Wind Turbine Wake Investigation from Surface Measurements during the 2010 and 2011 Crop Wind-Energy EXperiments (CWEX-10/11)

Recent numerical and wind tunnel simulations emphasize the three-dimensional structure of increased turbulence and reduced wind speed that each operating wind turbine produces in the lee of the rotor. Wake characterization is important for optimization of wind farm design and in performance and reliability of each turbine and the entire wind park. Field measurements are needed available to calibrate high resolution simulations of wind turbine wake interactions. The Crop Wind-energy EXperiment (CWEX) was designed to explore the meteorological characteristics of a large wind farm in an agricultural area. Flux stations were positioned between two lines of turbines in the summer of 2010 and near a turbine line in the summer of 2011 to document the differences in near-surface flow fields upstream and downstream of turbines (prevailing wind direction from the south) in the southwest corner of a large 200-turbine wind farm in Central Iowa. Two profiling lidars were positioned upwind and downwind of a single turbine for two months in 2011 to document the wake profiles of mean wind speed and turbulent kinetic energy (TKE). Nacelle-based measurements of wind speed and wind direction verified the likely presence of a wake above the flux stations. We report turbine-wake influences to arise from (1) reduction in the wind speed and modified turbulence conditions different from ambient that intersect the surface, (2) reduction in the vertical exchange between the surface and boundary layer from overhead wakes interrupting the ambient turbulence that scales with height, or (3) modification in the static pressure fields around each line of turbines to create small-scale pressure gradients, localized flows, and changes to the vertical exchange of surface fluxes. The lidar and nacelle wind direction data allowed us to define which turbine or turbines were responsible for wake conditions overhead each surface station for any wind direction. Comparison of surface-layer aerodynamic and microclimate properties measured by surface flux stations windward and leeward of the turbine lines allow us to begin constructing a conceptual model of mechanisms influencing near-surface flow and changes in microclimate conditions when wakes reach to the surface. For example, wind speed covariances and TKE across the surface wake under different stability and wind speed conditions lead to a deeper understanding of changes in mean wind speed and surface fluxes in the near-turbine environment. Our findings will lead to a more realistic process-conceptual model and provide validation data for numerical and wind tunnel simulations of the complex forcing and flow structure in wind farms. We also are learning how to better deploy a limited set of instruments to better reveal essential dynamical features of the complex flow in a wind farm.