Stochastic characteristics for the vortical structure of a 5-MW wind turbine wake

In this study, we analyze the near-wake characteristics of a 5-MW single wind turbine using a large-eddy simulation with the actuator line method. It is observed that stable helical structures of wake had breaking process due to flow instability with high-frequency turbulence in the analysis of turbulence energy spectra and auto-covariance. For vortical structure detection, the swirling strength criterion showed good performance when describing the tip vortices and their various streaklines. In the stable region, the stable helical structure of the tip vortices was observed with little wake recovery and flow instability. However, in the unstable region, the tip vortices had various streakline advection patterns such as horizontal, upward, and downward. In the statistical analysis of the flow acceleration, the distribution and magnitude of the acceleration in the stable and unstable regions were compared in terms of the probability density function, root mean square, and kurtosis. At the hub height and at the opposite blade tips in the stable region, the kurtosis of the acceleration was approximately 20 and 40, respectively. At the height wherein the wake recovery was dominant in the unstable region, the value of the kurtosis was 67.