Wind Turbine Blade Flow Fields and Prospects for Active Aerodynamic Control: Preprint

As wind turbines continue to grow larger, problems associated with adverse aerodynamic loads will grow more critical. Thus, the wind energy technical community has begun to seriously consider the potential of aerodynamic control methodologies for mitigating adverse aerodynamic loading. Spatial and temporal attributes of the structures and processes present in these flow fields hold important implications for active aerodynamic control methodologies currently being contemplated for wind turbine applications. The current work uses complementary experimental and computational methodologies, to isolate and characterize key attributes of blade flow fields associated with axisymmetric and yawed turbine operation. During axisymmetric operation, a highly three-dimensional, shear layer dominated flow field yields rotational augmentation of both mean and standard deviation levels of aerodynamic forces. Under yawed operating conditions, pseudo-sinusoidal inflow angle oscillations elicit dynamic stall, which significantly intensifies aerodynamic load production. Both rotationally augmented and dynamically stalled flows possess attributes likely to pose central challenges for turbine flow control. Whether active control of turbine aerodynamics can help alleviate adverse aerodynamic loads will depend on comprehension and command of the issues documented herein. NOMENCLATURE Cn normal force coefficient c chord length (m) cp pressure coefficient ((p-p∞)/q) Hz Hertz LFA local inflow angle (deg) Employees of the Midwest Research Institute under Contract No. DE-AC3699GO10337 with the U.S. Dept. of Energy have authored this work. The U.S. Government retains, and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes. m meter msec millisecond p static pressure (Pa) p∞ freestream static pressure (Pa) q dynamic pressure (Pa) r radial distance from hub (m) R blade length (m) RPM revolutions/minute s second U∞ test section speed (m/s) Uloc local inflow velocity (m/s) x chord location (m) Ψ blade azimuth angle (deg) Ω blade rotation rate (rad/s) γ rotor yaw angle (deg) INTRODUCTION Wind turbine aerodynamics remains a challenging and crucial research area for wind energy. Clearly, steady-state aerodynamic performance is essential to turbine energy capture, since blade aerodynamic forces produce mechanical energy that is subsequently converted to electrical energy. However, more recent inquiry has focused on adverse time varying aerodynamic loads that wind turbines frequently suffer during routine service. These undesirable aerodynamic loads impose excessive stresses on turbine blades and gear boxes, and appreciably shorten machine service life. At present, the wind energy technical community is contemplating the utility of various aerodynamic control methodologies for mitigating adverse aerodynamic loading. Wind turbine blade aerodynamic phenomena can be broadly categorized according to the operating state of the machine, and two particular aerodynamic phenomena assume crucial importance. At zero and low rotor yaw angles, rotational augmentation determines blade aerodynamic response. At moderate to high yaw angles, dynamic stall dominates blade aerodynamics. As described herein, the spatial and temporal attributes of the structures and processes present