Lokalno opterećenje i odziv strukture broda uslijed udaranja o valove

Grgić, Mate (2015) Lokalno opterećenje i odziv strukture broda uslijed udaranja o valove. = Local load and response of a ship structure due to slamming. Doctoral thesis , Sveučilište u Zagrebu, Fakultet strojarstva i brodogradnje, UNSPECIFIED. Mentor: Žanić, Vedran and Malenica, Šime.

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Abstract (Croatian)

U kontekstu hidrodinamičkih opterećenja tankostjenih struktura, na lokalnoj odnosno globalnoj razini odziva udaranje o valove predstavlja vrlo bitan dio s obzirom da može uzrokovati ozbiljna oštećenja lokalnih elemenata konstrukcije kao i prouzrokovati prolazne globalne vibracije trupa plovnog objekta. U radu je razmatran direktni pristup proračunu tlakova uslijed udaranja o valove u preliminarnoj fazi projektiranja koji se zasniva na hidrodinamičkom modelu potencijalnog strujanja fluida. Napravljen je opsežan uvid u postojeće metode proračuna pri čemu je detaljno objašnjena poopćena Wagnerova metoda (PW) koja danas predstavlja jedan od osnovnih načina rješavanja problema udaranja. Osnovna jednadžba problema izražava se pomoću Greenovog trećeg identiteta uz korištenje Greenove funkcije Rankinovog tipa čije rješenje se dobiva numerički pomoću metode rubnih elemenata. Za potrebe ovog istraživanja osnovna metoda PW je u potpunosti implementirana u proračunski kod što je omogućilo detaljnu analizu svih njenih karakteristika. Analiza je uključivala ispitivanje konvergencije rješenja na temelju varijacije ulaznih parametara te određivanje optimalne veličine proračunske domene na slobodnoj površini. Uočeni nedostaci postojeće PW metode nastojali su se ukloniti razvojem unaprijeđene PW metode. U cilju postizanja točnije raspodjele potencijala brzine na oplakanoj površini tijela napravljena je desingularizacija istog. Potencijal brzine podijeljen je u dva dijela: na regularni i singularni dio. Pri tome je uvedena nova funkcija kojom se opisuje ponašanje rješenja u blizini kontaktne točke i koja predstavlja singularni dio potencijala. Također, uvedena je i nova Greenova funkcija, Kelvinovog tipa, s kojom nestaje potreba za diskretizacijom slobodne površine s obzirom da novo uvedena funkcija zadovoljava dinamički rubni uvjet na slobodnoj površini. Također, napravljena je analiza lokalnog odziva strukture uslijed udaranja o valove na primjeru jednostavne roštiljne konstrukcije brodskog dna. Proračun udarnih tlakova napravljen je poboljšanom poopćenom Wagnerovom metodom koji su zatim prebaceni na strukturni model konacnih elemenata korištenjem prostorne i vremenske interpolacije. S obzirom na krutu lokalnu strukturu problem odziva riješen je na kvazistatički način zanemarivanjem inercijskih sila strukture. Opterećivanje modela napravljeno je u nizu zamrznutih vremenskih trenutaka te je proračun proveden za svaki trenutak odvojeno. Na osnovu tako dobivenih rezultata analizirana je propagacija naprezanja u vremenu te pojava značajnih reznih sila pri nailasku na diskontinuitet u strukturi.

Abstract

In the context of the hydrodynamic loading and structural response of thin– walled structures, slamming type of loading makes an essential part on both the local and global level, where it can cause severe damage to the local structural elements and transient vibrations of the hull. In order to define an adequate load calculation methods due to slamming in the preliminary design stage, as well as analysis of the local response of the structure, a direct approach in calculation of hydrodynamic loading is considered using potential flow analysis. A comprehensive insight into the existing calculation methods is evaluated and among them the important group of so called generalized Wagner approach is explained in detail. The generalized Wagner models assume the simplification of free surface boundary conditions while the boundary condition on the wetted surface of the body remains in an original form. The velocity potential is represented by Green’s third identity which is solved numerically using boundary element method. A basic approach of generalized Wagner method is implemented in a form of programming source code to allow a detailed analysis of all its characteristics. The analysis includes solution convergence evaluation and determination of a size of the free surface domain by varying the input parameters of the model (a number and a length of the segments on the wetted part of the body and the free surface). These imperfections in the existing PW methods are partialy removed with a development of an enhanced Generalized Wagner method. In order to achive more accurate pressure distribution on the wetted surface of the body the velocity potential is desingularized. A function of the velocity potential is divided into two separate parts: a regular and a singular one. The singular part is defined by the function which describes a behaviour of the solution close to the contact points. In order to simplify a calculation a new Green’s function of Kelvin’s type is introduced. Using the new approach there is no need for a discretization of the free surface because the boundary condition on that part of the domain is satisfied exactly. Also, a local response of a stiffened panel due to slamming is analysed. As an example a part of a bottom structure of a ship is used. The calculation of the impact loads is performed using the enhanced generalized Wagner method. Then, the pressures are mapped into a structural FEM model using a spatial and temporal interpolation. Due to the rigidity of the local structure, the structural analysis is performed using quasi-static approach neglecting inertial forces. The model is loaded by static impact pressure at several time steps and the response is calculated for each of them separately. Using the FEM results the propagation of significant shear forces along the the stiffened structure is analysed.

Item Type: Thesis (Doctoral thesis)
Uncontrolled Keywords: udaranje broda o valove; poopćena Wagnerova metoda; poboljšana poopćena Wagnerova metoda; desingularizacija potencijala; Kelvinova Greenova funkcija; metoda rubnih elementa; preslikavanje tlakova; lokalni odziv konstrukcije
Keywords (Croatian): slamming; Generalized Wagner Method; Enhanced Generalized Wagner Method; desingularization of the velocity potential; Kelvin’s Green function Boundary element method pressure mapping; local response of a structure
Subjects: TECHNICAL SCIENCE > Shipbuilding
Divisions: 600 Department of Naval Engineering and Marine Technology > 620 Chair of Marine Structures Design
Date Deposited: 07 Mar 2016 10:39
Last Modified: 07 Mar 2016 10:44
URI: http://repozitorij.fsb.hr/id/eprint/5189

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