Helical vortices in rotor wakes

This thesis reports on an experimental study on the vortical structures in the wake of a rotor. Two optimum design rotors, modelling a horizontal axis wind turbine and an experimental two-bladed rotor, are used to generate helical vortex structures. The vortex structures in the wake are measured using high temporal and spatial resolution particle image velocimetry. The dynamics and evolution of the independent helical tip and root vortices are characterised with a parametric study of the rotor speed variation, and measurements are compared to theoretical predictions of the blade circulation. The instability transition points leading to reorganisation of the vortices are identified, and the mutual inductance is shown to develop at the helical pitch length-scale. Time-resolved measurements of the two-bladed rotor wake identify the dominant growth rate of the vortex evolution, and compared to predictions from linear stability analyses show good agreement, revealing that the growth rates collapse across the studied tip speed ratios when scaled by the helical pitch length-scale. A high spatial and temporal measurement of the tip vortex is undertaken, approximating a Lagrangian measurement frame of the tip vortex in the measurement plane, revealing the evolution of the vortex core. The results show helicoidal short-wave perturbations on the vortex core that exhibit characteristics of the elliptical instability, and the dominant wavenumber of the perturbation is presented.