Dynamics and instabilities of helical vortex pairs – TWIN-HELIX

The aim of this project is to gain new basic knowledge on the dynamics and instabilities of vortex pairs having a large-scale helical geometry, which have relevance to applications involving flows around rotors. These applications include the wake generated by a helicopter and the flow behind a horizontal-axis wind turbine. In both cases, important fluid mechanical issues exist, related to the safety, comfort/nuisance or efficiency of operation. The helicopter vortex wake is known to undergo a hazardous transition to a so-called Vortex Ring State in situations of steep descent, and the interaction of the blade tip vortex with a following blade is a well-known source of undesirable helicopter noise. The spatial evolution of a wind turbine wake has a direct influence on the performance and fatigue of a second turbine placed downstream, which is today a common configuration.

The TWIN-HELIX project focuses on the study of a novel generic configuration involving helical vortices. One or several regularly spaced helices have been used in the past to model rotor wakes, and the study of their dynamics and evolution has recently made significant progress. Here, the single vortex is replaced by a closely spaced pair of counter- or corotating vortices. The interactions between the two, which scale on the new parameters related to the pair, in particular the separation distance, are expected to influence and modify significantly the overall evolution of the helical flow, as observed in several empirical studies in the past.

An original approach based on the use of theoretical modelling combined with dedicated experiments is developed, in order to identify the main parameters governing the dynamics and instabilities of helical vortex pair systems. In a first phase, previous knowledge on the evolution of straight vortex pairs and “standard” helical vortices is used to explore possible scenarios for helical vortex pair dynamics, accompanied by a preliminary experimental survey using a finned wing. In a second phase a comprehensive theoretical analysis of the helical vortex pair geometry is carried out, determining the scope of possible base flows, as well as their stability with respect to various perturbation mechanisms: pairing, merging, long- and shortwave instabilities. Based on these results, configurations expected to have the most promising characteristics for full-scale applications are selected for an in-depth experimental investigation, using a single finned rotor blade and including detailed comparisons with theoretical predictions. A further comparison with results from three-dimensional numerical simulation results obtained by DTU Wind Energy in Copenhagen is also planned for these configurations.

It is expected that the TWIN-HELIX project will lead to new physics-based ideas and concepts for modifying or controlling rotor wake behaviour, which could help increase the safety and reduce the noise of helicopter flight, and increase the efficiency and lifetime of wind turbines. The practical implementation of these ideas could be explored in future follow-up work.