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Distributed lock-in drives broadband vortex-induced vibrations of a long flexible cylinder in shear flow

Published online by Cambridge University Press:  01 February 2013

Rémi Bourguet ,
George Em Karniadakis  and
Michael S. Triantafyllou
Rémi Bourguet*
Affiliation:
Institut de Mécanique des Fluides de Toulouse, Université de Toulouse and CNRS, Toulouse, 31400, France
George Em Karniadakis
Affiliation:
Brown University, Providence, RI 02912, USA
Michael S. Triantafyllou
Affiliation:
Massachusetts Institute of Technology, Cambridge, MA 02139, USA
*
†Email address for correspondence: [email protected]
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Abstract

A slender flexible body immersed in sheared cross-flow may exhibit vortex-induced vibrations (VIVs) involving a wide range of excited frequencies and structural wavenumbers. The mechanisms of broadband VIVs of a cylindrical tensioned beam of length-to-diameter aspect ratio 200 placed in shear flow, with an exponentially varying profile along the span, are investigated by means of direct numerical simulation. The Reynolds number is equal to 330 based on the maximum velocity, for comparison with previous work on narrowband vibrations in linear shear flow. The flow is found to excite the structure at a number of different locations under a condition of wake–body synchronization, or lock-in. Broadband responses are associated with a distributed occurrence of the lock-in condition along the span, as opposed to the localized lock-in regions limited to the high inflow velocity zone, reported for narrowband vibrations in sheared current. Despite the instantaneously multi-frequency nature of broadband responses, the lock-in phenomenon remains a locally mono-frequency event, since the vortex formation is generally synchronized with a single vibration frequency at a given location. The spanwise distribution of the excitation zones induces travelling structural waves moving in both directions; this contrasts with the narrowband case where the direction of propagation toward decreasing inflow velocity is preferred. A generalization of the mechanism of phase-locking between the in-line and cross-flow responses is proposed for broadband VIVs under the lock-in condition. A spanwise drift of the in-line/cross-flow phase difference is identified for the high-wavenumber vibration components; this drift is related to the strong travelling wave character of the corresponding structural waves.

JFM classification

Aerodynamics: Flow-structure interactions Vortex Flows: Vortex shedding Wakes/Jets: Vortex streets
Type
Papers
Information
Journal of Fluid Mechanics , Volume 717 , 25 February 2013 , pp. 361 - 375
Copyright
©2013 Cambridge University Press

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