Search

The vortex-induced vibrations of two side-by-side flexible cylinders in a uniform flow were studied using a three-dimensional direct numerical simulation at Reynolds number Re = 350 with an aspect ratio of 100, and a center-to-center spacing ratio of 2.5. A mixture of standing-traveling wave pattern was induced in the in-line (IL) vibration, while the cross-flow (CF) vibration displayed a standing-wave characteristic. The ninth vibration mode prominently occurred in both IL and CF directions, along with competition between multiple modes. Proximity effects from the neighboring cylinder caused the primary frequency to be consistent between IL and CF vibrations for each cylinder, deviating from the IL to CF ratio of 2:1 in isolated cylinder conditions. Repulsive mean lift coefficients were observed in both stationary and vibrating conditions for the two cylinders due to asymmetrical vortex shedding in this small gap. Comparatively, lift and drag coefficients were notably increased in the vibrating condition, albeit with a lower vortex shedding frequency. Positive energy transfer was predominantly excited along the span via vortex shedding from the cylinder itself and the neighboring one, leading to increasing lower-mode vibration amplitudes. The flip-flopping (FF) wake pattern was excited in the stationary and vibrating cylinders, causing spanwise vortex dislocations and wake transition over time, with the FF pattern being more regular in the stationary cylinder case. [ABSTRACT FROM AUTHOR]

This research investigates the cross-wind aeroelastic effects of tall buildings with hexagonal cross-sections by using aeroelastic models with multiple-degree-of-freedom (MDOF). Cross-wind displacement and acceleration responses at the top of each model are measured using the wind tunnel test. The aerodynamic damping ratios for the two representative wind directions are identified by analyzing the measured responses using the random decrement technique. Results show that large-amplitude vortex-induced vibrations occur for one of the representative wind directions where the vertex of the hexagonal model is against the approaching wind, while there is no significant VIV observed for the other representative wind direction where the face of the hexagonal model is perpendicular to the approaching wind. The most dangerous wind direction is then identified based on the discussion. Two expressions for the cross-wind aerodynamic damping ratio are established for the two wind representative wind directions. The two equations can be used in engineering practice to estimate the cross-wind aerodynamic damping ratio of hexagonal tall buildings. [ABSTRACT FROM AUTHOR]

Energy converters based on vortex-induced vibrations (VIV) have shown great potential for harvesting energy from low-velocity flows, which constitute a significant portion of ocean energy. However, solid-solid triboelectric nanogenerators (TENG) are not wear-resistant in corrosive environments. Therefore, to effectively harvest ocean energy over the long term, a novel solid-liquid triboelectric nanogenerator based on vortex-induced resonance (VIV-SL-TENG) is presented. The energy is harvested through the resonance between VIV of a cylinder and the relative motions of solid-liquid friction pairs inside the cylinder. The factors that affect the output performance of the system, including the liquid mass ratio and the deflection angle of the friction plates, are studied and optimized by establishing mathematical models and conducting computational fluid dynamics simulations. Furthermore, an experimental platform for the VIV-SL-TENG system is constructed to test and validate the performance of the harvester under different conditions. The experiments demonstrate that the energy harvester can successfully convert VIV energy into electrical energy and reach maximum output voltage in the resonance state. As a new type of energy harvester, the presented design shows a promising potential in the field of 'blue energy' harvesting. [ABSTRACT FROM AUTHOR]

