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The wake of rotors is, similar to that of the fixed-wing aircraft, characterized by a superposition of vortex structures. The formation of concentrated blade tip vortices is a direct physical consequence of the pressure equalization between the flow on the pressure and suction side. Due to the combination of the free inflow velocity and the rotational movement, a characteristic helical geometry results in the case of applications involving rotors. These floating concentrated blade-tip vortices can cause negative effects through interactions with objects downstream. In the case of wind turbines, the wake can lead to undesired load changes at a turbine positioned further downstream and damage it by resulting vibrations. Regarding helicopters or unmanned aerial vehicle, strong interactions between the blade tip vortices and the rotor blades occur, especially during landing approach. This so-called blade-vortex interaction leads to an impulsive, strong noise emission through a combination of aerodynamic and aeroacoustic effects, which is perceived as disturbing by humans. Considering the predicted sharp increase in UAV traffic, noise-reducing concepts are becoming increasingly important within present reasearch projects.
Within the framework of the German-French research project TWIN-HELIX a novel approach is pursued to reduce the harmful influence of leaf-tip vortices. In a first step, the concept envisages replacing the single concentrated blade tip vortex by two emerging. In the following, a benefical interaction of the two vortices is expected. Due to instability phenomena and the eventual merging of both vortices, a resulting vortex with a widened core radius and weakened vortex strength shall be created. The project includes a detailed theoretical, numerical and experimental analysis of the developed concept.
The cooperative research project is funded by ANR and DFG and is carried out in collaboration with the Institut de Recherche sur les Phénomènes Hors Equilibre of the University of Marseille.