Flexible Flag Research Articles

Metasurface-enhanced multifunctional flag nanogenerator for efficient wind energy harvesting and environmental sensing

Wind energy, as a widely distributed and renewable energy resource, plays a crucial role in promoting self-powered wireless sensor networks and wind speed sensing. However, current research predominantly focuses on specific application purposes, lacking comprehensive investigations into integrated solutions. In this paper, a metasurface-enhanced multifunctional flag-type triboelectric nanogenerator (FTENG) is introduced, which enables efficient wind energy harvesting as well as accurate wind speed sensing over a wide range of wind speeds via the synergistic effect of metasurface treatment on the flagpole and flexible flag fixation approach. The flexible fixation maximally utilizes the upstream wake of metasurface configurations, boosting the energy harvesting power density by up to 23 times. The metasurface-enhanced FTENG with careful optimization, striking a balance between energy harvesting and sensing capabilities, achieves a linearity of 0.992 over a wind speed range of 2.3–14.4 m/s and a peak power density of 250 mW/m2. Finally, the FTENG demonstrates its excellent abilities to light LEDs, power the wireless sensor node (WSN), and serve as a self-powered environmental sensor. This work opens up an impactful possibility for developing versatile self-powered electronics by employing metasurface-enhanced wind energy harvesting techniques.

Investigating the influence of flexible flags on heat transfer through experimental analysis

ABSTRACTThe impact of flexible flags vortex generators (FFVGs) positioned on the channel surface on friction factors and the transfer of heat were examined experimentally in this work. Within the scope of the study, the width (w), length (L), height from the ground (c), and position on the channel surface of the determined flexible flags were emphasized. FFVGs are placed in four different positions on the channel surface. The experiments were achieved with four various the number of Reynolds (from 10000 to 30000) using air as the fluid. The temperature changes on channel surface were observed by the thermal imaging method. The Nusselt numbers, friction factor, heat transfer coefficient and heat performance factor were investigated in the study. In the study carried out, Taguchi methodology was employed to establish the optimal values of the parameters used and the L16 (45) orthogonal array was selected. The Taguchi method took into account the number of Nusselt, the factor of friction, and the factor of thermal enhancement as performance factors. The results of flexible flags’ heat transfer in various positions are compared to those of a smooth plate. As a result, the thermal performance factor was increased to 1.38 and the Nusselt number to 2.02, respectively, in comparison to the smooth canal, by using flexible flags.

An Autoencoder‐Based Deep‐Learning Method for Augmenting the Sensing Capability of Piezoelectric Microelectromechanical System Sensors in a Fluid‐Dynamic System

Herein, an innovative deep‐learning architecture is proposed to enhance the sensing capabilities of a microelectromechanical system (MEMS) used in fluid dynamic applications. The MEMS sensor comprises a polyvinylidene fluoride flexible (PVDF) piezoelectric flag and a bluff body, with vortex generation influenced not only by the bluff body's geometry but also by the input fluid speed. As a result, mechanical vibrations are induced in the piezoelectric flag, leading to charge displacement and the generation of electrical voltage signals. Through the developed deep learning method, accurate extraction of wind speed and successful classification of turbulence are achieved. Experimental tests in a wind tunnel, involving various wind speeds and bluff body geometries, demonstrate the robust correlation between the extracted continuous manifold in Fourier spectra and wind speed. By incorporating a feed‐forward network alongside the autoencoder, wind speed information even under strong turbulence is extracted. Moreover, the deep learning method's ability to classify different bluff bodies, independent of wind speed, is investigated. The findings reveal a unique capability to fingerprint turbulence and distinguish them for various applications. This research showcases the potential of our deep learning‐based MEMS systems for enhancing fluid dynamic sensing and classification tasks.

A self-powered electro-coagulation system afforded by flexible electromagnetic flag wind generators for efficient removal of arsenic from water

In the past few years, human activities, such as mining and metallurgical industries, have made the arsenic concentration in natural waters elevate. To provide clean and safe water to people worldwide, tremendous endeavors have been made to accomplish low-cost and effective treatment of arsenic-containing water. Among the methods employed, electro-coagulation has been the one that stands out attributable to its high efficiency and low cost. However, its high energy consumption has become an obstacle to further applications, particularly in the regions where electricity is limited. Here, an innovative self-powered electro-coagulation system (SPECS) driven by flexible electromagnetic flag wind generators (FEFWGs) is developed to realize the power-free removal of dissolved arsenic in water. The FEFWGs exhibit an excellent output performance under a moderate wind speed (6.3 m s−1), with a peak open-circuit voltage of 3.7 V and a peak short-circuit current of 13.6 mA. Using the SPECS to purify polluted water, a large volume of wastewater containing 5 ppm of arsenic can be purified with our SPECS in 20 min, which is more efficient than that of existing reports.

