Effects of wind speed changes on wake instability of a wind turbine in a virtual wind tunnel using large eddy simulation

Large Eddy Simulation (LES) of the National Renewable Energy Laboratory (NREL) 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 analyze 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 20 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 11 rotor diameters downstream of the wind turbine and complete breakdown was observed at 13 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.