Turbulence transport phenomena in the wakes of wind turbines

A true physical understanding of the subtleties involved in the recovery process of the wake momentum deficit downstream of utility-scale wind turbines in the atmosphere has not been obtained to date. While the wind energy community has now a better understanding of some of the effect of the atmospheric stability state on wind turbine power production and wake recovery within an array of wind turbines, available field data are, in general, not acquired at sufficient spatial and temporal resolution that would allow to dissecting some of the mysteries of wake turbulence. It is here that the Actuator Line Method (ALM) has evolved to become the technology standard in the wind energy community for modeling the wakes of single wind turbines as well as arrays of wind turbines and wind farms immersed in an atmospheric boundary-layer flow. This work presents the ALM embedded into an OpenFOAM-LES solver (ALM/LES) and applies it to two small wind farms, the first one consisting of an array of two NREL 5-MW turbines separated by seven rotor diameters in neutral and unstable atmospheric boundary-layer (ABL) flow and the second one consisting of five NREL 5-MW wind turbines arranged in two staggered arrays of two and three turbines, respectively, in unstable ABL flow. Detailed statistics involving power spectral density (PSD) of turbine power along with standard deviations reveal the effects of atmospheric turbulence and its space and time scales. Furthermore, the effect of turbulence generated directly by upstream wind turbines on the power response of downstream wind turbines is quantified. High-resolution surface data extracts in addition to selected Reynolds-stress statistics provide new insight into the complex recovery process of the wake momentum deficit governed by turbulence transport phenomena. Copyright