Coupling performance of two tandem and side-by-side inverted piezoelectric flags in an oscillating flow

The flow-induced vibration characteristics and energy extraction performance of two flexible inverted piezoelectric flags arranged in tandem and side-by-side configurations in an oscillating flow are studied. An immersed boundary-lattice Boltzmann method is employed for Reynolds number of 100, mass ratio of 2.9 and non-dimensional bending stiffness of 0.26 which correspond to the maximum flapping amplitude for a single inverted flag in a uniform flow. 2D simulations are conducted by varying the ratio R of the frequency of the oscillating flow to the fundamental natural frequency of the flag and the horizontal velocity amplitude (Au) of the flow. Three coupling regimes at Au=0.5 are identified for both tandem and side-by-side flags: chaotic oscillations regime I (0.1≤R≤1), large periodic and symmetric oscillation regime IIa1.1≤R≤1.5andIIb(2.1≤R≤3.0), and small periodic and asymmetric oscillation regime III(1.6≤R≤2). The maximum mean electrical power coefficient C¯P occurs in regime IIa at R=1.5 with, α (piezo-mechanical coupling parameter) = 0.5, and β (piezo-electric tuning parameter) = 1.5. C¯P is 0.1 for a tandem upstream flag, 0.068 for the tandem downstream flag and 0.1 for both side-by-side flags, and is respectively 120%, 300% and 213%, higher than that of the corresponding flag in the uniform flow. This improvement is attributed to the higher flapping angular amplitude (180°), the higher ratio of the flapping frequency of the flags to the oscillating frequency of the flow (virtually constant at 0.5), and constructive vortex interaction in regime IIa.

Experimental investigation of the effect of flexible/rigid flag on heat transfer

This experimental work investigated the heat transfer characteristics of rectangular channels equipped with a flexible flag vortex generator (FFVGs) and a rigid flag vortex generator (RFVGs). The FFVGs and RFVGs are mounted at the entrance of the channel using four different obstacle geometries. The experiments were performed in a rectangular duct with FFVGs and RFVGs under four different Reynolds numbers (9965, 19,930, 29,985, and 34,875), four different obstacle geometries (triangle-shaped obstacle (T-SO), cylinder-shaped obstacle (C–SO), rectangular-shaped obstacle (R–SO) and half cylinder-shaped obstacle (HC–SO), four different FFVGs and RFVGs lengths (L) (50 mm, 100 mm, 150 mm, and 200 mm), four different FFVGs and RFVGs width (w) (10 mm, 20 mm, 30 mm, and 45 mm), and two different FFVGs and RFVGs channel height clearance (c) (1 mm, and 4 mm). To reduce the number of trials, the Taguchi technique was utilized, thereby reducing the number of trials from 1024 to 16. The highest thermal performance factor was determined as approximately 1.32 in Case 12, which was observed in Case 12 with, l*=0.36, w*=0.036, c*=0.014, Re = 34,875, FFVGs, and R–SO. The heat transfer results of the FFVGs and RPVGs were compared with those of a smooth plate. The best heat transfer performance was obtained with the FFVGs. The presence of the vortex generators produced higher heat transfer coefficients than the smooth plate surfaces.

