Evolution and instabilities of helical vortices in rotor wakes

Andras Nemes, Postdoctoral Scholar, St. Anthony Falls Laboratory, University of Minnesota

The primary feature in the near wakes of open rotor systems including wind turbines, helicopter rotors and marine propellers are helical vortex structures formed by the rotating blades. In all these application these helical vortices constitute an undesirable characteristic of the wake – contributing to increased turbulence in the far wake, fatigue loading on downstream structures, noise generation and dangerous flight aerodynamics. In this talk we review some recent research into helical vortices and present water channel experiments investigating the evolution of the vortex structures behind a model rotor using particle image velocimetry. Measurements recover the primary features of the near wake featuring tip and root vortices and their evolution as a function of tip speed ratio of the rotor. The dynamics of the tip vortex configuration is then related to the instability mechanisms of helical vortex filaments finding that the symmetry breakdown is dominated by the mutual inductance of neighboring helices. The destabilisation of the helical vortices is shown to be related to the growth rates predicted by inviscid linear stability analysis. Finally, results of phase-locked and Lagrangian measurements of the tip vortices are shown which reveal the presence and mode structures of shortwave instabilities on the vortex core.