Dynamics of a flag behind a bluff body

Kim, Hyeonseong; Kang, Su-Jin; Kim, Daegyoum

doi:10.1016/j.jfluidstructs.2017.03.001

Cited by 40 publications

(11 citation statements)

References 22 publications

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“…which indicates the relative magnitudes of the inertial fluid force and the bending force of the elastic body, has been suggested by several studies (e.g. Alben et al 2002;Shelley & Zhang 2011;Kim et al 2013;Kim, Kang & Kim 2017). In (4.1), B is the bending stiffness, L is a characteristic length, and f is a geometric parameter that determines the effective cross-sectional width of the elastic body.…”

Section: Resultsmentioning

confidence: 99%

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Three-dimensional reconfiguration of an elastic sheet with unidirectional side flaps

et al. 2022

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Motivated by drag-based propulsion of crinoids, the shape reconfiguration of a feather-like elastic structure under both steady and unsteady translational motions is investigated. The simplified elastic structure consists of a centre rod to which numerous side flaps are attached by elastic hinges. These side flaps fold in only one direction to realize a dramatic reduction in the area of the structure during the recovery stroke. Compared with experimental measurements, analytical methods developed to couple the dynamics of the centre rod and the side flaps successfully predict the drag force and three-dimensional reconfiguration of the elastic structure during both power and recovery strokes. A dimensionless speed given by the ratio of inertial fluid force to elastic bending force is proposed for the coupled deflections of the centre rod and side flaps, and is found to determine primarily the reconfiguration of the elastic structure. A reconfiguration number defined specifically for our model provides an appropriate characterization of the effect of side-flap folding on drag force reduction. Moreover, the ratio of drag forces between the power and recovery strokes is evaluated to find model conditions for the optimal force ratio.

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Section: Resultsmentioning

confidence: 99%

“…2002; Shelley & Zhang 2011; Kim et al. 2013; Kim, Kang & Kim 2017). In (4.1), is the bending stiffness, is a characteristic length, and is a geometric parameter that determines the effective cross-sectional width of the elastic body.…”

Section: Resultsmentioning

confidence: 99%

Three-dimensional reconfiguration of an elastic sheet with unidirectional side flaps

et al. 2022

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“…To quantitatively analyse the interaction between a flag and a surrounding fluid, many previous studies have adopted a dimensionless flow velocity U * = U(ρ f l 3 /B) 1/2 , which represents the ratio of the fluid force exerted on the flag to its internal bending force (e.g. Connell & Yue 2007;Eloy et al 2008;Kim et al 2013Kim et al , 2017. However, the gravitational force is also important in our flag model.…”

Section: Mode Distribution and Critical Velocitymentioning

confidence: 99%

“…Numerous studies on the critical conditions and dynamic characteristics of a flag subjected to a free stream parallel to the flag have been conducted, including experiments (Huang 1995;Zhang et al 2000;Kumar, Arekar & Poddar 2021), high-fidelity numerical simulations (Farnell, David & Barton 2004a;Connell & Yue 2007;Huang, Shin & Sung 2007;Sawada & Hisada 2007;Wang & Tian 2019) and low-order theoretical modelling (Guo & Païdoussis 2000;Argentina & Mahadevan 2005;Alben & Shelley 2008;Eloy et al 2008;Michelin, Smith & Glover 2008). In addition, the dynamics of a flag interacting with a nearby structure have been examined using a system of multiple flags (Zhu & Peskin 2003;Farnell, David & Barton 2004b;Jia et al 2007;Ristroph & Zhang 2008;Schouveiler & Eloy 2009;Kim, Huang & Sung 2010;Tian et al 2011;Mougel, Doaré & Michelin 2016) and a single flag considering the ground effect or an upstream bluff body (Akaydın, Elvin & Andreopoulos 2010;Dessi & Mazzocconi 2015;Kim, Kang & Kim 2017). A wall located close to a flag has also been considered, in which a considerable distance between the flag and the wall exists, precluding direct contact between them during the flutter of the flag.…”

