32 research outputs found
Exploiting Axisymmetry to Optimize CFD Simulations—Heave Motion and Wave Radiation of a Spherical Buoy
Full text linkPublisher Copyright: © 2024 by the authors.Simulating the free decay motion and wave radiation from a heaving semi-submerged sphere poses significant computational challenges due to its three-dimensional complexity. By leveraging axisymmetry, we reduce the problem to a two-dimensional simulation, significantly decreasing computational demands while maintaining accuracy. In this paper, we exploit axisymmetry to perform a large ensemble of Computational Fluid Dynamics (CFDs) simulations, aiming to evaluate and maximize both accuracy and efficiency, using the Reynolds Averaged Navier–Stokes (RANS) solver interFOAM, in the opensource finite volume CFD software OpenFOAM. Validated against highly accurate experimental data, extensive parametric studies are conducted, previously limited by computational constraints, which facilitate the refinement of simulation setups. More than 50 iterations of the same heaving sphere simulation are performed, informing efficient trade-offs between computational cost and accuracy across various simulation parameters and mesh configurations. Ultimately, by employing axisymmetry, this research contributes to the development of more accurate and efficient numerical modeling in ocean engineering.Peer reviewe
Excitation Forces on Point Absorbers Exposed to High Order Non-linear Waves
No full textWave energy devices are often located in intermediate or shallow water close to the free water surface where the wave exposure is extreme. The structures typically consist of large thin curved plates, spherical shells, or cylindrical structures which are sensitive to pressure loads. The lack of proper methods to calculate design pressure distributions has led to structural failures such as buckling in the shells in wave energy prototypes. As a step towards understanding the complex loading from high order non-linear waves, this paper presents a practical approach to estimate wave excitation forces accounting for both nonlinearity and diffraction effects. The method is validated by laboratory experiments using a hemispherical point absorber with a 6-axis force transducer, but the technique is believed to be applicable for most types of submerged or semisubmerged floating devices. The applied method is based on calculation ofa peak force coefficient defined as experimentally measured forces on the structure divided by forces estimated by the chosen theoretical method. Methods used include an integration of the undisturbed wave pressure over the surface of the structure, corresponding to the Froude-Krylov force, and a numerical solution to linear potential diffraction theory. Since the two methods have mutual limitations in describing higher order waves and diffraction, a combination of the two will be introduced
Intraday liquidity management: a tale of games banks play
Full text linkOver the last few decades, most central banks, concerned about settlement risks inherent in payment netting systems, have implemented real-time gross settlement (RTGS) systems. Although RTGS systems can significantly reduce settlement risk, they require greater liquidity to smooth nonsynchronized payment flows. Thus, central banks typically provide intraday credit to member banks, either as collateralized credit or priced credit. Because intraday credit is costly for banks, how intraday liquidity is managed has become a competitive parameter in commercial banking and a policy concern of central banks. This article uses a game-theoretical framework to analyze the intraday liquidity management behavior of banks in an RTGS setting. The games played by banks depend on the intraday credit policy of the central bank and encompass two well-known paradigms in game theory: "the prisoner's dilemma" and "the stag hunt." The former strategy arises in a collateralized credit regime, where banks have an incentive to delay payments if intraday credit is expensive, an outcome that is socially inefficient. The latter strategy occurs in a priced credit regime, where postponement of payments can be socially efficient under certain circumstances. The author also discusses how several extensions of the framework affect the results, such as settlement risk, incomplete information, heterogeneity, and repeated play.Payment systems ; Banks and banking, Central ; Bank liquidity ; Game theory ; Credit
Hydrodynamic forces on monopiles with secondary structures. Numerical approach
No full textThe main purpose of this project was to investigate the usability of numerical models in order to determine the hydrodynamic forces on a monopile with a secondary structure. The numerical models used was BEM models and CFD models.In the analysis, a analytical solution by MacCamy and Fuchs was used to calibrate the BEM models because they are based on the same theory and should trend towards the same results. This was done with a convergence analysis. Skewness of the elements was also considered.For the CFD models the Volume of Fluid (VOF) method was used and using the program Star-CCM+ it is possible to generate waves in the domain. Models with and without the secondary structure was analysed using nine different wave conditions with varying wave height and wave period. The wave conditions are based on waves used in physical model tests.Finally the different methods are compared to each other and physical model tests. The comparison shows that BEM might not be a suitable method to determine forces because drag is a factor when considering monopiles. CFD consider drag and gives results closer to the model tests.But there are still further studies to be made in order to determine the viability of numerical models, but there is potential