Experimental study on fluid mixing and pressure drop of mini-mixer with flexible vortex generator

In this paper, a mini-mixer with flexible vortex generator (mMixer with FVG), which consists of a cylinder located at the mixing channel centerline and a vertical flexible flag mounted at the bottom wall of the mixing channel is fabricated to experimentally study the influences of the Reynolds number (Re) and the flexible flag height on the mixing efficiency and pressure drop. The performances of a simple mini-mixer without FVG (SmMixer) and a mini-mixer with cylinder (mMixer with C) are also measured as comparisons. The results indicate that as Re increases, the mixing mode within mMixer with FVG changes from molecular diffusion to vortex-induced mixing, and the mixing efficiency of mMixer with FVG first decreases under the mode of molecular diffusion-induced mixing and then increases under the mode of vortex-induced mixing. Furthermore, the mixing efficiency increases as the flexible flag height increases. Compared with SmMixer and mMixer with C, the FVG can significantly enhance the mixing at large Re. In addition, the pressure drop of mMixer with FVG always increases with the increase of Re and the flexible flag height, and the pressure drop of mMixer with FVG is higher than the other two kinds of mini-mixers.

Power‐Free Electrochemical Degradation of Tetracyclines Contributed by Flexible Magnetoelectric Flag Wind Generators

AbstractElectrochemical degradation is a commonly used strategy to remove organic contaminations in water. However, in some remote areas where electricity is insufficient, electrochemical degradation may become ineffective due to the absence of a power supply. Here, a unique self‐powered electrochemical system (SPES) is proposed to accomplish the power‐free degradation of tetracyclines (TCs) and organic dyes in water with the assistance of flexible magnetoelectric flag generators (MFGs). These MFGs convert wind energy into electricity. Under a wind speed of 6.3 m s−1, a 22 × 30 cm2 MFG yields a peak open‐circuit voltage of 3.5 V and an ultra‐high peak short‐circuit current of ≈13 mA. A suchlike MFG charges large capacitors of 5 F to 1 V in 184 min. Powered by a series of MFGs, 5 ppm of tetracycline hydrochloride (TCH) or organic dyes in water are totally degraded with SPES in only 10 min. Moreover, the MFG is fully foldable and portable, which is well‐suited for the migrating feature of the nomadic people in remote areas. The study opens up a new solution to the persistent problem of TCs contamination in natural waters in some remote regions where electricity is lacking.

Influence of geometrical parameters on the hysteresis of flutter onset in confined configurations

The interaction between a flow and a flexible structure can provide fascinating insight into the vortex shedding phenomenon and any propagation and mixing characteristics, which relate to a plethora of applications such as heat transfer, snoring, musical instruments, or propulsion mechanisms. In this investigation, the influence of confinement on the flapping behaviour of a flexible flag is explored. In particular, hysteresis, one of the least understood aspects of flapping flags, and its sensitivity on both the flexural rigidity of the flag and the confinement ratio is addressed. For the same test-section dimensions and flag material, variations in the flag thickness and flag length enable a range of mass ratios (M^*) and confinement ratios (C^*) to be studied. Pressure measurements and high-speed imaging allow quantification of the hysteretic behaviour. The results demonstrate that although confinement ratio does not contribute to the existence of hysteresis, the relation between the critical reduced velocities and the mass ratio is sensitive to the degree of confinement.Graphical abstract

A coupled flow and beam model for fluid–slender body interaction

In this study, a coupled numerical model for simulation of 3D flow interacting with a slender body is proposed. A discrete-forcing immersed boundary method is implemented to couple the fluid motion and structure deformation. To model the dynamic response of a slender structure, an efficient nonlinear beam model is developed. A new surface reconstruction method is proposed to accurately represent the occupation of the moving structure with different cross-sections in the fixed Cartesian grids of the fluid domain. A quadratic interpolation based on the least-squares method is proposed to realize the local fluid velocity near the relocated solid surface. To validate the model, several fluid–structure interaction problems including a flexible plate attached to a circular cylinder in a flow, a flexible flag in a flow, etc. are simulated. The numerical results are in good agreement with reference results from either experiments or numerical modelling. The model is further employed to investigate the vortex-induced vibration of a flexible riser under unidirectional and oscillatory flow conditions as well as impulse force analysis for a swinging carangiform swimmer in different swing frequencies, the latter has a varying cross-sectional shape along the body’s central axis.

