Experimental investigation of a rotor blade tip vortex pair

Abstract

This paper presents the results of an experimental study of two closely spaced vortices generated by a rotating blade with a modified tip geometry. The experiments are carried out in two water channel facilities and involve a generic one-bladed rotor operating in a regime near hover. It is equipped with a parametric fin placed perpendicular to the pressure surface near the tip, which generates a co-rotating vortex pair having a helical geometry. Based on previous results obtained with a fixed wing, a series of small-scale experiments is first carried out, to validate the method of vortex pair generation also for a rotating blade, and to obtain a qualitative overview of its evolution going downstream. A more detailed quantitative study is then performed in a larger facility at three times the initial scale. By varying the fin parameters, it was possible to obtain a configuration in which the two vortices have almost the same circulation. In both experiments, the vortex pair is found to merge into a single helical wake vortex within one blade rotation. Particle image velocimetry measurements show that the resulting vortex has a significantly larger core radius than the single tip vortex from a blade without fin. This finding may have relevance in the context of blade–vortex interactions, where noise generation and fatigue from fluid–structure interactions depend strongly on the vortex core size.

Generation of a wingtip vortex pair using a pressure-side fin

“…In the current investigation using a fixed-wing geometry, instabilities and merging of the tip and fin vortices were not observed in the near-wake region considered. In a related study [53][54][55] involving a rotating wing equipped with a similar fin, designed using the results presented here, a strong three-dimensional short-wave instability, followed by a rapid merging, was found. This interesting new behaviour appears to be related to the difference in the spanwise circulation distributions between fixed and rotating wings.…”

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

“…The main goal of this modification is the generation of a final merged tip vortex which reduces the negative effects associated with blade-vortex interactions, such as noise emission and vibrations. In previous experimental studies [34,35], it was shown that the vortex pair generated by this configuration is influenced by centrifugal instabilities, which result in a beneficial rapid growth of the core radii prior to the onset of the merging procedure. To provide new insights into these unsteady and highly three-dimensional flow phenomena, the time-resolved LPT experiments were conducted.…”

“…Previous experimental studies (stereoscopic particle image velocimetry measurements and dye visualizations) involving comprehensive parameter variations identified promising configurations generating unstable vortex pairs of equal strength which ultimately merge into a single vortex [33,34]. The experimental data revealed that these vortex pairs were influenced by centrifugal instabilities, indicated by a rapid core growth right after emerging from the trailing edge.…”

“…1a). Additional information regarding the rotor geometry and the blade design process is found in [34]. To prevent recirculation regions at the rotor edges, a small free-stream velocity of u ∞ = 11.4 cm/s is required within the test section.…”