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Coupling performance of tandem flexible inverted flags in a uniform flow

Published online by Cambridge University Press:  28 December 2017

Haibo Huang Open the ORCID record for Haibo Huang [Opens in a new window] ,
Heng Wei  and
Xi-Yun Lu Open the ORCID record for Xi-Yun Lu [Opens in a new window]
Haibo Huang
Affiliation:
Department of Modern Mechanics, University of Science and Technology of China, Hefei, Anhui 230026, China
Heng Wei
Affiliation:
Department of Modern Mechanics, University of Science and Technology of China, Hefei, Anhui 230026, China
Xi-Yun Lu*
Affiliation:
Department of Modern Mechanics, University of Science and Technology of China, Hefei, Anhui 230026, China
*
†Email address for correspondence: xlu@ustc.edu.cn
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Abstract

The interaction of tandem inverted flexible flags in a uniform flow is investigated. For the inverted flags, their ends are fixed with their heads freely flapping. A direct numerical simulation is performed for which the Reynolds number is of order 200. Large flapping amplitude as well as large drag force is preferred because more energy may be harvested if more bending energy is generated. For the simple case of two tandem inverted flags, the drag force and flapping amplitude of the rear flag are found to be smaller than those of an isolated inverted flag due to the destructive merging mode of vortices. However, it is still unknown whether more bending energy can be generated when coupled inverted flags are arranged properly. To explore the possibility, inverted flags are proposed to be arranged as two rows, which indicate two lines of inverted flags perpendicular to the direction of the incoming flow, and flags in the front and rear rows are in-line or staggered. First the results for infinite flags with periodic boundary condition are presented. In both the in-line and the staggered arrangements, due to the interactions between the front–rear flags, the flapping amplitude or the maximum bending deformation and bending energy of a flag in the rear row can be enhanced, which may be significantly higher than those of an isolated case. Meanwhile, the bending energy of a flag in the front row is close to that of an isolated case. Second, results for finite inverted flag groups show that antiphase synchronization is preferred. When the group number is large enough, the bending energies of the front and rear flags in the inner groups are close to those in the infinite case. This finding may be helpful for the designing of an efficient energy harvesting device using inverted flags.

JFM classification

Flow Control: Drag reduction Vortex Flows: Vortex interactions
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 837 , 25 February 2018 , pp. 461 - 476
Copyright
© 2017 Cambridge University Press 

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Huang et al. supplementary movie 1

Supplemental material: Vorticity contours (upper and lower boundaries are periodic) with K = 0.3, Gx = 2.0, Gy=0 for the in-line arrangement. The red and blue contours represent the positive (anti clockwise) and negative (clockwise) vortices.

Download Huang et al. supplementary movie 1(Video)
Video 3.8 MB