Piezoelectric Flags Research Articles

Experimental investigation of the wake of tandem cylinders using pivoted flapping mechanism for piezoelectric flag

In this paper, the current of ocean at a depth with the velocity of 0.1–0.3 m/s has been studied. The effect of the mounting mechanism of the piezoelectric flag on the energy efficiency of the energy harvester is investigated. The flag is mounted uniquely, and the results are analyzed to explain the underlying mechanism associated with variations in power output. The experiments described extend our previous work and it is the experimental investigation of the baseline case for our previous work. It was found that using a pivoted mounting mechanism caused a significant improvement in flapping amplitude (51%) and dominant frequency (23%). The variation in the flag's distance from the bluff body (Gd) is also investigated. The maximum output for a passively mounted flag was obtained at Gd=2D due to the existence of a vortex core position at the same point where the flag's head was positioned. The same mechanism is implemented for tandem cylinder arrangement. The study found that increasing the distance between the bluff bodies resulted in the suppression of vortex shedding of the upstream cylinder by the rear cylinder. However, this behavior was found to be random and was thoroughly explored in this study. The obtained results were also verified through the flow visualization technique. A comparative analysis is made to show the optimal performance of the harvester by fine-tuning parameters such as the gap between cylinders and piezo flag, flow rate, and flag-mounting technique. The comparative analysis with benchmark cases demonstrated a rise in the A/L ratio by 30.25% and in the flapping frequency by 19.09% in the case of 120° cut C-shaped inverted cylinder upstream, indicating a higher percentage gain of 66% more energy harvesting. An increase of 51.18% in the A/L ratio and 22.79% in flapping frequency with the use of a pivot-mounted piezoelectric flag was observed, indicating higher energy harvesting performance compared to the baseline case (circular cylinder with fixed flag head).

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.

Flag-type hybrid nanogenerator utilizing flapping wakes for consistent high performance over an ultra-broad wind speed range

Fluttering of regular flags and flapping of inverted flags in the wind serve as the foundational principles of flag-type nanogenerators (FNGs). However, FNGs relying on a single aerodynamic behavior exhibit significant power output only within a limited spectrum of wind speeds, posing a challenge to their robustness in scenarios with intensely fluctuating wind. In this paper, we propose a novel hybrid scheme aimed at harnessing the synergistic potential of two aerodynamic behaviors to enhance the performance of FNGs and broaden their operational wind speed ranges. A flag-type triboelectric-piezoelectric hybrid nanogenerator (FTPNG) is developed with the integration of flapping piezoelectric flags (PEFs) and a fluttering triboelectric flag (TEF). To overcome the limited operational wind speed range, flapping PEFs are configured in an array format, optimized through fluid-solid coupled simulations. The rear TEF leverages the fluttering motion of a polytetrafluoroethylene (PTFE) membrane, which intermittently contacts and separates from conductive textiles positioned on the inner surface of the baffles. A noteworthy feature is the innovative “back-to-back” design, which utilizes the flapping wakes generated by PEFs to intensify the fluttering of the PTFE membrane, resulting in a remarkable boost in power generation of up to 132 times and achieving a maximum peak power output of 5400 μW. The FTPNG offers consistent high performance, with an average output of exceeding 200 μW over an ultra-broad wind speed range of 4.7–14.6 m/s, while the complete operational range is 3.7–15 m/s. It also attains a considerable average power output of 850 μW at 7.8 m/s, marking a significant advancement compared to other FNGs. Finally, in demonstration tests, the FTPNG can light 252 LEDs and showcases the capabilities of PEF array and TEF to independently power a wireless sensor node (WSN), highlighting its significant potential for applications in the Internet of Things and various self-powered systems.

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.

