Thesis
Wave transformation over obstacles and its influence on a tidal turbine
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2022
- Thesis identifier
- T16475
- Person Identifier (Local)
- 201653253
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Tidal and marine energy has been explored extensively in recent years. Part of the studies made in the area includes the resource available, the design challenges, the environmental impact and other important areas for research and development. One of these research areas focuses on the mechanical response of tidal turbines to different operational environments. The operational environment can depend on factors such as wave weather, turbulence and flow velocity. The changes in these factors will affect the mechanical performance of the device. This thesis explores the changes in the mechanical performance markers, which are the turbine torque and thrust, to swell wave systems with moderate to low amplitude. The work also explores how a non-even seabed, characterised here as a regular bathymetry rise, can affect the wave parameters such as wave height and wavelength, changing the unsteady flow before and after the geographical obstacle and resulting in a different mechanical turbine response. The thesis first explores the wave propagation over an uneven bottom using classic theory and expanding into the subject with experiments and validation of the theories using physical scenarios resembling normal turbine deployment in the ocean. As the classical theory of wave propagation is frequency non-variant, theories are explored to expand comprehension of the phenomena by addressing frequency variant solutions under larger depths relative to the wave wavelength, which in our case cover periods from 6s to 14s at depths between 28m and 42m. The formulations explored are validated against experiments in a wave tank. Wave conditions are designed to scale wave heights, depths, and periods to the mentioned depth to ensure a good representation of the dynamic behaviour of the waves propagating in the tank. The experiments show how frequency variant theories can predict more closely the changes in the wave amplitude and wavelength when a wave propagates over obstacles of limited length. Amplitude changes, and also changes in the wavelength and velocity, are measured and detected. However, the latter two prove to be harder to measure. The changes in both the amplitudes and lengths correlate in many cases to each other in the experiments. When the wave path has been obstructed, the length for larger waves is reduced and their amplitude increases consistently for larger waves. Validation shows a close relationship between frequency variant theories and the results obtained in the test campaign. The formulations validated are then used to modify a Blade Element Moment Theory (BEMT) model which calculates the mechanical torque and thrust of a tidal turbine. A sensitivity analysis is carried out using the BEMT model to understand how different parameters such as the wave height, current, wave period, and depth ratio between the original depth and the obstacle affect the mechanical turbine response. The results show how wave height is the most important parameter and current velocity is the second. The formulations are then used to model a specific location for tidal turbine deployment in a real case study with data gathered from real buoy systems. Simulation for the case study occurs in three types of wave weather using long swells with low amplitude, smaller period swells with steeper heights, and a mixed case using some wind-wave components with larger heights. The model output shows the large influence of swells on the mechanical performance of the turbine, being the largest contributor to the torque and thrust variation with the thrust and the torque being 60% and 40% larger, than conditions with shorter periods but steeper waves. The study also shows how the inclusion of a simple irregular seabed interacting with large wave components can affect the expected mechanical response in our model. The results show how these small changes, paired with the wave propagation under larger wave heights, change the simulated mechanical response by 50% of their original value when we compare the different responses to each type of wave weather. Work made here seeks to highlight the importance of other environmental factors different than extreme wave loads, current velocity or turbulence that can affect the mechanical behaviours of a tidal turbine device. Particularly it aims to expand our understanding of how the sudden changes in a field of waves produced by local bathymetry irregularities, could modify the forces on a tidal turbine device by integrating with the waves which later propagate into a tidal turbine. These factors could prove to be important to the turbine fatigue or loads depending on their relative impact compared to other already explored factors such as extra waves.
- Advisor / supervisor
- Johnstone, Cameron
- Resource Type
- DOI
- Embargo Note
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