Paulsen, Bo Terp

I denne afhandling præsenteres numeriske beregninger af fuldt ulinære bølgelaster på bundfæstede cylindere. Da studiet har til formål at tilvejebringe ny viden om bølgelaster på fundamenter til havvindmøller, betragtes udelukkende cylinder dimensioner og bølgeparametre relevant for dette.De numeriske beregninger blev gennemført ved hjælp af tre numeriske modeller. Det være sig den fuldt ulinære Navier-Stokes/VOF løser der distribueres som en del af open-source CFD-programmet OpenFOAM R, den fuldt ulineære potentialløser OceanWave3D samt en multi-domæne løser udviklet som en del af dette Ph.D. studie. I multi-domæne løseren beskrives det ydre bølgefelt ved en fuldt ulineær potential-løsning, mens det indre bølgefelt, i nærheden af en given struktur, er givet ved Navier-Stokes ligninger kombineret med en beskrivelse af den frie overflade.Alle numeiriske modeller blev grundigt valideret i form a grid konvergens eller ved sammenligninger med forsøg. I særdeleshed blev den nyudviklet multi-domæne løser grundigt valideret mod eksperimentelle målinger. Det være sig overflade elevationer og kræfter fra regulære bølger , irregulære bølger og retningsspredte irregulære bølger.Der præsenteres et grundigt studie af bølgeanslag på cylindere fra stejle regulære bølger. Her indgår en detaljeret beskrivelse af både kræfter og overflade elevationer, som funktion af den dimensionsløse vanddybde, bølgestejlhed og cylinder diameter. Ydermere beskrives effekten af en tillagt strøm på bølgeanslaget.Fra de numeriske bergninger blev de højere harmoniske kræfter bestemt og sammenlignet med Morisons ligning og analytiske kraftformuleringer udledt til tredje orden i bølgestejlheden.De fysiske processor der fører til skabelsen af en ekstra kraftpåvirkning kaldet “secondary load cycle” beskrives grundigt. Beskrivelsen underbygges af en simple analytisk potentialløsning udledt i to horisontale dimensioner.Diskussionen af bølgelaster på cylindere udbygges yderligere til at inkludere fasefokuserede bølger fra henholdsvis en eller to retninger. Her diskuteres effekten af bølgestejlheden samt retningsspredningen. Idet der for de stejleste bølger ses “secondary load cycles” relateres der til studiet af regulære bølger og det sandsynliggøres at observationer gjort her er alment gyldige.En grundig diskussion af eksperimentel bestemmelse af hydrodynamiske kræfter bliver desuden givet. Der ses betydelige forskelle mellem målte og beregnede højere harmoniske kræfter og det vises at dette skyldes egensvingninger i forsøgsopstillingen. Afslutningsvis blev anvendelsen af multidomæne løseren diskuteret i en bredere ingeniørmæssig kontekst. Her bliver en simple men robust metode til at identificere kræfter der kan være forkert estimeret af Morsions ligning præsenteret. Det foreslås at disse kræfter efterbergnes med multi-domæne løseren.The present thesis considers numerical computations of fully nonlinear wave impacts on bottom mounted surface piercing circular cylinders at intermediate water depths. The aim of the thesis is to provide new knowledge regarding wave loads on foundations for offshore wind turbines. Hence, the dimensions of the cylinders and the chosen wave parameters were inspired by typical monopile foundations for offshore wind turbines.The numerical computations are carried out using three numerical solvers. That is, the fully nonlinear Navier-Stokes/VOF solver provided as a part of the open-source CFD-toolbox OpenFoam R, the fully nonlinear potential flow solver OceanWave3D and finally a fully nonlinear domain decomposed solver, which was developed as part of this project. In the domain decomposed solver, the outer wave field is described by the potential flow solver, whereas the inner wave field, in the vicinity of a given structure, is described by the Navier-Stokes/VOF solver.All numerical models are carefully validated either in terms of convergence by grid refinement or by comparisons to experimental measurements. Special attention is paid to the newly developed domain decomposed solver, which is carefully validated against experimental measurements of regular-, irregular- and multi-directional irregular waves. The ability of the numerical model to accurately reproduce experiments is also investigated.Wave impacts on a bottom mounted circular cylinder from steep regular waves are presented. Here, the inline forces and the motion of the free surface is described as a function of the non-dimensional wave steepness, the relative water depth, the relative cylinder diameter and a co-existing current. From the computations, higher harmonic forces are determined and compared against the Morison equation and established analytical force formulations accurate to the third order in wave steepness.The physics related to the strongly nonlinear load phenomena “secondary load cycles” is described and an explanation of the wave load phenomena is provided. To further support the explanation a simple inviscid kinematic model flow is derived.The discussion of wave impacts on circular cylinders is further extended to uni- and bi-directional phase-focused waves. Here, the influence of the nondimensional wave steepness and wave directionality is discussed. For the steepest wave impacts “secondary load cycles” are observed and the physics of the impact and the mechanisms related to the “secondary load cycle“ are discussed and compared to the observations made for regular waves.Additionally, attention is paid to experimental determination of hydrodynamic forces. Significant differences between experimentally measured and computed higher harmonic forces are observed and the differences are explained in terms of the eigenmotion of the test setup. Finally, the application of the domain decomposed solver is discussed in an engineering context. Here, a simple and robust way of identifying forces, which may be inaccuvii rately estimated by the Morison equation, is presented. It is suggested that these impacts are recomputed by the domain decomposed solver

Online Research Database In Technology

Efficient computations of wave loads on offshore structures

http://orbit.dtu.dk/ws/files/77800007/Ph.D._Thesis_Bo_Terp_Paulsen..PDF

Efficient computations of wave loads on offshore structures

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Online Research Database In Technology