High-speed volumetric particle tracking measurements of unstable helical vortex pairs

We present results from an experimental study investigating pairs of helical vortices generated by a one-bladed rotor in hovering conditions. Time-resolved volumetric Lagrangian particle tracking measurements are conducted in a water tunnel to analyze the three-dimensional development of the vortex system. The vortex pairs are generated by a specific tip design, which allows splitting the single tip vortex into two vortices, whose characteristics depend on the geometric fin parameters. The objective of this procedure is the modification of the tip vortex structure, in order to minimize negative effects caused by fluid-structure interactions in applications involving rotors. Certain vortex configurations are affected by centrifugal instabilities, which result in an immediate pronounced growth of the vortex cores. As a consequence of the instability, secondary vortex structures are formed between the unstable cores. The presence of these structures results in an accelerated break-up of the cores, causing them to merge. In order to investigate the influence of the trailing vorticity layer shed from the inner part of the blade, two blade designs with different radial circulation distributions are considered. The measurements are able to track the evolution of the vorticity layer and the secondary structures, providing new insights into the instability of closely spaced vortex pairs with varying circulation ratios.