Parametric aerodynamic and aeroelastic study of a deformable flag-based energy harvester for powering low energy devices

Extensive wind tunnel experiments were performed to find the optimal and most influential parameters for energy harvesting from piezoelectric membranes (thin elastic cantilevered plates), herein called flags. Four different piezoelectric flags were tested in conjunction with the seven different bluff bodies at various wind speeds and streamwise gaps between the bluff body and flag. Stiffness of the flags was also varied along with its length to assess the impact on harvested power. Various flapping modes are observed for different flags which correspond to the amount of harnessed power. The results revealed that altering the stiffness of flag can produce an increase of 29 and 31 times in power generation for long and short flags, respectively. It was found that the 120-degree bluff body produced the largest power with the capability to generate a strong wake region. Further analysis of the best performing flag (short and stiff) for various bluff bodies shows a remarkable increase of 661.2 % in generated power. This membrane shows rhythmic periodic flapping with higher modes of deformation in comparison to its other flag counterparts. These alternate flags show out-of-plane and twisting motion which wastes strain energy and causes a cancelation effect, reducing total energy output. The results show that in designing an efficient and reliable energy harvester, the flag’s stiffness and length as well as the shape of the bluff body play an important role. Furthermore, in comparison with the flutter phenomenon (i.e. a dynamic instability of the flag in the absence of a bluff body), the results with a bluff body demonstrate that higher energy generation can be achieved at a lower wind speed, thus paving the way for small and practical energy harvesting devices specifically in remote areas.

Synergistic analysis of wake effect of two cylinders on energy harvesting characteristics of piezoelectric flag

In this study, the effect of asymmetric wake flow regime of two side-by-side cylindrical bluff bodies on power output is experimentally examined by using a piezoelectric flag. Different synchronization modes of the flag with wake flow are observed. It is demonstrated that the streamwise gap between the flag and cylinders (Gx), and the center-to-center gap between cylinders have a significant impact on the flag's dynamical behavior that results in a fluctuation in the power output of the piezoelectric flag. The levels of output power are analyzed by varying the Gx and the cross-stream or lateral gap (N/d) between the two cylinders. N/d values from 1.0 to 2.0 for different values of Gx (2.0 ≤ Gx ≤ 4.0) are experimentally tested. The comparison of the flapping response at each point is made to ascertain the impact of the harvester's dynamic behavior on the output energy. The power generated at each point is recorded for all cases and a comparative analysis is made to find the optimal configuration. Limited research is conducted in the past to enhance the energy output by using the bluff body with the improved wake dynamics. Hence, two cylinders are employed in a uniform flow and crosswise gap between cylinders is varied to change the characteristics of the wake region. The cylinder arrangement with N/d = 1.0, shows continuous oscillations and higher output power persisting for 2.0 ≤ Gx ≤ 4.0. The monotonic rise in power output is observed till Gx = 4.0. The stated configurations with N/d = 1.0 gives a significant advantage over a single-cylinder-based energy harvester as a kinetic source of fluid energy harvester from the flowing fluid. The output power became almost doubled with an increase of 95% approximately using side-by-side arrangement.

Energy harvesting of inverted piezoelectric flags in an oscillating flow

The energy harvesting potential of flexible structures (e.g. flags) made of piezoelectric materials has drawn rapidly increasing attention in recent years. In this work, we numerically study the energy harvesting performance of an inverted piezoelectric flag in an oscillating flow 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 in a uniform flow. 2D simulations are conducted by varying the ellipticity (e), 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: chaotic oscillations regime I (0.1≤R≤1 ), large periodic and symmetric oscillation regime IIa1.1≤R≤1.5and IIb(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 Au=0.5, α (piezo-mechanical coupling parameter) = 0.5, and β (piezo-electric tuning parameter) = 1.5. C¯P is 0.10 for a single inverted flag, and is 148%, higher than that of the corresponding flag in the uniform flow. This improvement is attributed to the higher flapping angular amplitude (180°), higher ratio of the flapping frequency to the oscillating frequency (virtually constant at 0.5) of the flags, and constructive vortex interaction in regime IIa.

An SSHC Interface Circuit for Energy Harvesting of Piezoelectric Flags

Piezoelectric flags have functions of both classic flags and energy harvesting, and are becoming a new research focus. Interface circuits that convert wind energy to electrical energy are the key component of piezoelectric flags. A new structure for piezoelectric flags was designed to generate vibration by wind induction. After theoretical analysis, only SEH (standard energy harvesting) and SSHC (synchronized switch-harvesting-on capacitors) interface circuits were found suitable for piezoelectric flags. Simulation in Multisim was performed to compare SEH and SSHC in different load resistance. Experiments were carried out using different wind speeds. The on-time and delay-time of each switch were controlled by the proposed control algorithm. Both simulation and experimental results indicate that the output voltage with SSHC is higher than the output voltage with SEH. When the resistance is 1700 kΩ and the wind speed is 24 m/s, the output power of SSHC can be increased by 45.63% compared with the SEH circuit.

