Abstract
Tidal stream turbines (TSTs) produce a rotating downstream wake. This paper describes the characteristics of the swirl flow in the wake of a TST with a view of comparing these against classical swirl theory and investigating whether swirl is an important factor in wake recovery prediction.Using computational fluid dynamics the paper describes the characteristics of velocities, pressure drop, viscosity and swirl number of 2, 3 and 4 bladed TSTs. To provide confidence in the results the characteristics are compared to the findings in the literature for a set of generic swirl generators. The swirl numbers for the TSTs in a 3.08m/s tidal (plug) flow were found to be between 0.14 and 0.28, which describes a weak or very weak swirl flow. Whilst the characteristics are in agreement with theory it also means that the swirl component of the wake is not coupled with the axial component and cannot be used to estimate the wake length. However, peak swirl number for the 4 bladed turbine is close to the threshold of 0.3 at which axial velocity starts to become coupled with tangential velocity and therefore wake recovery may be related to S for some turbine designs.
Highlights
Peak swirl number for the 4 bladed turbine is close to the threshold of 0.3 at which axial velocity starts to become coupled with tangential velocity and wake recovery may be related to S for some turbine designs
This paper investigates whether the wakes of Tidal stream turbines (TSTs) fit into classical swirl theory and determines if the swirl number can be used to estimate the length of wake recovery
This paper investigated whether the wakes of TSTs fit into classical swirl theory and whether the swirl number can be used to predict wake recovery
Summary
It is widely understood and accepted that in order to ensure security of supply, the energy mix must diversify to include a wider variety of resources [1]. A recent study in the Strangford Lough, by an independent science group led by Royal Haskoning and including the Northern Ireland Environment Agency, Queen’s University Belfast, the Sea Mammal Research Unit and others, has shown that there were no adverse effects on the wildlife and environment following the installation of the SeaGen turbine within those waters [8]. This endorsement should provide impetus for the deployment of arrays of tidal devices by a number of companies. Whilst initial work focussed on the swirl characteristics of a 3 bladed turbine [14], this paper considers the swirl characteristics in the wake of 2, 3 and 4 bladed rotors
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