Renewal of water in monopiles for offshore wind
No full textDer er i dag stor fokus på miljøet, samt besparelse af konstruktions materialer, og derfor også fokus påbesparelse i offshore sektoren. Dette projekt omhandler en metode for at reducere materiale forbrugetved konstruktionen af monopæls fundamenter for offshore brug. Materiale forbruget reduceres ved atsænke korrosionen af konstruktions materialerne. Reduktionen af korrosionen fungere ved at neutraliserepH-værdien af det indelukket vand inde i monopælene. Det indelukket vands pH-værdi sænkes ved atkorrosionen frigiver hydrogen ioner til vandet. Den sænket pH-værdi resulter i en øget korrosion. Detteprojekt undersøger muligheden for at udskifte det indelukket vand med hav vandet omkring, hvor pHværdien er 8.Udskiftningen forgår ved at et eller flere huller er boret i monopælen, hvor vandet kan strømme indog ud gennem. Strømningen gennem hullerne bliver drevet af tryk forskelle mellem det indelukkevand og havvandet. Systemet er lavet passivt ved at strømningen kun drive af trykforskelle skabt afhavforholdende . Der er i denne rapport undersøgt for tre forskellige havforhold; tidevand, bølger ogstrøm.Målet er at opnå en total udskiftning af det indelukket vand for at øge pH-værdien i hele monopælen.Udskiftningen af vandet er undersøgt gennem CFD modeller i både to og tre-dimensioner. Initialt er laveten simplificering af modellerne for at undgå at simulere havet omkring monopælen. Denne simplificeringviser at havet kan reduceres til at lille område udenfor hullet, og randbetingelserne kan bestemme vedhjælp af energi ligningen.Alle tre havforhold er først undersøgt for at bedømme om en fuld udskiftning er mulig. Denneundersøgelse er lavet i to-dimensionselle modeller. Simplificeringen fra tre-dimensioner til todimensioner resulter i store konsekvenser grundet geometrien af monopælen og hullerne. Der er derforreduceret på tidevands og bølge perioden for at fastholde den samme procentmæssige udskiftning af detindelukket vand. Undersøgelsen af de tre havforhold, viser at ved en konstant strøm, opnås en megetlille udskiftning. I modsætning viser både bølger og tidevand at resultere i en mulighed for at udskiftetalt vandet. Reduktionen af perioden viser sig at have stor effekt på bølge simuleringerne, grundet denallerede korter periode for en bølge i modsætning til tidevandet. Grundet dette er der kun fortsat medtidevandet i to-dimensioner.En videre undersøgelse af tidevandet viser at udskiftning er meget påvirket af temperatur variationenmellem det indelukke vand og hav vandet. Grundet tidevandet lange periode er hastigheden umiddelbartfor langsom til at undgå lagdeling grundet densitet og temperatur forskellene. De to-dimensionelle modelviser derfor at en højere hastighed er nødvendig. For at øge hastigheden gennem hullerne er den tredimensionelle undersøgelse fokuseret på bølger.Før der er lavet analyser af de tre-dimensionelle modeller er strømningen undersøgt ved hjælp afeksperimentelle forsøg. Disse eksperimenter er sammenlignede med simulerings modeller. Det er ergjort for at validere k − e turbulens modellen brugt for simuleringerne.Tre-dimensions modellerne er kun udført for bølger, da den har vist at give mulighed for høj udskiftningved høj strømningshastighed. De tre-dimensionelle modelle viser udskiftningen er mulig ved brug afflere huller placeret modsat af hinanden.Resultatet bliver derfor at en fuld udskiftning er mulig, og pH-værdien kan komme op og ligge næstenstabilt med det omkring liggende hav. Denne neutralisering af pH-værdien betyder at op imod 80 % afdet nuværende anode materiale kan spares væk grundet den reduceret korrosion.This thesis investigates a passive exchange system of the enclosed water inside a monopile, with the sea water . This increases the pH-value of the water inside and decrease the corrosion of the anodes.The exchange system is investigated through numerical CFD simulation of the flow inside the monopile, in both two- and threedimensional models. The exchange is through one or more holes drilled in the monopile.The models are simplified by a comparison between the simulation and the energy equation. The internal flow is validated through laboratory experiments, which are compared with simulated CFD models.It is found that hardly no exchange occurred from current, but a complete exchange might be possible from both tides and waves. The most promising results are from the wave conditions, as this introduce a higher velocity in the flow thought the holes.The results of this thesis suggest that a exchange system, such as this, could result in a reduction of up to 80 % of the anode material needed for the corrosion
CFD analysis of wave-chamber interaction on the Floating Power Plant hybrid wind-wave platform
No full textCFD analysis of current drag on the Floating Power Plant platform P80
