My Thesis: Combined wave-current scale model testing at FloWave

Somewhat belatedly, I’m posting a summary of my thesis on this blog. I’ve copied both the lay summary to explain the work in more general terms, together with the more technical abstract of my thesis.

Somewhat belatedly, I’m posting a summary of my thesis on this blog, to summarise what I spent over 3 years of my life working on. I think after finishing my corrections I had had enough of writing for a while. I also started my new post at the university around the same time, so was busy with that, and somewhat lost interest in this site.  I’ve copied below both the lay summary to explain the work in more general terms, together with the more technical abstract of my thesis. If you are really interested,  the full thesis is available in the Edinburgh Research Archive.

Lay summary

Humans are trying to harness energy from the seas to provide a clean and renewable source of electricity. This energy can be from the flow of the tides or movement of waves. To help with designing these, as well as using computers, small models are often tested in tanks to test new features. This is similar to testing new aeroplane models in a wind tunnel.

This research was carried out at The University of Edinburgh’s FloWave Ocean Energy Research Facility. Here, a small-scale version of the ocean can be produced in a large circular tank, like a swimming pool. This tank is fitted with paddles that can generate small waves. It also has pumps that can make the water flow like a tidal current in the sea. It is possible to make waves and currents at the same time, at any direction across the tank.
This project was to look at how to test models in the advanced waves and currents possible in the FloWave tank. It also looked at what clients can learn from testing models in waves and currents at the same time.

As a new tank, it is important to check how well the tank works. A previous project checked how well the tank makes waves. This project measured flow across the tank, how the flow varied between different places in the tank, plus changes in flow over time. The research also looked at how the shape of waves are changed by the flow of water. This included cases where there is an angle between the direction the waves and current are travelling in, which has not been studied much before. A way to make the right waves in the middle of the tank, at the same time as a current, was developed and tested. This is useful because the real sea has tidal currents as well as waves, so testing this is important.

The other part of the project looked at some of the problems people testing models in tanks like FloWave need to think about. Many of these have already been thought about before, and are written down in guidance documents. FloWave is different to most other tanks. It is circular and can make very realistic sea conditions. New guidance to cover this is therefore needed and was suggested. A list of questions was also put together, to highlight all the things clients need to think about before testing. This included what is in existing guidance, plus some extra questions about FloWave. These also explain what can be tested in the FloWave tank, and how much time this might take.

Results from this research project show the tank works properly. They show some of what is possible with waves and currents at the same time in the tank. They are also used to help with planning and running tests in the tank for clients.

Abstract

As part of a global drive to produce renewable electricity, devices are being designed to harness energy from the waves and tidal currents. Physical scale model testing is an important part of the development process for this and other technologies. The FloWave Ocean Energy Research Facility at The University of Edinburgh is designed to conduct these tests. Here it is possible to produce multi-directional waves combined with currents in the circular tank, re-creating the complexity of the ocean.

The research was driven by commercial requirements of the facility, aiming to highlight what can be learnt from testing at scale with complex conditions in a controlled environment. To enable this, it was first necessary to extend the characterisation of this new facility. Wave generation and reflections were assessed in a previous project. In this work, flow measurements taken throughout the test volume of the tank, allowed spatial and temporal variations in the currents to be determined. Waves and currents interact in a complex manner, compounded by the method of reproducing them in a tank. The influence of currents on waves in the basin was assessed. This included cases with an oblique angle between them, on which little has been published.

The other part of the project addressed issues to be considered when testing in a combined wave-current basin such as FloWave.

  • At many sites of interest for offshore renewable energy, waves are influenced by water depth. Implications of not scaling depth consistently were considered, and design diagrams produced to facilitate understanding and quantification of potential errors.
  • At FloWave, waves are generated in still water around the outside of the tank. A process was therefore developed and verified to produce the desired combined conditions in the central test area following their interaction with the current.
  • There is a wealth of published guidance on tank testing, for ships, offshore structures, and more recently renewable energy. This has been reviewed and suggestions offered to augment this by including testing in the more advanced conditions possible in a facility like FloWave.
  • Tools and guidance have been developed to highlight many of the issues to be considered by clients prior to testing at FloWave. This aims to facilitate planning of a test programme by highlighting potential knowledge gaps and recording decisions made. Flowcharts have been produced to represent this graphically, with a corresponding checklist of questions for clients, which have been trialled in a pilot study.

Outputs from this research are being used to help deliver both academic and commercial client tests at FloWave. The test area in currents was shown to be >50m2 with <10% variation in flow, and the combined wave-current conditions possible have been explored. Results that are important when designing client test plans.