a b s t r a c tDynamic stall (DS) on a wind turbine is encountered when the sectional angles of attack of the blade rapidly exceeds the steady-state stall angle of attack due to in-flow turbulence, gusts and yawmisalignment. The process is considered as a primary source of unsteady loads on wind turbine blades and negatively influences the performance and fatigue life of a turbine. In the present article, the control requirements for DS have been outlined for wind turbines based on an in-depth analysis of the process. Three passive control methodologies have been investigated for dynamic stall control: (1) streamwise vortices generated using vortex generators (VGs), (2) spanwise vortices generated using a novel concept of an elevated wire (EW), and (3) a cavity to act as a reservoir for the reverse flow accumulation. The methods were observed to delay the onset of DS by several degrees as well as reduce the increased lift and drag forces that are associated with the DSV. However, only the VG and the EW were observed to improve the post-stall characteristics of the airfoil.
Large Eddy Simulation (LES) of the NREL (National Renewable Energy Laboratory) Phase VI wind turbine inside a virtual wind tunnel, with the same test section as that of NASA Ames 24.4 m × 36.6 m, was carried out in order to analyse and better understand the wake instability and its breakdown behind the wind turbine. LES was performed using the commercial CFD software, ANSYS FLUENT, based on the dynamic Smagorinsky-Lilly model. The wind turbine was placed at a distance of two rotor diameters from the upstream boundary with a downstream domain of twenty rotor diameters in length The results of the simulation were compared with the experimental data published by the NREL and a good agreement was found between the two. Furthermore, the average turbulence intensities from the LES were compared with a semi-empirical model and very good agreement was observed, except for the regions of on-going wake instability and vortex breakdown. It was observed that the wake behind the wind turbine consists of a system of intense and stable rotating helical vortices.These vortices persisted for some distance downstream of the wind turbine and finally become unstable producing a sinuous shape. The downstream distance at which wake instability and vortex breakdown occur, was observed to be a function of the upstream wind speed. For example, for an upstream wind speed of 7 m/s, it was observed that the primary vortex structure became unstable at a downstream distance of four rotor diameters and complete breakdown occurred at approximately six rotor diameters. On the other hand, when the upstream wind speed was 15.1 m/s, wake instability occurred at approximately eleven rotor diameters downstream of the wind turbine and complete breakdown was observed at thirteen rotor diameters downstream of the wind turbine. Furthermore, it was observed that the turbulence intensity rapidly decreased during the process of wake instability and vortex breakdown; the location of the decrease is a function of the upstream wind speed. It is suggested that the distinction between the near and far wake can be identified as the average location between the start of the wake instability and the end of the process, at complete breakdown. Therefore the average location of this boundary is a function of the upstream wind speed. Hence for upstream wind speeds of 7 m/s, 10 m/s, 13.1 m/s and 15.1 m/s, the boundary between the near and far wake lies at five, seven, ten and twelve rotor diameters downstream respectively.
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