Several near-wake phenomena of a utility-scale wind turbine*

Flow visualization and PIV via natural snowfall serves as an effective tool to probe the flow field around utility-scale wind turbines (Hong et al. Nature Comm. 2014). Using this tool, we have collected several datasets over 2015-2018 in the near-wake (<D) of the 2.5 MW turbine at the EOLOS station. These data provide 2D velocity field on the streamwise-wallnormal planes over the entire vertical span of the wake at multiple spanwise locations, as well as quantitative information of all the key coherent structures generated from the turbine, including blade tip/root vortices, trailing vortex sheets, hub vortices and turbine tower structures, etc. The follow-up analysis reveals several interesting near-wake phenomena, such as intermittent wake contraction, flow acceleration around the nacelle, and oscillation of nacelle wake, etc. Particularly, the movement of the nacelle-induced flow structures is shown to cause significant change of mean flow and turbulent kinetic energy profiles of the near wake. With conditional sampling, the interconnection among the near-wake velocity field, coherent structure behavior and inflow/turbine operational conditions is also investigated.