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The influence of inter-device spacing on the performance of a tidal stream turbine

Ellis, Robert 2020. The influence of inter-device spacing on the performance of a tidal stream turbine. PhD Thesis, Cardiff University.
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The focus of this thesis was to investigate how the wake of an upstream horizontal axis tidal turbine influenced the performance of a downstream device. The upstream device used a 0.5m diameter rotor while the downstream device was 0.9m diameter rotor. This was achieved using the commercial CFD software ANSYS CFX and experimental testing for a 1/20th scale, 0.9m diameter device. ThedesignandcharacterisationoftheHATTisestablishedusingtwonumericalmethods: the blade element momentum theory and ANSYS CFX. The numerical models are validated using experimental testing at the Kelvin Hydrodynamics Laboratory and the IFREMER flume and are within 4% and 6% at peak power for the CP and CT respectively. The influence of Reynolds number on device performance is studied and for all testing conducted throughout this thesis the device was independent of the Reynolds number. Thestressblendededdysimulationturbulencemodel, ahybridRANS-LESturbulence model was used, giving a trade-off between turbulence resolution and computational time, for the wake and device interaction modelling. The results show that an increased level of ambient turbulence in the flow improved the wake recovery in both the near and far wake for comparable values of CT. The stress blended eddy simulation turbulence model provides a good prediction, within 6% between4-11Ddownstreamfortherecoveryofawakeforlowturbulencewhencomparedto experimental data using a volumetrically averaged approach. There is a greater difference between the experimental and numerical results when the inlet turbulence is increased. Thediscretisationschemeselectionhasalargeimpactonthepredictionofwakerecovery but causes minimal changes to the performance of the device. The bounded central difference scheme was found to be the preferred discretisation scheme. When using the high resolution scheme an upwind scheme was still dominant leading to greater smearing in flow variables and gave an under prediction of wake recovery. The work conducted throughout this thesis shows that the positioning of a turbine within the bypass flow can lead to an increase in the performance of the individual device whenoperatingatpeakpowerduetotheaccelerationoftheflowbetweenupstreamdevices. The unsteady loading seen by the fluctuation in the mean characteristics as characterised as a percentage of the mean were also reduced when located in the bypass flow.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Tidal stream energy; CFD (computational fluid dynamics); Tidal turbine; ANSYS FCX; Turbulence modelling; Tidal Turbine wake.
Date of First Compliant Deposit: 20 November 2020
Last Modified: 20 Oct 2021 01:07

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