Wind Turbine Aerodynamics

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The aerodynamics of horizontal-axis wind turbines still contains many topics where important improvements are to be made. This proposal focuses on four areas in the context of blade element theory (BET), the simple method of estimating wind turbine performance that is universally used for blade optimization, and is the basis for many control algorithms and operational models:***1. low tip speed ratio (TSR). The project will develop current work on the "nonlinear" terms that arise in BET equations for momentum and angular momentum in the wake for finite blade numbers particularly at low TSR. In conjunction, a model of the expanding wake at low TSR will be developed and applied to the related problem of accurate determination of the tip loss which accounts for the finite number of blades. This work will the main component of the DG project. The main collaborator will be Professor Valery Okulov (Dept Wind Energy, DTU, Denmark). It is proposed to extend Wood and Okulov's recently derived direct calculation of tip loss to improve its numerical behaviour in BET calculations and to investigate the effects of wake expansion and the application to cases where current tip loss models are inaccurate, such as diffuser-augmented wind turbines. ***2. Solidity. BET treats the blade elements as airfoils which is equivalent to assuming zero solidity, defined as the ratio of the blade element chord to the circumferential distance to the blade elements on adjacent blades. A wind tunnel model turbine with varying numbers of blades will be used to further investigate solidity and low TSR effects. An already-started computational fluid dynamics study of solidity for "cascades" of airfoils will be be used to interpret the experimental results.***3. Low Reynolds number (Re). Small wind turbines operate at low Re and are often compromised by the low lift:drag ratio at these conditions. Current wind tunnel tests at the UofC to determine the lift and drag of airfoils at low Re, are being extended to include the effects of icing which has not been studied at low Re. Concurrent CFD studies of both iced blades (and the mechanism of icing) will be used to compare with the experiments which use rapid-prototyping approximations to generic icing shapes.***4. Blades with unequal aerodynamic loading. Recently, the proposer showed theoretically for the first time that additional induced velocities occur in BET when the blades are aerodynamically unequal. This inequality can have many causes, such as wind shear and unequal (individual) pitch angles. These examples will be used to extend the theory of unequal loading and to trace its significance in terms of transient loads on the turbine drivetrain. Practical information on icing and unequal blade loading and its consequences for wind turbine dynamics will feed into a new project on turbine asset management in conjunction with a major wind farm owner.