Aspect ratio effects in wind energy harvesting using piezoelectric inverted flags

Ojo, Oluwafemi; Tan, David; Wang, Yucheng; Shoele, Kourosh; Ertürk, Alper

doi:10.1117/12.2519527

Cited by 4 publications

(3 citation statements)

References 34 publications

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“…The inverted flag problem was first studied experimentally by Kim et al (2013). They identified three distinct response modes of the structure due to its interaction with the flow: a stationary mode at low flow velocities, a two-sided flapping mode at intermediate flow velocities and a one-sided flapping or deflected mode at high flow velocities (Kim et al 2013;Sader et al 2016a;Gurugubelli & Jaiman 2019;Ojo et al 2019Ojo et al , 2021Park, Ryu & Sung 2019). These modes emerge from the balance between vortex-generated forces and the elastic restoring force in the flag (Yu et al 2019), resulting in self-sustained vibrations at relatively low flow speeds.…”

Section: Introductionmentioning

confidence: 99%

Aspect ratio-dependent hysteresis response of a heavy inverted flag

et al. 2022

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The bistable fluttering response of heavy inverted flags with different aspect ratios ( $AR$ ) is investigated to determine how the vortical structures affect the intermittent vibration response of the flag. A heavy inverted flag in a uniform flow may exhibit several response modes; amongst them are three major modes that occur over an extended velocity range: stationary, large-scale periodic oscillation and one-sided deflected modes. Significant hysteretic bistability is observed at the transition between these modes for all $AR$ , which is notably different from the conventional flag vibration with a fixed leading edge and free trailing edge where no hysteresis is observed at the lower $AR$ limit ( $AR<1$ ). The difference is associated with the distinct roles of vortices around the flag. Experiments with flags made of spring steel are conducted in a wind tunnel, where the flow speed is steadily increased and later decreased to obtain different oscillatory modes of the heavy inverted flags. The experimental results are used to validate the numerical model of the same problem. It is found that different critical velocities exist for increasing and decreasing flow velocities, and there is a sustained hysteresis for all $AR$ controlled by the initiation threshold and growth of the leading-edge and side-edge vortices. The effect of the vortices in the bistable oscillation regime is quantified by formulating a modal force partitioning approach. It is shown that $AR$ can significantly alter the static and dynamic vortex interaction with the flexible plate, thereby changing the flag's hysteresis behaviour and bistable response.

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Section: Introductionmentioning

confidence: 99%

Aspect ratio-dependent hysteresis response of a heavy inverted flag

et al. 2022

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“…The dynamics of a flag in uniform flow have been studied through various experimental [1,2], numerical [3][4][5][6][7], and analytical [8,9] techniques. Flag fluttering has also been employed to explore a wide range of biological and engineering applications including fish locomotion [10][11][12][13], wing fluttering [14], and energy harvesting [15,16].…”

Section: Introductionmentioning

confidence: 99%

“…When subjected to uniform flow, the flow-induced flapping of flags can be classified into three groups [17]: (a)-instability induced excitation (inverted flag configuration), (b)-movement-induced excitation (regular flag configuration), (c)-extraneously induced excitation (flag behind a bluff body). The periodic vortex shedding from the deflected position of an inverted flag is the primary factor that drives its sustained flapping dynamics [2,6,12]. In contrast, the regular flag dynamics are self-excited due to the dynamic feedback between the flag inertia forces, elastic deflection and fluid forces [18,19].…”

Section: Introductionmentioning

confidence: 99%

Flapping dynamics of an inverted flag behind a cylinder

Ojo¹,

Kohtanen²,

Jiang³

et al. 2022

Bioinspir. Biomim.

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The inverted flag configuration is inspired by biological structures (e.g., leaves on a tree branch), showing rich dynamics associated with instabilities at lower flow speeds than the regular flag configuration. In the biological counterpart, the arrangement of leaves and twigs on foliage creates a complex interacting environment that promotes certain dynamic fluttering modes. While enabling a large amplitude response for reduced flow speeds is advantageous in emerging fields such as energy harvesting; still, little is known about the consequence of such interactions. In this work, we numerically study the canonical bio-inspired problem of the flow-structural interaction of a 2D inverted flag behind a cylindrical bluff body, mimicking a leaf behind a tree branch, to investigate its distinct fluttering regimes. The separation distance between the cylinder and flag is gradually modified to determine the effective distance beyond which small-amplitude or large-amplitude flapping occurs for different flow velocities. It is shown that the flag exhibits a periodic large amplitude$-$low frequency response mode when the cylinder is placed at a sufficiently large distance in front of the flag. At smaller distances, when the flag is within the immediate wake of the cylinder, the flag undergoes a high frequency$-$small amplitude response. Finally, the flag's piezoelectric power harvesting capability is investigated numerically and experimentally for varying geometrical and electrical parameters associated with these two conditions. Two separate optimal response modes with the highest energy output have also been identified.

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