Passive Flow Control Method for Mitigation of Unsteady Load Excursions on a Wind Turbine Blade
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Stochastic flow conditions, such as large unsteady wind gusts and coherent structures in turbulent winds, cause detrimental blade loadings to horizontal axis wind turbines (HAWT). Such loadings lead to large blade root bending moments, increased blade fatigue damage, and inconsistent rotor torque and thrust. In the present work, robust, fast-response passive flow control (PFC) methods for mitigating adverse effects from unsteady aerodynamic loadings on a HAWT blade due were developed. The PFC methods examined were that of a leading-edge slot and a novel combination of two passive flow control devices, namely a leading-edge slot and a passively oscillating surface located at the slot exit (slot-flap). Wind tunnel tests were conducted at a Reynolds number of 0.3 x 10^6 and unsteady flow conditions were produced by an upstream disturbance generator. The disturbance generator utilized shed vortices from a rapidly deflecting upstream airfoil to simulate high-frequency, impulse-like load excursions on the main airfoil, representative of large wind gusts and coherent structures in turbulent winds. Results from the test series demonstrated reduced maximum load excursions of 7 to 9% using the PFC methods as compared to clean airfoil tests. Effectiveness of the PFC methods decreased for loading with multiple vortex interactions. To further examine the influence of the slot-flap oscillations, two additional slot-flap configurations were examined: (1) increased rigidity and thickness and (2) reduced length. Both variations indicated a further reduction of transient load excursions from 8 to 12% as compared to a clean airfoil. Coefficient of momentum (c_mu) was calculated for each slot-flap configuration, which was on the order of 1% and varied with angle of attack. The reduced excitation frequency (F+) ranged from 0.96 to 1.23 based upon the slot-flap configuration. The three slot-flap variations showed comparable c_mu and F+ average values and were within the uncertainty limits, indicating that the improved mitigation from the slot-flap variations is not attributed to c_mu and F+ contributions. Based upon this study, the location and oscillation amplitude of the slot-flap configurations are significant parameters in the resulting mitigation effects.
Weber, Jamie (2012). Passive Flow Control Method for Mitigation of Unsteady Load Excursions on a Wind Turbine Blade. Master's thesis, Texas A & M University. Available electronically from