In this study, two thin rectangular PVDFs were installed in the form of a cantilever on a FTEH (funnel-type energy harvester), and a CTEH (cymbal-type energy harvester) was fabricated in a form coupled to the upper part of the support. As a result of measuring the energy harvesting sensitivity according to the installation direction of the CTEH, a high voltage was measured in the structure installed on top of the support across all flow velocity conditions. A composite structure PVDF energy harvester combining CTEH and FTEH was fabricated and the amount of power generated was measured. As a result of measuring the open-circuit voltage of the PVDF energy harvester device with a composite structure to which the optimum resistance of CTEH of 241 kΩ and the optimum resistance of FTEH of 1474 kΩ were applied at a flow rate of 0.25 m/s, the output voltage compared to the RMS average value was 7 to 8.5 times higher for FTEH than for CTEH. When the flow rate was 0.5 m/s, the electrical energy charged for 500 s was measured as 2.0 μWs to 2.5 μWs, and when the flow speed was 0.75 m/s, it reached 2.5 μWs when charged for 300 s, generating the same amount when the flow rate increased by 50%. The time to do it was reduced by 66.7%. [ABSTRACT FROM AUTHOR]

A hydrodynamic wind–wave combined power generation system is a new type of energy device that uses wind and ocean current energy to generate electricity. In this paper, the hydrodynamics of a wind–wave combined power generation system was simulated in Fluent. The fluid–structure coupling simulation of the vortex vibration of the cylindrical oscillator was realized using UDF and dynamic mesh technology. The Vortex-Induced Vibration (VIV) characteristics of the cylindrical oscillator were analyzed, and the reliability of the numerical simulation method was verified by comparing the amplitude and trajectory of the eddy-excited vibration with the classic experiments of Jauvtis and Williamson. The VIV characteristics of cylindrical oscillators with different mass ratios were studied in terms of vibration response, motion trajectory, and the streamwise equilibrium position. The effect of the mass ratio on the hydrodynamics of a wind–wave combined power generation system was simulated using spring damping, achieving the goal of carrying out preliminary research work simulating the wind–wave combined power generation device. Some useful conclusions were obtained through calculation, which provided data support for the corresponding platform device. This study shows that in cylindrical oscillators with different mass ratios, the overall trend at the same reduced velocity is that the larger the mass ratio, the smaller the crossflow amplitude. The cylindrical oscillators with mass ratios of one and two appear in the upper branch, while cylindrical oscillators with mass ratios of three and four do not appear, and with the increase in the mass ratio, the frequency ratio in the lower branch tends toward one. At the same reduced velocity, the lower the mass ratio, the larger the corresponding downstream equilibrium position, and the higher the energy acquisition efficiency. [ABSTRACT FROM AUTHOR]

Small wind energy harvesting converts aeroelastic vibration into electricity and can provide independent power supplies for low-power-consumption sensors, which are not convenient for replacing chemical batteries frequently. As wind energy harvesters collect sustainable energy from the ambient environment, they are environmentally friendly and energy saving. The most widely adopted wind-induced vibration mechanisms for designing wind energy harvesters are vortex-induced vibration (VIV) and galloping. VIV-based piezoelectric energy harvesters (VIVPEHs) can stabilize the output voltage at low wind speeds, while galloping-based piezoelectric energy harvesters (GPEHs) can operate at high wind speeds and have wide bandwidths. This paper uses a spring to connect the two traditional wind harvesters to constitute a hybrid wind piezoelectric energy harvester (HWPEH). It is expected that the HWPEH can inherit the advantages of both traditional wind harvesters, i.e., it can reduce the cut-in wind speed, as the traditional VIVPEH, and have a broad working bandwidth, as the traditional GPEH. The effects of the mechanical and circuit parameters on the output voltage and power of the HWPEH are investigated and compared to traditional wind harvesters. It has been found that the aerodynamic behavior of the HWPEH can be customized by changing the masses, stiffnesses, shunt resistances, and damping coefficients. The proposed HWPEH can outperform traditional wind harvesters if the system parameters are well tuned. [ABSTRACT FROM AUTHOR]

