Passive oscillations of inverted flags in a uniform flow

Hu, Yao-Wei; Feng, Li-Hao; Wang, Jinjun

doi:10.1017/jfm.2019.937

Cited by 9 publications

(3 citation statements)

References 29 publications

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“…The interrogation window size was set to 32 × 32 pixels with a 75 % overlap. The approach of recognizing membrane deformation was elaborated in previous studies (He & Wang 2020; Hu, Feng & Wang 2020; He et al. 2022).…”

Section: Experimental Methodsmentioning

confidence: 99%

Aerodynamics and fluid–structure interaction of an airfoil with actively controlled flexible leeward surface

Guo

et al. 2023

J. Fluid Mech.

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Piezoelectric macro-fibre composite (MFC) actuators are employed onto the flexible leeward surface of an airfoil for active control. Time-resolved aerodynamic forces, membrane deformations and flow fields are synchronously measured at low Reynolds number (Re = 6 × 104). Mean aerodynamics show that the actively controlled airfoil can achieve lift-enhancement and drag-reduction simultaneously in the angle of attack range of 10° ≤ α ≤ 14°, where the rigid airfoil encounters stall. The maximum increments of lift and lift-to-drag ratio are 27.1 % and 126 % at the reduced actuation frequency of ${f^ + } = 3.52$ . The unsteady coupling features are further analysed at α = 12°, where the maximum lift-enhancement occurs. It is newly discovered that the membrane vibrations and flow fields are locked into half of the actuation frequency when ${f^ + } > 3$ . The shift of the dominant vibration mode from bending to inclining is the reason for the novel ‘half-frequency lock-in’ phenomenon. To the fluid–structure interaction, there are three characteristic frequencies for the actively controlled airfoil: $S{t_1} = 0.5{f^ + }$ , $S{t_2} = {f^ + }$ , and $S{t_3} = 1.5{f^ + }$ . Here, St1 and its harmonics (St2, St3) are coupled with the natural frequencies of the leading-edge shear layer, resulting in the generation of multi-scale flow structures and suppression of flow separation. The lift presents comparable dominant frequencies between St1 and St3, which means the instantaneous lift is determined by the flow structures of St1 and St3. The local membrane bulge and dent affect the instantaneous swirl strength of flow structures near the maximum vibration amplitude location, which is the main reason for the variation of instantaneous lift.

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Section: Experimental Methodsmentioning

confidence: 99%

Aerodynamics and fluid–structure interaction of an airfoil with actively controlled flexible leeward surface

Guo

et al. 2023

J. Fluid Mech.

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“…Recently, the concurrent operation of two or more inverted flags has started to gain attention, with two main configurations being considered: side-by-side (see Figure 1 b) [ 45 , 46 , 47 , 48 , 49 , 50 , 51 ] and tandem (see Figure 1 b) [ 51 , 52 , 53 ] configurations within the horizontal plane. Huertas-Cerdeira et al [ 45 ] experimentally studied the dynamic response of side-by-side inverted flags, with a transverse distance larger than double the length of the flag, demonstrating that the maximum angular amplitude is 36% higher than that of a single flag.…”

Section: Introductionmentioning

confidence: 99%

“…To the best of the authors' knowledge, there has not been an in-depth study into multiple flags arranged side-by-side on the same pole, hence creating the motivation for this work. Our aim is, therefore, to study the coupling interaction between inverted flags in a Recently, the concurrent operation of two or more inverted flags has started to gain attention, with two main configurations being considered: side-by-side (see Figure 1b) [45][46][47][48][49][50][51] and tandem (see Figure 1b) [51][52][53] configurations within the horizontal plane. Huertas-Cerdeira et al [45] experimentally studied the dynamic response of sideby-side inverted flags, with a transverse distance larger than double the length of the flag, demonstrating that the maximum angular amplitude is 36% higher than that of a single flag.…”

Section: Introductionmentioning

confidence: 99%

Wind Energy Harvesting with Vertically Aligned Piezoelectric Inverted Flags

Yang,

Cioncolini,

Revell

et al. 2023

Sensors

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Wind-energy-harvesting generators based on inverted flag architecture are an attractive option to replace batteries in low-power wireless electronic devices and deploy-and-forget distributed sensors. This study examines two important aspects that have been overlooked in previous research: the interaction between an inverted flag and a neighboring solid boundary and the interaction among multiple contiguous inverted flags arranged in a vertical row. Systematic tests have been carried out with metal-only ‘baseline’ flags as well as a ‘harvester’ variant, i.e., the baseline metal flag covered with PVDF (polyvinylidene difluoride) piezoelectric polymer elements. In each case, dynamic response and power generation were measured and assessed. For baseline metal flags, the same qualitative trend is observed when the flag approaches an obstacle, whether this is a wall or another flag. As the gap distance reduces, the wind speed range at which flapping occurs gradually shrinks and shifts towards lower velocities. The increased damping introduced by attaching PVDF elements to the baseline metal flags led to a considerable narrowing of the flapping wind speed range, and the wall-to-flag or flag-to-flag interaction led to a power reduction of up to one order of magnitude compared to single flags. The present findings highlight the strong dependence of the power output on the flapping frequency, which decreases when the flag approaches a wall or other flags mounted onto the same pole. Minimum flag-to-flag and flag-to-wall spacing values are suggested for practical applications to avoid power reduction in multi-flag arrangements (2-3H and 1-2H respectively, where H is flag height).

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Flow-structure interactions of multiple inverted flags in different arrangements

Feng

Wang

2021

Exp Fluids

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Passive oscillations of inverted flags in a uniform flow

Cited by 9 publications

References 29 publications

Aerodynamics and fluid–structure interaction of an airfoil with actively controlled flexible leeward surface

Aerodynamics and fluid–structure interaction of an airfoil with actively controlled flexible leeward surface

Wind Energy Harvesting with Vertically Aligned Piezoelectric Inverted Flags

Flow-structure interactions of multiple inverted flags in different arrangements

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