Wake Characteristics of a Single Turbine During the CWEX-10/11 Crop Wind-Energy EXperiments

In the summer of 2010 and 2011 for the Crop Wind-energy EXperiment (CWEX), flux stations measured differences in micrometeorology upstream and downstream of a single turbine within a large wind farm in Iowa. 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, wind direction, and power produced verified the likely presence of a wake above the downwind flux station. As described in the CWEX overview paper (Rajewski et al. 2013) the flux stations detected (1) turbine-wake events for wakes overhead but not intersecting the surface, (2) wakes with a direct surface influence, and (3) flow perturbations caused by the static pressure field around a line of turbines. We refine our conceptual model of wind turbine flow by comparing downwind-upwind flux and profile station differences for categories of waked and non-waked flow according to turbine hub-height speed and direction, ambient thermal stratification, and the operating status of the turbines. For nighttime stable conditions (some for which a low level jet is present) we measured both within the rotor depth and at the surface higher turbulence and stronger intermittency of the flow on the wake edges as compared to the wake core. We additionally observe frequent periods with 20-30° of directional shear from the surface to the top of the rotor as evidenced by a downwind flux station in non-waked flow with concurrent LiDAR measurement of a wake in the rotor layer. Momentum power spectra and co-spectra of 20-Hz surface data corroborate with previous wind tunnel and numerical simulations of wake turbulence with higher energy intensity but at reduced scales than for non-waked conditions. The spectra demonstrate a return to ambient flow when the wind farm is brought offline.