Abstract
Prediction of the unsteady aerodynamic flow phenomenon on wind turbines is challenging and still subject to considerable uncertainty. Under yawed rotor conditions, the wind turbine blades are subjected to unsteady flow conditions as a result of the blade advancing and retreating effect and the development of a skewed vortical wake created downstream of the rotor plane. Blade surface pressure measurements conducted on the NREL Phase VI rotor in yawed conditions have shown that dynamic stall causes the wind turbine blades to experience significant cycle-to-cycle variations in aerodynamic loading. These effects were observed even though the rotor was subjected to a fixed speed and a uniform and steady wind flow. This phenomenon is not normally predicted by existing dynamic stall models integrated in wind turbine design codes. This paper couples blade pressure measurements from the NREL Phase VI rotor to a free-wake vortex model to derive the angle of attack time series at the different blade sections over multiple rotor rotations and three different yaw angles. Through the adopted approach it was possible to investigate how the rotor self-induced aerodynamic load fluctuations influence the unsteady variations in the blade angles of attack and induced velocities. The hysteresis loops for the normal and tangential load coefficients plotted against the angle of attack were plotted over multiple rotor revolutions. Although cycle-to-cycle variations in the angles of attack at the different blade radial locations and azimuth positions are found to be relatively small, the corresponding variations in the normal and tangential load coefficients may be significant. Following a statistical analysis, it was concluded that the load coefficients follow a normal distribution at the majority of blade azimuth angles and radial locations. The results of this study provide further insight on how existing engineering models for dynamic stall may be improved through the integration of stochastic models to be able to account for the cycle-to-cycle variability in the unsteady wind turbine blade loads under yawed conditions.
Highlights
Horizontal axis wind turbines (HAWTs) are bound to operate under a 3D complex unsteady aerodynamic environment caused by various unsteady sources, such as variations of wind speed, atmospheric turbulence, wind shear, yawed flow, and the operational states of the wind turbine.The cumulative effect of these phenomena leads to significant variation of the aerodynamic loads on the blades, with azimuth angle and instantaneous change in the blade angle of attack
This paper presents a numerical approach in which a free-wake vortex model and the blade pressure measurements are used to derive the unsteady angle of attack distributions over successive
This paper presents a numerical approach in which a free-wake vortex model and the blade rotor rotations under yawed flow conditions
Summary
Horizontal axis wind turbines (HAWTs) are bound to operate under a 3D complex unsteady aerodynamic environment caused by various unsteady sources, such as variations of wind speed, atmospheric turbulence, wind shear, yawed flow, and the operational states of the wind turbine. The cumulative effect of these phenomena leads to significant variation of the aerodynamic loads on the blades, with azimuth angle and instantaneous change in the blade angle of attack. Energies 2016, 9, 460 especially on the inboard sections of the wind turbine blades [1]. Since these effects play an important role in predictions of wind turbine performance and fatigue lifetime of the structure, it is essential to have an in-depth understanding of the aerodynamic load phenomena before the structural response can be simulated. The National Renewable Energy Laboratory (NREL) in the USA [2] conducted a detailed experiment in the NASA Ames wind tunnel to produce very detailed measurements using high quality instrumentation under controlled conditions to avoid disturbances caused by the real environment
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