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Passive oscillations of inverted flags in a uniform flow

Published online by Cambridge University Press:  17 December 2019

Yao-Wei Hu Open the ORCID record for Yao-Wei Hu [Opens in a new window] ,
Li-Hao Feng Open the ORCID record for Li-Hao Feng [Opens in a new window]  and
Jin-Jun Wang Open the ORCID record for Jin-Jun Wang [Opens in a new window]
Yao-Wei Hu
Affiliation:
Fluid Mechanics Key Laboratory of Education Ministry, Beijing University of Aeronautics and Astronautics, Beijing100191, China
Li-Hao Feng*
Affiliation:
Fluid Mechanics Key Laboratory of Education Ministry, Beijing University of Aeronautics and Astronautics, Beijing100191, China
Jin-Jun Wang
Affiliation:
Fluid Mechanics Key Laboratory of Education Ministry, Beijing University of Aeronautics and Astronautics, Beijing100191, China
*
†Email address for correspondence: lhfeng@buaa.edu.cn
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Abstract

The passive oscillations of inverted flags are investigated both experimentally and theoretically in this paper. First, the force and energy distributions of inverted flags, which contain elastic and inertia components, are analysed based on the experimental data. Two main differences between inverted and conventional flags are found: (1) the elastic energy of a conventional flag is concentrated near the free end, while the fixed end of an inverted flag presents the largest elastic energy; and (2) the elastic component is several orders of magnitude greater than the inertia component for an inverted flag, while they are of the same magnitude for a conventional flag. Second, a linear analysis shows that the critical flow velocities obtained from the experiments at small mass ratios are scattered around the theoretical curve of wavenumber $k=1.875$, which is in contrast with $k=4.694$ of a conventional flag. For large mass ratios, the mass ratio has a certain influence on the critical velocity rather than being irrelevant. For two parallel inverted flags, both the experimental and theoretical results indicate that the range of the in-phase flapping mode becomes smaller with an increase in the separation distance, and a multiple flapping state may occur. For $n\geqslant 2$ parallel inverted flags, the theoretical results show that two of all coupled flapping modes are dominant with most parameters. These findings could contribute to a better understanding of the passive oscillations of inverted flags.

JFM classification

Aerodynamics: Aerodynamics
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 884 , 10 February 2020 , A32
Copyright
© 2019 Cambridge University Press

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