Heat transfer enhancement by a flexible inverted flag with an inclination angle

The heat transfer system, including an inclined inverted flag that plays a role of a vortex generator, is proposed in the present study. A two-dimensional simulation is performed to analyze the effects of the inclination angle and the bending rigidity of the inverted flag on thermal performance. To consider the fluid-flexible body–thermal interaction, an immersed boundary method is adopted. The four regimes are observed depending on the inclination angle and the bending rigidity, that is, large-amplitude flapping (LAF), small-amplitude flapping (SAF), deflected (D), and straight (S) modes. The SAF and LAF modes are observed to be favorable in terms of the heat transfer efficiency, which considers the heat flux and mechanical energy loss. A scaling analysis is performed to explain the correlation between the flapping kinematics and the thermal quantities. A scaling parameter is newly defined based on the momentum transfer to the inverted flag due to a vortical impulse and shows a proportional relation to the mean drag force with a slope of 0.166. The heat transfer efficiency is observed to be proportional and inversely proportional to the parameter in the SAF and LAF modes, respectively. The optimized heat transfer system is obtained at the angle of 12° and the bending rigidity of 0.7, where the efficiency is enhanced up to 112.8% over the baseline flow.

Heat transfer enhancement in a channel flow using two wall-mounted flexible flags with a confined cylinder

A heat transfer system to improve the thermal performance of a channel flow with two symmetrically wall-mounted flexible flags is proposed by arranging a confined cylinder in an upstream region of the flags. The effects of the horizontal gap distance between the cylinder center and the wall-mounted position of the downstream flags (Gx) and the bending rigidity of the flags (γ) on the vortex dynamics are examined based on numerical simulations of the fluid-structure-thermal interaction using a penalty-immersed boundary method. Four distinct flapping modes of the two downstream flags are identified in the regime map of (Gx, γ): an out-of-phase flapping mode with symmetric motion of the flags (OS mode) and an out-of-phase flapping mode with non-symmetric motion of the flags (ON mode), and an in-phase flapping mode (I mode) and combined flapping mode in which the I mode and ON mode are presented alternately with time (ION mode). Heat transfer systems consisting of two flags under the OS and ON modes achieve improved thermal efficiency compared to that for a clean channel system (CS) due to vortex-to-vortex interactions promoting fluid mixing between the core cold fluid and near-wall hot fluid. However, the thermal efficiency for the systems in the I and ION modes decreases due to the small increase in the heat transfer rate for the I mode and a large deficit of the mechanical energy loss for the ION mode, respectively. When the values of Gx, γ and the Reynolds number are optimal, the present system in the OS mode results in an increase of the heat transfer rate by approximately 177% with an accompanying increase of the thermal efficiency up to 24% compared to that for the CS (despite an increase of the mechanical energy loss of 1021%).

Hydrodynamic Coupling of Inverted Flags in Side-by-Side, Left Triangular and Right Triangular Configurations in a Uniform Flow

The hydrodynamic coupling of inverted flexible flags in side-by-side, left triangular and right triangular configurations in a uniform flow is investigated numerically. Various arrangements can be characterized by the lateral and longitudinal gaps, i.e., H and G, respectively. For the side-by-side configurations, three typical modes are identified in flapping region, i.e., in-phase quasi out-of-phase (QOP) and out-of-phase (OP) modes, corresponding to 2P, irregular and 4(P + S) wake patterns, respectively. Further analysis on the mean drag coefficient (Cd̄) and bending energy (Eb̄) indicates that the side-by-side flags can not harvest more bending energy from surrounding flows than the isolated one, except for a narrow range of H ∈ (1.8, 2.5). Moreover, an additional inverted flag is placed in front or rear of the side-by-side flags to explore possible favorable arrangements for bending energy enhancement. Three typical modes have been identified in the H − G planes for both left and right triangular configurations, i.e., the regular-coupled (RC), irregular-coupled (IC) and decoupled (DC) modes, respectively. For left triangular configurations, the overall system may experience significant performance enhancement in some favorable configurations, corresponding to harvest enhancement region (HER) in the G − H plane; For the case in HER, especially, the Eb̄ of the rear flag may increase by up to 30% than isolated flag. While for right triangular configurations, no HER is observed. The unsteady flow characteristics are further explored to reveal the mechanism of hydrodynamic coupling in system. The present study may shed some light on understanding the dynamics of this multi-flag system.

Dynamics of flags over wide ranges of mass and bending stiffness

The flutter of a thin flexible plate or flag is a benchmark problem in fluid-structure interactions with applications to energy harvesting and flow sensing. Here we study flutter dynamics across a wide range of bending stiffnesses, and down to very small values of flag mass, e.g. very flexible fabric flags. We find that the flapping amplitude, frequency, and wavenumber are power laws of the flag mass and stiffness.