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.

Experimental hydrodynamic investigations on the effectiveness of inverted flag-based piezoelectric energy harvester in the wake of bluff body

The flapping motion and energy harvesting performance of an inverted flag in the wake of a bluff body are experimentally studied. The experimental measurements are carried out by changing the bending rigidity, streamwise gap (which represents the distance from the bluff body to the tip of the flag), and Reynolds number to explore their impact on the flapping dynamics and power generated by the piezoelectric flag. The optimal values of the Reynolds number, bending rigidity, and streamwise gap are determined based on the power generated. Variation in the flapping modes ranging from continuous to deflected mode is observed. The results show that the inverted flag having a high peak-to-peak amplitude is preferred for piezoelectric energy harvesting as it produces high strain energy. It is also demonstrated that the vortices shed by the upstream bluff body have a strong effect on the flapping amplitude of the downstream inverted flag. This research would help in determining the most effective streamwise position of the inverted flag behind the bluff body and bending rigidity for generating voltage at low Reynolds numbers.

Hermes - Wind Energy Harvesting Wireless System for Sensing AoA and Wind Speed

In this letter, we present Hermes - a novel, low-cost, wireless, batteryless, energy harvesting system for aerial vehicles for sensing wind speed and Angle of Attack (AoA) concurrently. Hermes comprises a set of piezoelectric films which flutter due to incoming wind and the characteristics of this aeroelastic flutter are utilized for determining the wind speed and AoA of the head-wind. Note that in our work we restrict the notion of flutter to high frequency oscillations due to incoming air flow. Hermes consists of five piezoelectric flags that are mounted on rigid clamps specifically placed at different angles. We designed Hermes to maximize the sensing performance and energy harvesting capability simultaneously, without compromising either accuracy or harvesting efficiency. Our current prototype can harvest the power of 440 μW on average. Over a wide range AoA from -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> to 30 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> , the estimation of the wind speed is within 0.7 km/h error with 90% probability, and AoA error is within 1.2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> with 90% probability. Since Hermes necessitates no wires and batteries and is a low-cost sensor, it is well suited for a range of UAVs, gliders, and aircraft, which require flexible sensor placement and do not require new wiring, which is often complex in aircraft. Hermes is the first of its kind that exploits piezoelectric energy harvesting to simultaneously sense AoA and wind speed. This work is expected to open up new avenues for interdisciplinary research on embedded computing devices for aerospace applications.

Direct Scaling of Measure on Vortex Shedding through a Flapping Flag Device in the Open Channel around a Cylinder at Re∼103: Taylor's Law Approach.

The problem of vortex shedding, which occurs when an obstacle is placed in a regular flow, is governed by Reynolds and Strouhal numbers, known by dimensional analysis. The present work aims to propose a thin films-based device, consisting of an elastic piezoelectric flapping flag clamped at one end, in order to determine the frequency of vortex shedding downstream an obstacle for a flow field at Reynolds number in the open channel. For these values, Strouhal number obtained in such way is in accordance with the results known in literature. Moreover, the development of the voltage over time, generated by the flapping flag under the load due to flow field, shows a highly fluctuating behavior and satisfies Taylor’s law, observed in several complex systems. This provided useful information about the flow field through the constitutive law of the device.