Section: Introductionmentioning

confidence: 99%

Gravity-coupled flutter and contact of a flag near a wall

Lee,

Jung,

Lee

et al. 2023

J. Fluid Mech.

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The stability and postcritical behaviour of a horizontal flag undergoing gravity-induced deformation and periodic contact with a nearby horizontal rigid wall are experimentally investigated. The results elucidate the combined effects of gravity and contact on flutter, and reveal design principles for application to triboelectric energy harvesting. By varying the free-stream velocity, flag thickness and distance between the flagpole and the wall, the dynamics of the flag are classified into quasistatic equilibrium, flutter, partial contact and saturated contact modes. Considering the significance of gravitational effects, a new dimensionless flow velocity is proposed to identify the distribution of the dynamic modes, and its definition varies according to whether the wall is placed above or below the flag. The critical conditions for transitions between the dynamic modes are determined from the balance of fluid dynamic and gravitational effects. The distance from the flagpole to the wall is found to be more critical for transitions in the lower-wall configuration than in the upper-wall configuration. The peak contact force as well as the oscillation amplitude and frequency at postequilibrium exhibits remarkably different trends depending on the location of the wall. The peak contact force imposed on the wall by the fluttering flag weakens as the distance to the wall increases in the case of an upper wall, whereas it becomes stronger in the case of a lower wall.

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“…Diverse configurations of a flapping flag with one fixed end and one free end have been suggested for applications to piezoelectric energy harvesters. These are mostly based on the deformation of single or multiple flags (Doaré & Michelin 2011;Tosi, Dorschner & Colonius 2021), a flag with an upstream bluff body (Allen & Smits 2001;Akaydin, Elvin & Andreopoulos 2010;Mittal & Sharma 2022), single or multiple inverted flags (Kim et al 2013;Shoele & Mittal 2016;Ryu, Park & Sung 2018;Mazharmanesh et al 2020;Tavallaeinejad et al 2020) and an inverted flag with an upstream bluff body (Kim, Kang & Kim 2017). Furthermore, for triboelectric energy generation, the contact and separation of a flag with a nearby rigid wall (Bae et al 2014;Meng, Zhu & Wang 2014;Zhang et al 2020;Lee, Kim & Kim 2021) or bluff body (Zhang et al 2020) and the mutual contact and separation of two side-by-side flags (Chen et al 2020) have been investigated.…”

Section: Introductionmentioning

confidence: 99%

Wall-bounded periodic snap-through and contact of a buckled sheet

Mahravan,

Lahooti,

Kim

2023

J. Fluid Mech.

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Fluid flow passing a post-buckled sheet placed between two close confining walls induces periodic snap-through oscillations and contacts that can be employed for triboelectric energy harvesting. The responses of a two-dimensional sheet to a uniform flow and wall confinement in both equilibrium and post-equilibrium states are numerically investigated by varying the distance between the two ends of the sheet, gap distance between the confining walls and flow velocity. Cases with strong interactions between the sheet and walls are of most interest for examining how contact with the walls affects the dynamics of the sheet and flow structure. At equilibrium, contact with the wall displaces the sheet to form a nadir on its front part, yielding a lower critical flow velocity for the transition to snap-through oscillations. However, reducing the gap distance between the walls below a certain threshold distinctly shifts the shape of the sheet, alters the pressure distribution and eventually leads to a notable delay in the instability. The contact between the oscillating sheet and the walls at post-equilibrium is divided into several distinct modes, changing from sliding/rolling contact to bouncing contact with increasing flow velocity. During this transition, the time-averaged contact force exerted on the sheet decreases with the flow velocity. The vortices generated at the extrema of the oscillating sheet are annihilated by direct contact with the walls and merging with the shear layers formed by the walls, resulting in a wake structure dominated by the unstable shear layers.

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Dynamics of a flag behind a bluff body

Cited by 40 publications

References 22 publications

Three-dimensional reconfiguration of an elastic sheet with unidirectional side flaps

Three-dimensional reconfiguration of an elastic sheet with unidirectional side flaps

Gravity-coupled flutter and contact of a flag near a wall

Wall-bounded periodic snap-through and contact of a buckled sheet

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