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Modelling high axial induction flows in tidal stream turbines with a corrected blade element model

Date: October 14, 2013 at 17:31 GMT

Abstract: The Blade Element Momentum Theory approach to turbine design is a numerically fast tool for design iteration of wind and tidal stream turbines (TST). The method works well around the optimum design point, but theoretical predictions diverge from experiments under a number of conditions.

High induction, where the axial flow towards the turbine is significantly slowed, results in physically meaningless results with standard BEMT. The Buhl correction allows the axial induction factor, a, to be above the theoretical upper limit of 0.5 and to arrive at results that more closely resemble experimental conditions. The correction has been applied to wind turbines operating in air, and this is implemented here for a TST operating in water.

The combined effect of tip and hub loss correction and high induction correction are compared to alternative models for the same rotor. The implementation is numerically robust across all tip speed ratios and avoids the local minima in the solution space found by traditional BEMT implementation.

Results for a number of rotor configurations are presented together with a parametric study of the axial induction factor across the whole rotor diameter and all operating speeds. The conclusions from these results are that the axial load on the rotor is increased by an amount that is not insignificant in design terms, and hence the thrust load on the foundation system of a tidal turbine will also increase above the standard BEMT predictions. 

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