Effect of Structural Parameters of the Piezoelectric Film-Based Wind Energy Harvesting Flags on their Power Generation Wind Speed Ranges

Effect of Structural Parameters of the Piezoelectric Film-Based Wind Energy Harvesting Flags on their Power Generation Wind Speed Ranges

Hydrodynamic energy harvesting analysis of two piezoelectric tandem flags under influence of upstream body’s wakes

In this study, the effects of the interaction of piezoelectric flags in the tandem configuration on energy harvesting in a wake flow are experimentally investigated. The flags are placed behind the bluff body and their flapping behavior is examined in terms of the flapping frequency and amplitude (peak to peak motion). The experiments are performed in a low-speed water tunnel by varying the flow velocity and streamwise gap behind an inverted C-shape cylinder to determine the influence of wake flow on the oscillating amplitude, flapping frequency, and harvested power by the piezoelectric flags. Threshold values for energy harvesting of streamwise gap and water speed are found to be the same for both flags, 1.5 and 0.18 m/s, respectively. The results show that inverted drafting is observed in flags in which the flapping amplitude of the rear flag is increased by excitation from the vortices and wake of the front flag. This interaction boosts the energy harvester efficiency based on the flapping frequency and the random excitations with high amplitudes. It is observed that as the streamwise gap in-between the flags changes, the influence of the front flag on downstream flag alters, and dynamical behavior of front flag show variation when the distance between bluff body and front flag varies. The highest power is also obtained for the rear flag at a gap of 1.75 and water speed of 0.26 m/s. The tandem configuration produces 116% more power and significantly improves the energy harvesting efficiency as compared to the single flag energy harvester.

Experimental investigation of energy harvesting behind a bluff body

The effect of base suction on flapping and energy harvesting through the piezoelectric membrane in the wake of a cylindrical bluff body is studied in this paper by carrying out a series of wind tunnel experiments. The effect of S/D (ratio of the distance between the cylinder and membrane to the cylinder diameter), L/D (ratio of the membrane length to the cylinder diameter), and flow speed on the flapping dynamics and energy generation from the polyvinylidene fluoride membrane placed behind the bluff body was studied. Different flapping modes were found from optimal coupling to fully deformed. The flapping motion of the membrane was determined by using a high-speed camera, and an oscilloscope was used for the measurement of the generated voltages. Also, the flapping dynamics and output voltages were extensively studied in the sub and post-critical regions by varying the S/D ratio, L/D ratio, and flow velocity. An increase of 38% in energy harvesting is observed when the piezomembrane was placed at L/D = 1 and S/D = 2 with free stream velocity U = 10 m/s. An increase or decrease in voltages is attributed to the observation that the amount of harvested energy changes by varying the stated parameters. Optimum energy can be harvested by fine-tuning of flow and geometrical parameters and adjusting the piezoelectric flag in the specified range.

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.

Numerical Simulation for Energy Harvesting of Piezoelectric Flag in Uniform Flow

Numerical Simulation for Energy Harvesting of Piezoelectric Flag in Uniform Flow

Fluid-solid-electric energy transport along piezoelectric flags

The fluid–solid–electric dynamics of a flexible plate covered by interconnected piezoelectric patches in an axial steady flow are investigated using numerical simulations based on a reduced-order m...

A novel method for improving the energy harvesting performance of piezoelectric flag in a uniform flow

ABSTRACTA novel method of piezoelectric layer configuration is proposed for preventing cancellation of the positive and negative charges of piezoelectric flag in flow, which will effectively improve the energy harvesting performance. Vibration stain modes of piezoelectric flag are identified and performance of the piezoelectric flag with continuous piezoelectric layer (CPL) and segmented piezoelectric layers (SPL) are investigated. The output power of piezoelectric flag with the SPL is much more than that of the CPL at 2nd and higher order vibration mode. The higher vibration mode of piezoelectric flag, the more advantage of the SPL for energy harvesting is obtained.

Electro-hydrodynamic synchronization of piezoelectric flags

Hydrodynamic coupling of flexible flags in axial flows may profoundly influence their flapping dynamics, in particular driving their synchronization. This work investigates the effect of such coupling on the harvesting efficiency of coupled piezoelectric flags, that convert their periodic deformation into an electrical current. Considering two flags connected to a single output circuit, we investigate using numerical simulations the relative importance of hydrodynamic coupling to electrodynamic coupling of the flags through the output circuit due to the inverse piezoelectric effect. It is shown that electrodynamic coupling is dominant beyond a critical distance, and induces a synchronization of the flags’ motion resulting in enhanced energy harvesting performance. We further show that this electrodynamic coupling can be strengthened using resonant harvesting circuits.