Detection of tip-vortex signatures behind a 2.5 MW wind turbine

The near-field signature of the vortical structures shed from the blade tips behind a 2.5 MW horizontal axis wind turbine in a stably-stratified atmospheric boundary layer (ABL) was quantified for the first time. This study utilizes wind velocity measurements from sonic anemometers installed on a meteorological tower, which offers continuous characterization of wind conditions in the field, at elevations coinciding with the bottom, hub and top tip heights of the turbine. Using the stringent criteria on wind speed, direction and steadiness, we are able to subsample the dataset in which the sonic anemometers are positioned near the edge of the turbine wake. The spectral analysis of this dataset shows a distinct peak at the turbine rotational frequency (f(T)) for hub and top tip height measurements. Based on recent literature, we infer that this peak is the signature of tip-vortices, and the shift of this signature from blade-passing frequency (3f(T)) to f(T) is likely to be caused by vortex grouping phenomena. Slight changes of mean wind direction in other data samples result in the absence of the spectral peak, suggesting the very local extent of tip vortices. (c) 2015 Elsevier Ltd. All rights reserved.