Comprehensive experimental study on bluff body shapes for vortex-induced vibration piezoelectric energy harvesting mechanisms

• Both bluff body shape and flag configuration are two crucial factors. • The bluff body shapes with higher drag coefficients can harvest more energy. • Short full active flags generate more energy in higher wind speed. • A new experimental approach in flexible VIV piezoelectric energy harvester. • Electrode pattern are as important as upstream bluff body shapes. Vortex-induced vibration (VIV) is an appropriate mechanism to harvest energy from the low-speed wind energy by flexible piezoelectric flags as transducers. To enhance the low-speed wind energy harvesting, this work proposes a novel study on finding the best possible combination of bluff body shape and flag configuration. This study considered several bluff body shapes in different cross sections and flag configurations as two crucial parameters for finding an appropriate combination. In high flexible piezoelectric flag, zero strain or electrical canceling point along the length of the flag is an important parameter that could be considered in energy harvester design. To this purpose, wind tunnel experiments were conducted to investigate the combination of proper bluff body shape and flag configuration to improve the harvester performance. The proposed bluff body shapes are classified by drag and lift coefficients which are calculated by a computational fluid dynamics (CFD) analysis. Then, several flag configurations in different active area length were clamped to these bluff bodies and tested in the wind tunnel in low wind speed range. The analysis in time and frequency domain of the acquired voltage lead to the conclusion that in low wind speed the bluff body with higher drag coefficients can excite more the longer full active piezoelectric flags, consequently generating more energy. However, for the same bluff body shapes, short full active flags generate more energy in higher wind speed. This study could develop a new experimental approach on finding the most favorable combination of bluff body shape and flag configuration which is as important as just considering bluff body shape to improve the efficiency of the low-speed wind piezoelectric energy harvesting system.

Numerical studies on the hydraulic and mixing performances of fluid flow around a cylinder in microchannel with vertical flexible flag

Oscillating flexible flag due to the force exerted by periodic flow behind a cylinder is a significant technique to enhance mixing. To analyze its hydraulic and mixing performances, this paper numerically studies the pressure drop and mixing efficiency of fluid flow in a microchannel mounted with a cylinder and vertical flexible flag (CCFF) with different height and deformability in a wide range of Reynolds number Red based on the cylinder diameter. The simple channel without cylinder and flag (SC) and the channel with cylinder and rigid flag (CCRF) are also studied as comparisons. It is found that the mixing within CCFF occurs mainly due to the molecular diffusion at small and moderate Red, and its pressure drop and mixing efficiency increase with the increasing flag height. At large Red, the mixing in CCFF is highly enhanced by the vortices detached from the vibrating flag, and its pressure drop and mixing efficiency first increase and then reach a slight decrease or quasi-steady value with the increasing flag height. Compared with SC, the pressure drop of CCFF is always larger than that of SC, however, the mixing efficiency of CCFF is larger than that of SC only when Red or the flexible flag height is large enough. Comparing with CCRF, the pressure drop and mixing efficiency of CCFF are close to or smaller than those of CCRF at small and moderate Red, however, at large Red, CCFF has smaller pressure drop and larger mixing efficiency, and the maximum decrease of pressure drop could be up to 61.09% and the maximum increase of mixing efficiency could be up to 11.46% compared with CCRF.

Direct numerical simulation of flapping flags in grid-induced turbulence

A fully resolved direct numerical simulation approach for investigating flexible bodies forced by a turbulent incoming flow is designed to study the flapping motion of a flexible flag at the moderate Reynolds number. The incoming turbulent flow is generated by placing a passive grid at the inlet of the numerical domain and the turbulence level of the flow, impacting that the flag can be controlled by changing its downstream distance from the grid. The computational framework is based on the immersed boundary method for dealing with arbitrary geometries and implemented using a graphics-processing-unit accelerated parallelization to increase the computational efficiency. The grid-induced turbulent flow is first characterized by means of comparison with well-known results for decaying turbulence and a scale-by-scale analysis. Then, the flag-in-the-wind problem is revisited by exploring the effect of the turbulence intensity on self-sustained flapping. While the latter is still manifesting under strong fluctuations, the main features of the oscillation (including its amplitude and frequency) are altered by turbulence, whose fingerprint can also be qualitatively detected by spectral analysis. Besides their relevance for advancing the fundamental understanding of the fluid–structure interaction in turbulence, these findings have potential impact for related applications, e.g., aeroelastic energy harvesting or flow control techniques.

