This describes how the experimental vortex centers were determined.

The locations of the vortex centers were automatically determined
in the DNW large field of view (but low resolution) PIV frame
(without rotation into the vortex axis system), and correspond to
the extrema values of the vorticity computed from the simple
averaged data. It was checked that these vortex center positions
were correctly in the vortex area, and not in the vorticity
sheets that surround the vortex. If this was the case, then the
dimensions of the PIV frame were reduced such to cut off the
vorticity from the piv frame until the vortex center was
identified correctly.

Then, these vortex center position data were transformed from
the PIV coordinate system to the wind tunnel frame, and finally
from the wind tunnel frame to the rotor hub frame.
