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Closely spaced co-rotating helical vortices: long-wave instability

Published online by Cambridge University Press:  05 August 2022

A. Castillo-Castellanos Open the ORCID record for A. Castillo-Castellanos [Opens in a new window]  and
S. Le Dizès Open the ORCID record for S. Le Dizès [Opens in a new window]
A. Castillo-Castellanos*
Affiliation:
IRPHE, Aix Marseille Université, CNRS, Centrale Marseille, Marseille, France
S. Le Dizès
Affiliation:
IRPHE, Aix Marseille Université, CNRS, Centrale Marseille, Marseille, France
*
†Email address for correspondence: [email protected]
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Abstract

We consider as base flow the stationary vortex filament solution obtained by Castillo-Castellanos et al. (Phys. Rev. Fluids, vol. 6, 2021, 114701) in the far wake of a rotor with tip-splitting blades. The cases of a single blade and of two blades with a hub vortex are studied. In these solutions, each blade generates two closely spaced co-rotating tip vortices that form a braided helical pattern in the far wake. The long-wave stability of these solutions is analysed using the same vortex filament framework. Both the linear spectrum and the linear impulse response are considered. We demonstrate the existence of different types of instability modes. A first type corresponds to the local pairing of consecutive turns of the helical pattern, which is well described by the instability of a uniform helical vortex with a core size given by the mean separation distance of the vortices in the pair. A second type corresponds to the pairing of consecutive turns of the vortex pair and is observed only for densely braided patterns, which is well described by the instability of two interlaced helical vortices by straightening out the baseline helix. A third type of unstable modes modifies the separation distance between the vortices in each pair and amplifies specific (linear) wavelengths. These unstable modes also spread spatially with a weaker rate.

JFM classification

Vortex Flows: Vortex instability
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 946 , 10 September 2022 , A10
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Copyright
© The Author(s), 2022. Published by Cambridge University Press

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References

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