Instability and merging of a helical vortex pair in the wake of a rotor

We present results from an experimental investigation of two closely spaced helical vortices. The two co-rotating vortices are generated in a water channel by a one-bladed rotor, whose tip geometry is modified by the addition of a perpendicular fin on the pressure side. The fin parameters are tuned so that the two vortices have approximately equal circulation. Dye visualisations and flow field measurements from Particle Image Velocimetry show that the vortices merge into a single tip vortex within less than one rotor rotation. This fast merging is caused by a rapid increase of the core radii of the initial vortices, which indicates the presence of an instability phenomenon. Analysis of the vortex velocity profiles identifies it as a centrifugal instability, linked to the presence of opposite-signed vorticity in the blade tip region. We further explore the origin of this vorticity by modifying the streamwise fin position. Measurements suggest that the interaction between the secondary fin vortex and the blade surface is partly responsible for the generation of the unstable velocity profile. The vortex resulting from the merging has a significantly larger core size than the one developing without a fin, indicating that this configuration could be beneficial in the context of Blade-Vortex Interaction noise for helicopters, or for the reduction of fatigue loads on wind turbine rotors caused by the interaction with concentrated tip vortices in the wake of upstream turbines.