Abstract
Actuator line computations of two different tidal turbine rotor designs are presented over a range of tip speed ratios. To account for the reduction in blade loading on the outboard sections of these rotor designs, a spanwise flow correction is applied. This spanwise flow correction is a modified version of the correction factor of Shen et al. (Wind Energy 2005; 8: 457-475) which was originally developed for wind turbine rotors at high tip speed ratios. The modified correction is described as ‘directionally dependent’ in that it allows a more aggressive reduction in the tangential (torque producing) direction than the axial (thrust producing) direction and hence allows the sectional force vector to rotate away from the rotor plane (towards the streamwise direction). When using the modified correction factor, the actuator line computations show a significant improvement in the accuracy of prediction of the rotor thrust and torque, when compared to similar actuator line computations that do not allow the sectional force vector to rotate. Furthermore, the rotation of the sectional force vector is attributed to the changing surface pressure distribution on the outboard sections of the blade, which arises from the spanwise flow along the blade. The rotation of the sectional force vector can also be used to explain the reduction in sectional lift coefficient and increase in sectional drag coefficient that has been observed on the outboard blade sections of several rotors in the literature
Highlights
The majority of low order rotor models (such as the blade element momentum (BEM), actuator line and actuator surface methods) operate using the flow independence principle
The flow independence principle assumes that the forces acting on each aerofoil section of a rotor blade may be computed by only considering the flow components in the plane of the aerofoil section and that the forces experienced are the same as those experienced by the corresponding two-dimensional aerofoil
They allow the tangential force per unit span to drop off faster than the axial force per unit span, so the sectional force vector can reduce in magnitude and rotate towards the streamwise direction
Summary
The majority of low order rotor models (such as the blade element momentum (BEM), actuator line and actuator surface methods) operate using the flow independence principle. The accuracy of the existing correction factors has not been comprehensively assessed when applied to these rotors In this investigation a modified version of the existing correction factor of Shen et al will be adopted instead and calibrated to two different tidal turbine rotor designs. New actuator line computations will subsequently be carried out using the calibrated correction factor, to demonstrate the resulting improvement in rotor thrust and torque prediction. Before carrying out these computations, the spanwise flow correction factor will be introduced, along with the tidal turbine rotor designs that are used for the calibration. THIS ARTICLE HAS BEEN SUBJECT TO SINGLE-BLIND PEER REVIEW BY A MINIMUM OF TWO REVIEWERS
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