Dynamics of helical vortices – HELIX

The HELIX project focuses on the study of simplified generic vortex configurations to gain the missing physical understanding of helical vortex dynamics, which will complement the existing, mostly empirical knowledge. An original approach based on the intense use of theoretical modelling, combined with dedicated experiments and numerical simulations, will be developed in order to identify the main parameters governing the instabilities and transitions of helical vortex systems. The same strategy has already been applied with success by the project partners in the framework of previous European collaborations on aircraft trailing vortices, where significant new results on fundamental mechanisms of transition were obtained.

The project also involves collaborations with the helicopter manufacturer EUROCOPTER and the Fluid Mechanics Group at the Technical University of Denmark, leading experts in wind turbine aerodynamics. They will assist the partners of the HELIX project in relating the fundamental results obtained using generic models to the full-scale applications. It is expected that this exchange will lead to new ideas and concepts for identifying and controlling rotor wake behaviour, which could help improve the safety of helicopter flight and the efficiency and lifetime of wind turbines.

Since the start of the project HELIX in January 2013, the following results have been obtained:

- Development of theory to describe generic helical flows. An asymptotic solution for a helical vortex was found in the general framework of thin-cored vortices.

- The local-pairing instability of a single helical vortex, predicted theoretically more than 40 years ago, was observed and characterised quantitatively in an experimental study in a water channel.

- A numerical code for the simulation of rotor wakes, involving the Actuator Line Method, was parallelised and used to characterise in more detail the flow studied experimentally.

- Improvement of a numerical code with helical symmetry, used for the simulation of systems of helical vortices.

The project will generate new fundamental results on elementary vortex configurations. These results concern basic mechanisms of vortex dynamics (three-dimensional instabilities, merging, global transition, etc..) that are found in a variety of flows - so they have a general relevance, beyond helical configurations studied here. The HELIX project will provide new knowledge about the effects of curvature and torsion of the vortices, which so far have not been studied extensively.

The stability analysis of helical vortices will investigate the sensitivity of these flows with respect to disturbances. It should therefore provide new ideas for the development of future strategies for the control of rotor wakes, in order to improve the safety of helicopter flight, or to increase the efficiency and lifetime of wind farms.

JOURNAL PAPERS

Bolnot, H., Leweke, T., Le Dizès, S.: Spatio-temporal development of the pairing instability in an infinite array of vortex rings. Fluid Dynamics Research (2013) – submitted

CONFERENCES

Leweke, T., Bolnot, H., Quaranta, H., Le Dizès, S.: Local and global pairing in helical vortex systems. In “Proceedings of the International Conference on Aerodynamics of Offshore Wind Energy Systems and Wakes (ICOWES 2013)”, Shen, W. Z. (editor), pp. 94-101 (2013) – 17-19 June 2013, Lyngby/Copenhagen, Denmark

Delbende, I., Rossi, M.: Dynamics of the three helical vortex system and instability. In “Proceedings of the International Conference on Aerodynamics of Offshore Wind Energy Systems and Wakes (ICOWES 2013)”, Shen, W. Z. (editor), pp. 224-235 (2013) – 17-19 June 2013, Lyngby/Copenhagen, Denmark

Ali, M., Abid, M.: Helical vortex wake: How far is the infinity? In “Proceedings of the International Conference on Aerodynamics of Offshore Wind Energy Systems and Wakes (ICOWES 2013)”, Shen, W. Z. (editor), pp. 236-246 (2013) – 17-19 June 2013, Lyngby/Copenhagen, Denmark

Leweke, T., Bolnot, H., Le Dizès, S.: Experiments on the pairing instability in helical vortices. IUTAM Symposium on “Vortex Dynamics: Formation, Structure and Function”, Abstract Book, pp. 98-99 – 10-14 March 2013, Fukuoka, Japan

THESES, REPORTS

Quaranta, H. U.: Instabilities of helical vortices representative of wind turbine and helicopter wakes. Master thesis, Ecole Polytechnique/ENSAM, Paris (2013)

The aim of this project is to gain new basic knowledge on the dynamics and instabilities of helical vortex systems, with relevance to applications involving flows around rotors. These 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 either safety 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 spatial evolution of a wind turbine wake has a direct influence on the performance of a second turbine placed downstream, which is today a common configuration. Despite a large amount of accumulated data, these phenomena are still poorly understood today, preventing major advances in these domains.

The HELIX project focuses on the study of simplified generic vortex configurations to gain the missing physical understanding of helical vortex dynamics, which will complement the existing, mostly empirical knowledge. An original approach based on the intense use of theoretical modelling, combined with dedicated experiments and numerical simulations, will be developed in order to identify the main parameters governing the instabilities and transitions of helical vortex systems. The same strategy has already been applied with success by the project partners in the framework of previous European collaborations on aircraft trailing vortices, where significant new results on fundamental mechanisms of transition were obtained.

The project also involves collaborations with the helicopter manufacturer EUROCOPTER and the Fluid Mechanics Group at the Technical University of Denmark, leading experts in wind turbine aerodynamics. They will assist the partners of the HELIX project in relating the fundamental results obtained using generic models to the full-scale applications. It is expected that this exchange will lead to new ideas and concepts for identifying and controlling rotor wake behaviour, which could help improve the safety of helicopter flight and the efficiency and lifetime of wind turbines.