Dynamics and instabilities of helical vortex pairs
Final Report Abstract
Engineering applications involving rotors, such as e.g. helicopters or wind turbines, are characterized by the presence of multiple helical vortices in the rotor wake, depending on the blade number. These vortices are strongly relevant regarding the flow around rotors and their long-lasting presence can lead to undesired flow phenomena related to the safety, comfort/nuisance, or operation efficiency. 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 (Blade-Vortex-Interaction, BVI). The spatial evolution of a wind turbine wake has a decreasing influence on the performance and simultaneously increases the fatigue mechanisms of a second turbine placed downstream, representing a common configuration used in current wind farms. Specific vortex parameters, such as vortex core radii and peak tangential velocities, affect the strength of the BVI or a possible fluid-structure interaction mechanism. Thus, various approaches, involving active and passive systems, have been proposed and analyzed to reduce the hazardous effect of the stable vortices. The primary objective was to widen the vortex core and to decrease the tangential velocity maximum. The TWIN-HELIX approach targeted this issue through enhanced instabilities inside the vortex cores. Fluid vortices are affected by various instability mechanisms, which cause an accelerated destabilization of the distinct core structure. Consequently, an artificial triggered instability or an enforced growth rate of a given mechanism represents a promising method to address the issues named above. In the TWIN-HELIX framework, the single helical vortex emerging from each blade tip is replaced by a closely spaced pair of counter- or co-rotating vortices using a special blade tip design including an additional fin. This concept of splitting the single concentrated tip vortex into two separate vortices is inspired by a particular blade tip geometry (“vane tip”), which has shown promising results in past investigations concerning BVI mitigation. The TWIN-HELIX projects aimed at a deeper physical understanding of the interaction process between the helical vortex pair, involving phenomena as pairing, merging and mutual instabilities. Comprehensive experimental, numerical and theoretical methods revealed new insights in the interaction process, involved instability phenomena, and influence of the blade shear layer. Promising configurations were identified, which led to a rapidly destabilizing vortex pair eventually merging into a single vortex with strongly increased core sizes in relation to the unmodified reference. The findings of the project provide a novel approach for rotor blade tip design to reduce the negative effects associated with long-lasting helical vortex structures in the rotor wakes.
Publications
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A strongly-coupled model for flexible rotors. Journal of Fluids and Structures, 89, 219-231.
Durán Venegas, E.; Le Dizès, S. & Eloy, C.
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Generalized helical vortex pairs. Journal of Fluid Mechanics, 865, 523-545.
Durán Venegas, E. & Le Dizès, S.
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Experimental investigation of a rotor blade tip vortex pair. CEAS Aeronautical Journal, 13(1), 97-112.
Schröder, Dominic; Aguilar-Cabello, Jorge; Leweke, Thomas; Hörnschemeyer, Ralf & Stumpf, Eike
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Experiments on helical vortex pairs in the wake of a rotor. AIAA Scitech 2021 Forum. American Institute of Aeronautics and Astronautics.
Schröder, Dominic; Leweke, Thomas; Hörnschemeyer, Ralf & Stumpf, Eike
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Instability and merging of a helical vortex pair in the wake of a rotor. Journal of Physics: Conference Series, 1934(1), 012007.
Schröder, D.; Leweke, T.; Hörnschemeyer, R. & Stumpf, E.
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Joukowski’s wake model for tip-splitting rotors. Journal of Physics: Conference Series, 1934(1), 012005.
Castillo-Castellanos, A.; Durán Venegas, E. & Le Dizés, S.
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Structure and stability of Joukowski's rotor wake model. Journal of Fluid Mechanics, 911.
Durán Venegas, E.; Rieu, P. & Le Dizès, S.
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Generation of a wingtip vortex pair using a pressure-side fin. Aerospace Science and Technology, 130, 107860.
Schröder, Dominic; Leweke, Thomas; Hörnschemeyer, Ralf & Stumpf, Eike
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High-speed volumetric particle tracking measurements of unstable helical vortex pairs. Experiments in Fluids, 64(8).
Schröder, Dominic; Leweke, Thomas & Stumpf, Eike
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Time-resolved volumetric particle tracking measurements of an unstable helical vortex pair. AIAA SCITECH 2023 Forum. American Institute of Aeronautics and Astronautics.
Schröder, Dominic; Leweke, Thomas & Stumpf, Eike
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Implementation and Testing of a Novel Rotor Design Methodology. Deutsche Gesellschaft für Luft- und Raumfahrt - Lilienthal-Oberth e.V.
Pissarski, N.A.; Schröder, D. & Stumpf, E.
