  Atmospheric turbulent velocity fluctuations are known to increase wind
turbine structural loading and accelerate wake recovery, but the impact of
vortical coherent structures in the atmosphere on wind turbines has not yet
been evaluated. The current study uses flow imaging with natural snowfall with
a field of view spanning the inflow and near wake. Vortical coherent structures
with diameters on the order of 1 m are identified and characterized in the flow
approaching a 2.5 MW wind turbine in the region spanning the bottom blade tip
elevation to hub height. Their impact on turbine structural loading, power
generation, and wake behaviour are evaluated. Long coherent structure packets
(larger than 200 m) are shown to increase fluctuating stresses on the turbine
support tower. Large inflow vortices interact with the turbine blades, leading
to deviations from the expected power generation. The sign of these deviations
is related to the rotation direction of the vortices, with rotation in the same
direction as the circulation on the blades leading to periods of power surplus,
and opposite rotation causing power deficit. Periods of power deficit coincide
with wake contraction events. These findings highlight the importance of
considering coherent structure properties when making turbine design and siting
decisions.
