Analysis of the helical vortices measured behind a model wind turbine in a water channel
are reported. Phase-locked measurements using planar particle image velocimetry are
taken behind a Glauert rotor to investigate the evolution and breakdown of the helical vortex
structures. Existing linear stability theory predicts helical vortex filaments to be susceptible to
three unstable modes. The current work presents tip and root vortex evolution in the wake for
varying tip speed ratio and shows a breaking of the helical symmetry and merging of the …