  Recent field experiments conducted in the near-wake (up to 0.5 rotor
diameters downwind of the rotor) of a 2.5 MW wind turbine using snow-based
super-large-scale particle image velocimetery (SLPIV) (Hong et al., Nature
Comm., vol. 5, 2014, no. 4216) were successful in visualizing tip vortex cores
as areas devoid of snowflakes. The so-visualized snow voids, however, suggested
tip vortex cores of complex shape consisting of circular cores with distinct
elongated comet-like tails. We employ large-eddy simulation (LES) to elucidate
the structure and dynamics of the complex tip vortices identified
experimentally. The LES is shown to reproduce vortex cores in good qualitative
agreement with the SLPIV results, essentially capturing all vortex core
patterns observed in the field in the tip shear layer. We show that the
visualized vortex patterns are the result of energetic coherent dynamics in the
rotor tip shear layer driven by interactions between the tip vortices and a
second set of counter-rotating spiral vortices intertwined with the tip
vortices. We further show that the mean flow within the region where such rich
coherent dynamics occur satisfies the instability criterion proposed by
Leibovich and Stewartson (J. Fluid Mech., vol. 126, 1983, pp. 335--356),
indicating that the instability uncovered by the SLPIV and the LES is of
centrifugal type. This study highlights the feasibility of employing snow voids
to visualize tip vortices and demonstrates the enormous potential of
integrating SLPIV with LES as a powerful tool for gaining novel insights into
the wakes of utility scale wind turbines.
