­ves­ti­gated in most val­i­da­tion stud­ies. how­ever, the pro­peller be­hind the ship dom­i­nates flows and re­duces the ef­fect of the tur­bu­lence model. For most ap­pli­ca­tions in the ma­rine in­dus­try, the stan­dard k- or k- tur­bu­lence mod­els are ad­e­quate. But other mod­els are avail­able to bet­ter pre­dict sec­ondary flows, the Reynoldsstress model (RSM) cur­rently be­ing one of the most pop­u­lar op­tions. In the fu­ture, large-ed­dysim­u­la­tion (LES) analy­ses are likely to end the de­bate on tur­bu­lence mod­el­ling. LES di­rectly cap­tures the larger, sig­nif­i­cant ‘fin­ger-print’ vor­tices of the flow di­rectly and uses sub­grid-scale tur­bu­lence mod­els for the small, ‘back­ground noise’ tur­bu­lence. cur­rently only a few re­search in­sti­tu­tions have the com­pu­ta­tional re­sources nec­es­sary to carry out LES cal­cu­la­tions. how­ever, th­ese re­sources are ex­pected to be­come avail­able to the in­dus­try over the next ten to fif­teen years through a gen­eral growth of com­put­ing power and cloud-based busi­ness mod­els.

CLAS­SI­FI­CA­TION SO­CI­ETY AP­PROVED

Free-sur­face flows are of great in­ter­est to naval ar­chi­tects. Mea­sur­ing the wave re­sis- tance of a ship can help them de­ter­mine which small or mod­er­ate changes in hull shape could sig­nif­i­cantly re­duce the over­all re­sis­tance of the ves­sel and im­prove its per­for­mance. Other ap­pli­ca­tions of freesur­face flows in­clude sea­keep­ing, slam­ming and slosh­ing. Mod­ern cFD meth­ods al­low the sim­u­la­tion of highly non­lin­ear free sur­face flows. Such sim­u­la­tions are now so well pre­dicted that they are widely ac­cepted by clas­si­fi­ca­tion so­ci­eties for load de­ter­mi­na­tion in strength analy­ses.

EAS­IER TO USE

cFD tools have be­come more user-friendly as re­flected in the use of in­te­grated de­sign en­vi­ron­ments. The in­te­grated de­sign en­vi­ron­ment com­bines many as­pects of cFD soft­ware in­clud­ing free-form hull de­scrip­tion us­ing para­met­ric mod­el­ling, in­ter­faces to most mod­ern cFD solvers, sev­eral op­ti­miza­tion al­go­rithms, and soft­ware to han­dle process man­age­ment and user in­ter­faces. The de­sign en­gi­neer can then work on sim­u­la­tion driven de­signs us­ing one in­ter­face from model gen­er­a­tion to post-pro­cess­ing.

Grow­ing com­puter power and fully au­to­matic pro­ce­dures have opened the door for for­mal op­ti­miza­tion as the nat­u­ral step be­yond sim­u­la­tion-based de­sign. Lines op­ti­miza­tion (also lo­cal bow op­ti­miza­tion for re­fits in times of slow-steam­ing) saves typ­i­cally 5% be­yond the sim­u­la­tion­based ap­proach. Trim op­ti­miza­tion saves typ­i­cally 3% be­yond the clas­si­cal ap­proach based on crew ex­pe­ri­ence.

LEAVE IT TO THE EX­PERTS

De­spite the grow­ing power of cFD soft­ware, it re­mains a tool. The speed and qual­ity of re­sults achieved de­pends on the per­son us­ing the tool. Ef­fec­tive cFD re­sults are achieved through a com­bi­na­tion of knowl­edge, un­der­stand­ing and skill­ful cFD tech­niques. De­spite progress in num­ber crunch­ing, ex­per­tise and com­pe­tence re­main at the core of good en­gi­neer­ing. This ar­ti­cle first ap­peared in Ship­Build­ing In­dus­try, Vol. 9 Is­sue 5. Many thanks to Yel­low & Finch Pub­lish­ers – Ship­Build­ing In­dus­try, for al­low­ing us to re­use it.

Model of a com­plete propul­sion sys­tem in STAR-CCM+

Newspapers in English

Newspapers from India

© PressReader. All rights reserved.