No full textDenne rapport omhandler påvirkningen af dragkræfter på et nyt design af en flydende, kombineretbølgegenerator og vindturbinefundament af Floating Power Plant. Funktionaliteten af platformenP80 afhænger af dens orientering i forhold til bølgeretningen, da den frit kan rotere omfortøjningspunktet. Det er derfor nødvendigt at bestemme strømningens indvirkning påorienteringen af P80.P80’eren er blevet undersøgt via CFD analyser for at undersøge de virkende strømningskræfternepå strukturen. Fuldskala simuleringer er blevet kørt ved Reynolds tal på Re≈50e6. Et nøgleresultataf denne rapport er drag koefficienter, som sammenfalder med retningen af strømningen ogortogonalt på samme, når strømningsretningen angrebsvinkel ændres, med henblik på at bestemmekrøjningsmomentet, som strømningen forudsager.Eksperimenter er blevet lavet i renden på AAU på nedskalerede modeller af en floating power plant.Resultater fra disse eksperimenter er blevet anvendt til at validere CFD modellen, som er anvendttil at bestemme dragkræfter forårsaget af strømningen.Andre udfald af projektet inkluderer strømningskræfter på en nedskaleret model af en floatingpower plant og et af dens ben. Disse kræfter er beregnet ved brug af CFD og er begge understøttetaf eksperimentale målinger. Eksperimenterne er udført som en del af dette projekt af forfatterne tildenne rapport og er beskrevet i rapporten.This report concerns drag forces acting on a newdesign of a semi-submersible, combined waveenergy converter and wind turbine generatorby Floating Power Plant. The functionality ofthe platform depends on its orientation towardsincoming waves, as the mooring turret enablesthe platform P80 to rotate freely. It is thereforeof interest to determine the current forces effecton the orientation of the P80.The P80 has been the subject of CFD analysisto investigate the current induced forces actingon the structure. Full scale simulations havebeen performed at Re≈50e6. A main outcomeof this report is drag coefficients in line with andtransverse to the current direction for differentcurrent angles of attack, in order to determinethe yawing moment created by the current.Experiments have been performed in the flumeat AAU on small scale models of a floatingpower plant. Results from these experimentshave been used to validate the CFD model,which is used for determining current induceddrag forces.Other outcomes are current induced forces ona model scale of a FPP and a single Leg ofthis model. These forces are calculated withCFD and are both supported by experimentalmeasurements. The experiments are performedas part of the project by the authors of this reportand presented in detail in this report
Wave Action on Offshore Monopile Foundations: A study on run-up, slamming & influence of secondary structure
No full textThis thesis report documents the analysis of several aspects of wave action on offshore monopile foundations. The first part deals with the run-up phenomenon on monopiles and strives to arrive at a modified run-up factor using previously conducted laboratory tests. The second part focuses on slamming forces and pressures on platforms on monopiles. Both solid and grated platforms with different solidity are analysed. Results from run-up and laboratory results for forces and pressures are used to obtain slamming coefficient and grate multiplication factor for platforms. In the final part of the report, results for wave forces from the recent experiments conducted in AAU for monopile foundation models are analysed. The objective is to study the effect of secondary structures on monopiles by attaching a simple cylindrical element to the model. The experimental results are also compared with the existing theories. Finally the conclusions from the entire study are presented
Wave Excitation Tests on a Fixed Sphere
Full text linkWave excitation tests on a fixed sphere with the center at the still water level were carried out with three different physical wave basin setups. The tests were completed as a continued effort of the working group OES Wave Energy Converters Modelling Verification and Validation to increase confidence in numerical models of wave energy converters by generation of accurate benchmarks datasets for numerical model validation. An idealized testcase with wave excitation of a fixed sphere to be used with the benchmarks was formulated. The three investigated physical wave basin setups included: 1) a six degree-of-freedom load cell mounted to the top of the sphere, 2) a bending beam force transducer mounted to the top of the sphere, and 3) a system of six pretensioned wires mounted to the top and bottom of the sphere with force transducers attached to each wire. The aim of the present paper is to identify the best representation of the idealized testcase. To this end, the three experimental setups are inter-compared in terms of dynamic properties, sensitivity, and disturbances of the water phase from the presence of measurement equipment. Low inter-experiment variability was disclosed, i.e., 5-8% depending on wave-nonlinearity, indicating accurate representations of the idealized testcase across all setups. Setup 3 was found to be the more accurate representation and further work with this setup to release a public benchmark dataset was planned
Facebook: Et spændende forsøg med bølge- og vindenergi i universitets forskningslaboratorium
No full textKim Borup fra AAU Build fortæller her om et spændende forsøg med bølge- og vindenergi i universitets forskningslaboratorium. Læs mere om hvad der foregår på Build, og hvilken betydning det kan få for fremtidens byggeri.Find også artiklen i medlemsbladet på www.smvaalborg.dk - Se link dertil under presse
