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Numerical models for the motion and forces of point-absorbing wave energy converters in extreme waves
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Center for Natural Disaster Science (CNDS), Villavägen 16, SE-752 36 Uppsala, Sweden.
Department of Mechanical Engineering, Harbin Institute of Technology, Harbin, China.
School of Engineering, Plymouth University, UK.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
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2017 (English)In: Ocean Engineering, ISSN 0029-8018, E-ISSN 1873-5258, Vol. 145, p. 1-14Article in journal (Refereed) Published
Abstract [en]

Reliable simulation tools are necessary to study the performance and survivability of wave energy devices, since experiments are both expensive and difficult to implement. In particular, survivability in nonlinear, high waves is one of the largest challenges for wave energy, and since the wave loads and dynamics are largely model dependent, each device must be studied separately with validated tools. In this paper, two numerical methods based on fully nonlinear computational fluid dynamics (CFD) are presented and compared with a simpler linear method. All three methods are compared and validated against experimental data for a point-absorbing wave energy converter in nonlinear, high waves. The wave energy converter consists of a floating buoy attached to a linear generator situated on the seabed. The line forces and motion of the buoy are studied, and computational cost and accuracy are compared and discussed. Whereas the simpler linear method is very fast, its accuracy is not sufficient in high and extreme waves, where instead the computationally costly CFD methods are required. The OpenFOAM model showed the highest accuracy, but also a higher computational cost than the ANSYS Fluent model.

Place, publisher, year, edition, pages
2017. Vol. 145, p. 1-14
National Category
Marine Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-328485DOI: 10.1016/j.oceaneng.2017.08.061ISI: 000414886600001OAI: oai:DiVA.org:uu-328485DiVA, id: diva2:1135732
Funder
Natural‐Disaster ScienceSwedish Research Council, 2015-04657Available from: 2017-08-24 Created: 2017-08-24 Last updated: 2018-02-22Bibliographically approved
In thesis
1. Wave Loads and Peak Forces on Moored Wave Energy Devices in Tsunamis and Extreme Waves
Open this publication in new window or tab >>Wave Loads and Peak Forces on Moored Wave Energy Devices in Tsunamis and Extreme Waves
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Surface gravity waves carry enormous amounts of energy over our oceans, and if their energy could be harvested to generate electricity, it could make a significant contribution to the worlds power demand. But the survivability of wave energy devices in harsh operating conditions has proven challenging, and for wave energy to be a possibility, peak forces during storms and extreme waves must be studied and the devices behaviour understood. Although the wave power industry has benefited from research and development in traditional offshore industries, there are important differences. Traditional offshore structures are designed to minimize power absorption and to have small motion response, while wave power devices are designed to maximize power absorption and to have a high motion response. This increase the difficulty of the already challenging survivability issue. Further, nonlinear effects such as turbulence and overtopping can not be neglected in harsh operating conditions. In contrast to traditional offshore structures, it is also important to correctly account for the power take off system in a wave energy converter (WEC), as it is strongly coupled to the devices behaviour.

The focus in this thesis is the wave loads and the peak forces that occur when a WEC with a limited stroke length is operated in waves higher than the maximum stroke length. The studied WEC is developed at Uppsala University, Sweden, and consists of a linear generator at the seabed that is directly driven by a surface buoy. A fully nonlinear CFD model is developed in the finite volume software OpenFOAM, and validated with physical wave tank experiments. It is then used to study the motion and the forces on the WEC in extreme waves; high regular waves and during tsunami events, and how the WECs behaviour is influenced by different generator parameters, such as generator damping, friction and the length of the connection line. Further, physical experiments are performed on full scale linear generators, measuring the total speed dependent damping force that can be expected for different loads. The OpenFOAM model is used to study how the measured generator behaviour affects the force in the connection line.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. p. 86
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1551
Keywords
OpenFOAM, CFD, Wave power, Tsunami waves, Extreme waves, Offshore
National Category
Marine Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-328499 (URN)978-91-513-0054-2 (ISBN)
Public defence
2017-10-20, Polhemsalen, 10134, Ångström, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2017-09-28 Created: 2017-08-24 Last updated: 2017-10-17

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