Time-resolved volumetric particle tracking measurements of an unstable helical vortex pair

We present the outcome of an experimental study of a helical vortex pair affected by the presence of centrifugal instabilities. Water tunnel experiments involving time-resolved volumetric Lagrangian Particle Tracking are used to investigate the vortex system of a one- bladed rotor in hovering conditions. A specific tip design provokes the generation of two separate vortices instead of one single tip vortex. The eventual goal of this modified tip design is the formation of a merged vortex with improved characteristic flow parameters to minimize blade-vortex interactions and the attached negative effects, such as noise generation. Previous studies of this rotor revealed that centrifugal instabilities occure for certain configurations, leading to an immediate rapid core growth of the initial vortices. The reason for this behaviour is found in the formation of secondary structures induced by the instability between the vortex cores. These structures cause an accelerated break-up of the stable core geometry, which results in the eventual merging. The experimental data provides insights in the temporal evolution and the three-dimensional character of the vortex pair affected by the instabilities