Heat transfer enhancement in a poiseuille channel flow by using multiple wall-mounted flexible flags

The flapping dynamics of multiple wall-mounted flexible flags vertically clamped in a heated channel and their effects on heat transfer performance were explored numerically. The fluid-structure-thermal interaction between the flapping flexible flags and the surrounding fluid was modeled with the penalty immersed boundary method. The vortex mixing process was examined to analyze their effects on heat transfer enhancement. The effects of varying the gap distance G/d, the reduced distance d/L, the bending rigidity γ, the channel height H/L and the Reynolds number Re on heat transfer enhancement were scrutinized to optimize cooling, where the flag length is L. The thermal enhancement factor was determined to assess both the heat transfer enhancement benefit and the pressure drop penalty. The asymmetric flag configuration G/d = 0.5 results in better cooling performance than the symmetric configuration G/d = 0 because of the resulting thermal mixing of the cold mainstream and the near-wall hot fluid. More flags can be clamped inside the channel by varying the reduced distance d/L. Multiple flags with γ = 0.04 provide the maximum convective heat transfer at Re = 600. Increasing the blockage ratio, i.e. the ratio of the cross-sectional height occupied by the flags to the channel height, enhances the friction factor.

Flapping dynamics of vertically clamped three-dimensional flexible flags in a Poiseuille flow

The flapping dynamics of vertically clamped three-dimensional flags in a Poiseuille flow was studied numerically by using the immersed boundary method. First, the flapping dynamics of a single flag was explored for comparison. Two distinct flow modes were observed: a flapping mode and a deflected mode. In the flapping mode, periodic vortices shed from the flag are formed, leading to alternating upstroke and downstroke flapping motions induced by the hydrodynamic and restoring forces. In the deflected mode, the flag is initially deflected by the hydrodynamic force and reaches a stationary state; the hydrodynamic force is balanced by the restoring force. For tandem flags, when the gap distance is small, the flags behave as one single flag with a higher bending rigidity. When the gap distance is intermediate, the front flag deflects the oncoming flow away from the rear flag. The flapping motion of the front flag is significantly confined by the presence of the rear flag, which results in an attenuation of more than 50% in its flapping amplitude. When the distance is large, the impact of the rear flag on the upstream flow field is negligible, so the front flag exhibits a flapping amplitude and frequency that are similar to those of a single flag. The vortices shed from the front flag induce the formation downstream of a low pressure region, which results in active flapping in the rear flag with a strong amplitude. There are two vortices shed from the tandem flags in each flapping period. When they are far apart, the phase difference is linearly dependent on the gap distance.

Energy harvesting of two inverted piezoelectric flags in tandem, side-by-side and staggered arrangements

The flow-induced vibration characteristics and energy extraction performance of flexible inverted piezoelectric flags in a uniform flow are studied by using an immersed boundary-lattice Boltzmann method for Reynolds number of 100, mass ratio of 2.9 and non-dimensional bending stiffness of 0.26 which correspond to the maximum flapping amplitude for a single inverted flag. The coupled fluid-structure-electric dynamics of two inverted piezoelectric flags in the tandem, side-by-side and staggered arrangements is investigated to determine the effects of the piezoelectric parameters (α and β) on the dynamics of the flags and to quantify the energy harvesting performance. Simulations are conducted by varying the streamwise gap distance (Gx/L) from 0.1 to 3.2 and cross-stream gap distance (Gy/L) from 0.4 to 2.5. It is found that both the flapping amplitude and the frequency of the flags reduce as the piezo-mechanical coupling parameter (α) increases due to the damping effect, particularly in the tandem arrangement. The maximum electrical power coefficient (C¯P) is produced in the staggered arrangement at α=0.5, β (piezo-electric tuning parameter) = 1.5, Gx/L=1.4 and Gy/L=2.2 where the C¯P of the downstream flag is 0.042, 16% and 12.5% higher than that of the upstream flag and that of the single flag, respectively. The improvement in C¯P of the downstream flag is attributed to the increase in the kinetic energy of the fluid flow created by the upstream flag as a bluff body.