Aerodynamics of a pitching wind turbine blade and the large-scale turbulence impact

Abstract

The impact of large-scale turbulence on aerodynamics is not well studied or documented, whereas the effect of high turbulence intensity is reported in many published papers. In particular, for a blade in pitching motion, which is typical in modelling non-uniform wind speed across the wind turbine height, highly unsteady incoming flows and yaw wind. This thesis conducts a research programme aimed at understanding and quantifying the effect of high pitching motion and, in particular, studying the impact of large-scale turbulence on the aerodynamic characteristics of a wind turbine blade. Firstly, large eddy simulation (LES) of the dynamic stall of a NACA 0012 aerofoil in pitching motion at various reduced frequencies k_red = 0.1, 0.15, 0.2, 0.3 and 0.4 in smooth inflow was investigated using both 2D and 3D LES. The high pitching frequencies are within the range of the frequencies of the unsteadiness of flows around the rotating wind turbine blade at different blade span locations and close to the blade structure frequencies. It was found that the lift hysteresis loop increase as the reduced frequency increases while the peak drag coefficients decrease with the increase of the reduced frequency, and the moment coefficient showed a strong dependency on the reduced frequency. Secondly, a similar set-up as the smooth inflow was studied, but with the imposition of an efficient and divergence-free inflow turbulence condition capable of generating synthetic largescale turbulence inside the domain. Two streamwise integral length-scales L_x = 1c and 1.5c are studied, which represent some energetic turbulent eddies at the height of the atmospheric boundary that wind turbine operates. It was found that the effect on the maximum lift coefficient at the dynamic stall angle near the maximum angle of attack is by an average of 20% and during the downstroke is by an average of 22%, while the maximum drag and minimum moment coefficients have an average of 21% and 60%, respectively compared to the smooth inflow for k_red = 0.1. A higher pitching motion of the blade (k_red = 0.2) does not improve the aerodynamic characteristics such as the lift, drag and moment coefficients, and lift-to-drag ratio either. Finally, the dispersive shear stress and turbulent shear stress in the wake are positive and negative - suggesting flow propulsion and resistance, respectively.

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ORCID for Zhengtong Xie: orcid.org/0000-0002-8119-7532

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