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The performance of a tidal stream turbine under wave and current interaction

Lloyd, Catherine ORCID: 2020. The performance of a tidal stream turbine under wave and current interaction. PhD Thesis, Cardiff University.
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This thesis examines the performance of a tidal stream turbine when operating under wave and current conditions. This was accomplished by investigating the loadings imparted on a 1/20th scale, 0.9m diameter tidal turbine, under multiple wave and current conditions. Experimental testing was undertaken as well as numerical modelling, achieved using the commercial CFD software package ANSYS CFX. Initial investigations analysed the performance and loadings on the turbine under uniform and profiled current only conditions. Experimentally, some of the velocity data sets were not normally distributed and therefore modal values were found to present a more accurate method of calculating the predominant flow conditions at each water depth. Average performance characteristics were found to be unaffected by the presence of a profiled current velocity. However, transient loadings such as the out of plane bending moment, thrust and torque, experienced greater variations in the data sets due to stanchion interaction. The amplitude of fluctuation in the loadings were heightened with increasing shear in the velocity profile, while the maximum and minimum turbine loadings coincided with the rotational frequency of the turbine. CFD simulation results showed good agreement with those observed experimentally. Further analysis explored the addition of waves which intensified the complexity of the flow conditions experienced by the turbine. The introduction of waves was found to have an insignificant effect on the average current velocity at each water depth and the average performance characteristics. However, the presence of an oscillatory flow effect induced by the waves greatly affects the transient turbine loadings. With the addition of waves, the fluctuation in the total turbine thrust and torque increased by roughly 35 times that of the current only cases. Peaks in the loadings were found to be aligned with those in the wave surface elevation. This indicates the importance of minimising large loading fluctuations to increase the fatigue life of a tidal stream turbine operating in these conditions. A CFD model was developed to simulate combined wave and current interaction with a turbine. Optimisation of the model geometry, mesh and setup are detailed in this thesis for simulations using regular waves within the limits of Stokes 2nd order theory. Comparisons to experimental data highlight the homogeneity between the data sets, justifying the use of CFD to accurately replicate experimentally generated flow conditions.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Tidal turbine; Wave-current interaction; Stokes 2nd Order Theory; Computational Fluid Dynamics; ANSYS CFX; Experimental validation.
Date of First Compliant Deposit: 23 June 2020
Last Modified: 07 Nov 2022 10:34

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