The impact of turbulence and turbine operating condition on the wakes of tidal turbines

Ebdon, Timothy 2019. The impact of turbulence and turbine operating condition on the wakes of tidal turbines. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

This thesis examines the impact that turbulence and turbine operating condition have on the wake of a horizontal axis tidal turbine, and the implications of this impact for array design. This is done via the use of computer simulations using a scale-resolving hybrid turbulence model known as detached eddy simulation, which combines a Reynolds-averaged Navier-Stokes model in the near-wall region with large eddy simulation-like behaviour in other regions. This allows turbulent uctuations to be resolved in the wake region whilst reducing the computational expense when compared to a pure large eddy simulation model. These computer simulations were supported by extensive ume measurements at three tip-speed ratios and three different turbulence conditions, which were used for both validation of the computational model, as well as data in their own right. It was shown that turbulence intensity has a significant impact on the wake; increasing wake recovery, reducing wake swirl and increasing the width of the downstream region impacted by the wake, whilst simultaneously reducing the magnitude of this impact. In addition, high turbulence intensities appear to have some impact on the mean performance characteristics of the turbine and cause large increases in the magnitude of the uctuations of these performance characteristics. These increases in uctuations become larger still as the turbulence length scale increases. Turbulence length scale was not found to have a significant impact on the wake, however, an impact could not be ruled out. The turbine operating condition was also found to have a profound impact on the resulting wakes. In the near wake, centreline velocity recovery was found to increase with increasing turbine thrust due to ow being diverted towards the turbine nacelle. For a volumetric averaged wake, greater power extraction was found to cause the greatest near-wake deficit. Wake swirl was found to be at a maximum at the point of peak turbine torque, whilst wake width was found to increase with increasing tip-speed ratio (and therefore turbine thrust). It was found that the detached eddy simulation turbulence model produced good agreement with the experimental results across a wide variety of turbulence conditions. This is expected to be of interest to array designers wishing to optimise array configurations due to its ability to accurately predict both wakes and turbine performance, including transient effects.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Engineering
Uncontrolled Keywords:	Tidal turbines; Wakes; Turbulence; Operating condition; DES; Length scale.
Date of First Compliant Deposit:	18 September 2019
Last Modified:	06 Jan 2021 02:42
URI:	https://orca.cardiff.ac.uk/id/eprint/125530

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