Experimental investigation of a rotor blade tip vortex pair

Jorge Aguilar-Cabello,Dominic Schröder,Ralf Hörnschemeyer,Thomas Leweke,Eike Stumpf

doi:10.1007/s13272-021-00555-1

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.

Highlights

IntroductionThe flow around rotors is characterised by the existence of helical vortices in its wake
1.1 Background and objectiveThe flow around rotors is characterised by the existence of helical vortices in its wake
We have presented the results of an experimental study of two closely spaced helical vortices, created by a generic rotor blade equipped with a parametric fin

Summary

Introduction

The flow around rotors is characterised by the existence of helical vortices in its wake. The helical tip vortices and their interaction with a following blade (blade–vortex-interaction, BVI) can generate unwanted noise and cause vibrations of the structure in certain flight regimes [1]. The fatigue load incurred by a wind turbine, as well as its performance, can be affected adversely by the wake of another turbine placed upstream [2]. In both cases, the strength of the local velocity gradients, with the corresponding rapid changes in local rotor blade air loads, is a crucial parameter. The circulation and core radius of the tip vortices, which both influence these gradients, are vital elements affecting the strength of the interactions between these vortices and the following blades [3]

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Conclusion