Twin-Helix 2Copyright: © ILR
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 (UAV), 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.
The TWIN-HELIX 2 research project builds on the results of the predecessor project TWIN-HELIX, in which a novel reduction method was investigated to mitigate the harmful influence of blade tip vortices. Using a special geometric design of the blade tip, the single, concentrated blade tip vortex is split into two separate vortices, which are subsequently subjected to mutual interaction mechanisms that ultimately result in a merged vortex with expanded core radius and weakened vorticity. The presence of instability phenomena can positively influence the described process and greatly reduce the relevant time intervals. The aim of the current project is to gain a comprehensive physical understanding of the instability phenomena occurring in this particular vortex system and their origin, in order to gain new insights for the geometric design of rotor blade tips. Within the project, extensive experimental investigations are combined with numerical calculations.