Flapping dynamics of an inverted flag in a uniform flow

Ryu, Jaeha; Saleel, C. Ahamed; Kim, Boyoung; Sung, Hyung Jin

doi:10.1016/j.jfluidstructs.2015.06.006

Cited by 114 publications

(60 citation statements)

References 20 publications

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“…These biological structures are also very narrow, which further restricts flapping. Indeed, elimination of inverted flag flapping at low Reynolds numbers is observed in recent computational simulations (Ryu et al 2015).…”

Section: Relation To the Flapping Of Biological Structuresmentioning

confidence: 72%

“…This quantitative behaviour of the Strouhal number with increasing flow speed, and its insensitivity to Reynolds number, are identical to observations of VIVs of a broad range of mechanical structures; see § 4.1.2. The low Reynolds number computational simulations of inverted flag flapping by Ryu et al (2015) exhibit similar behaviour, even though the Reynolds numbers are two orders of magnitude smaller.…”

Section: Strouhal Number Of Flappingmentioning

confidence: 75%

“…This indicates that flapping will not occur for small scale structures, which naturally exhibit small Reynolds numbers. This is borne out in the computational simulations of Ryu et al (2015) at low Reynolds number (Re 50). Interestingly, this is precisely the regime where cylinders do not exhibit periodic vortex shedding, a prerequisite for vortex-induced vibrations.…”

Section: Resultsmentioning

confidence: 99%

“…Re ≈ 10 4 -10 5 . Others have performed direct numerical simulations of the Navier-Stokes equations and thus focus on low Reynolds number flows (Re ≈ 100-200) (Ryu et al 2015;Tang, Liu & Lu 2015). While these computational studies were able to reproduce the phenomena observed experimentally, the dominant physical mechanisms were not described.…”

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confidence: 99%

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Large-amplitude flapping of an inverted flag in a uniform steady flow – a vortex-induced vibration

et al. 2016

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The dynamics of a cantilevered elastic sheet, with a uniform steady flow impinging on its clamped end, have been studied widely and provide insight into the stability of flags and biological phenomena. Recent measurements by Kim et al. (J. Fluid Mech., vol. 736, 2013, R1) show that reversing the sheet's orientation, with the flow impinging on its free edge, dramatically alters its dynamics. In contrast to the conventional flag, which exhibits (small-amplitude) flutter above a critical flow speed, the inverted flag displays large-amplitude flapping over a finite band of flow speeds. The physical mechanisms giving rise to this flapping phenomenon are currently unknown. In this article, we use a combination of mathematical theory, scaling analysis and measurement to establish that this large-amplitude flapping motion is a vortex-induced vibration. Onset of flapping is shown mathematically to be due to divergence instability, verifying previous speculation based on a two-point measurement. Reducing the sheet's aspect ratio (height/length) increases the critical flow speed for divergence and ultimately eliminates flapping. The flapping motion is associated with a separated flow -detailed measurements and scaling analysis show that it exhibits the required features of a vortex-induced vibration. Flapping is found to be periodic predominantly, with a transition to chaos as flow speed increases. Cessation of flapping occurs at higher speeds -increased damping reduces the flow speed range where flapping is observed, as required. These findings have implications for leaf motion and other biological processes, such as the dynamics of hair follicles, because they also can present an inverted-flag configuration.

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Section: Relation To the Flapping Of Biological Structuresmentioning

confidence: 72%

Section: Strouhal Number Of Flappingmentioning

confidence: 75%

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

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Large-amplitude flapping of an inverted flag in a uniform steady flow – a vortex-induced vibration

et al. 2016

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“…Figures 1a and 1b show a single measurement of such an inverted leaf (taken from Sader et al (2016a)) where largeamplitude flapping (limit cycle behaviour) is evident. Detailed studies of the related and canonical 'inverted flag' configuration-a cantilevered elastic sheet oriented such that the flow impinges on its free edge with its clamp downstream-provide an explanation for this leaf behaviour and show that orientation of the flag can have a dramatic effect (Kim et al 2013;Cossé et al 2014;Tang et al 2015;Ryu et al 2015;Sader et al 2016a). Kim et al (2013) reported the first detailed measurements on the stability and dynamics of the inverted flag, in both air and water.…”

Section: Introductionmentioning

confidence: 99%

Effect of morphology on the large-amplitude flapping dynamics of an inverted flag in a uniform flow

et al. 2019

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The stability of a cantilevered elastic sheet in a uniform flow has been studied extensively due to its importance in engineering and its prevalence in natural structures. Varying the flow speed can give rise to a range of dynamics including limit cycle behaviour and chaotic motion of the cantilevered sheet. Recently, the 'inverted flag' configuration-a cantilevered elastic sheet aligned with the flow impinging on its free edge-has been observed to produce large-amplitude flapping over a finite band of flow speeds. This flapping phenomenon has been found to be a vortex-induced vibration, and only occurs at sufficiently large Reynolds numbers. In all cases studied, the inverted flag has been formed from a cantilevered sheet of rectangular morphology, i.e., the planform of its elastic sheet is a rectangle. Here, we investigate the effect of the inverted flag's morphology on its resulting stability and dynamics. We choose a trapezoidal planform which is explored using experiment and an analytical theory for the divergence instability of an inverted flag of arbitrary morphology. Strikingly, for this planform we observe that the flow speed range over which flapping occurs scales approximately with the flow speed at which the divergence instability occurs. This provides a means by which to predict and control flapping. In a biological setting, leaves in a wind can also align themselves in an inverted flag configuration. Motivated by this natural occurrence we also study the effect of adding an artificial 'petiole' (a thin elastic stalk that connects the sheet to the clamp) on the inverted flag's dynamics. We find that the petiole serves to partially decouple fluid forces from elastic forces, for which an analytical theory is also derived, in addition to increasing the freedom by which the flapping dynamics can be tuned. These results highlight the intricacies of the flapping instability and account for some of the varied dynamics of leaves in nature. arXiv:1810.13308v1 [physics.flu-dyn]

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Flag Fluttering‐Triggered Piezoelectric Energy Harvester at Low Wind Speed Conditions

Özkan,

Erkan,

Basaran

2024

Energy Tech

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Flow‐induced vibrations are common occurrences in various fluid–solid interaction systems existing in nature. One way to transform this vibrational mechanical energy into a usable form is through energy harvesters. This study examines the performance of a piezoelectric energy harvester, where mechanical oscillations result from the fluttering flags made of various fabrics. Flags, made of commonly encountered fabrics, are attached to a cantilever beam inside a wind tunnel with a 30 × 30 cm test section, allowing the airflow to be adjusted between 0 and 10 m s−1. The alpaca flag, with dimensions corresponding to an aspect ratio of 2, can produce meaningful electricity even at a wind speed as low as ≈3.8 m s−1. As the wind speed increases, the fluttering frequency of this flag configuration becomes 10.9 Hz at a wind speed of 5 m s−1, coinciding with the natural frequency of the first mode shape. In this specific arrangement, the structural deformation of the piezoelectric patch on the beam surface is maximized, allowing the harvester system to produce a root mean square voltage of 5 V with a relatively high maximum power of 98 μW.

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Flapping dynamics of an inverted flag in a uniform flow

Cited by 114 publications

References 20 publications

Large-amplitude flapping of an inverted flag in a uniform steady flow – a vortex-induced vibration

Large-amplitude flapping of an inverted flag in a uniform steady flow – a vortex-induced vibration

Effect of morphology on the large-amplitude flapping dynamics of an inverted flag in a uniform flow

Flag Fluttering‐Triggered Piezoelectric Energy Harvester at Low Wind Speed Conditions

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