Vortex-induced vibration (VIV) is a process that wind energy converts to the mechanical energy of the bluff body. Enhancing VIV to harvest wind energy is a promising method to power wireless sensor nodes in the Internet of Things. In this work, a VIV-driven square cylinder triboelectric nanogenerator (SC-TENG) is proposed to harvest broadband wind energy. The vibration characteristic and output performance are studied experimentally to investigate the effect of the natural frequency by using five different springs in a wide range of stiffnesses ( 27   N / m < K < 90   N / m ). The square cylinder is limited to transverse oscillation and experiments were conducted in the Reynolds regime ( 3.93 × 10 3 – 3.25 × 10 4 ). The results demonstrate the strong dependency of VIV on natural frequency and lock-in observed in a broad range of spring stiffness. Moreover, the amplitude ratio and range of lock-in region increase by decreasing spring stiffness. On the other hand, the SC-TENG with higher spring stiffness can result in higher output under high wind velocities. These observations suggest employing an adjustable natural frequency system to have optimum energy harvesting in VIV-based SC-TENG in an expanded range of operations. [ABSTRACT FROM AUTHOR]

This paper investigates the dynamics of an electromagnetic vortex bladeless wind turbine (VBWT) with a tunable mechanism. The tunable mechanism comprises a progressive-rate spring that is attached to an oscillating magnet inside an electromagnetic coil. The spring stiffness is progressively adjusted as the wind speed changes to tune the turbine fundamental frequency to match the shedding frequency of the vortex-induced vibration (VIV) due to the wind flow crossing over the oscillating mast. Coupled nonlinear equations of motion of the tunable turbine are developed using the lumped-mass representation and Lagrange formulation. Numerical results show that the tunable turbine performs effectively beyond a threshold wind speed. An analytical expression of the threshold speed is derived based on the mechanical fundamental frequency of the turbine. The analytical results are in reasonable agreement with the numerical evaluations. At a given wind speed past the threshold, the tunable turbine has an optimum spring stiffness at which the output power is maximum. Numerical studies also show that the output power of the 2 m long tunable turbine is tens of times larger in comparison to a conventional bladeless turbine. For example, at a wind speed of 4.22 m/s, the output rms power of the tunable turbine is around 1105 mW versus 17 mW of the conventional VBWT. The power can be further maximized at an optimum external load. This research work demonstrated the feasibility and merits of the proposed tunable mechanism to enhance the overall performance of the bladeless wind turbine. [ABSTRACT FROM AUTHOR]

In this paper, a piezoelectric and electromagnetic hybrid wind energy harvester is proposed. The general design of the harvester comprises multiple cantilever piezoelectric energy harvesters (PEHs) and electromagnetic energy harvesters (EEHs) embedded inside the bluff body that is attached to the free end of PEHs. This research work investigates utilizing the room inside the bluff body to enclose harvesters to have a more compact and efficient harvesting system. A comprehensive coupled dynamic model of the harvester (HEH) is developed using Lagrange's formulation. The electromechanical and electromagnetic coupling coefficient equations are derived. The coupled equations of motion are solved analytically and numerically with an exact agreement. A parametric analysis is conducted to study the effect of the design parameters on the overall performance of the harvester in terms of output power and bandwidth. The proposed design evidently presents itself as a promising concept in utilizing the room inside a bluff body. [ABSTRACT FROM AUTHOR]

Air velocity of coal mine ventilation is an important consideration that may cause serious damage. This paper proposes a simple, low cost and effective air velocity monitor (AVM) for coal mine ventilation. The AVM uses the lock-in characteristic of vortex-induced vibration (VIV) to sense the air velocity. Amplitude of the VIV is converted into frequency signal of a vortex-induced triboelectric nanogenerator (VI-TENG) to improve the durability. Structure of the AVM are designed, and parameters of the AVM are optimized with experiments. For the upper and lower air velocity thresholds of 3.1 and 3.6 m/s, the optimized flexible beam length, slider weight, electrode space and electrode width are 42.5 mm, 0.4 g, 0.2 mm and 0.5 mm, respectively. Experiments also show that the output frequency of the VI-TENG could represent the amplitude of VIV well with the correlation coefficient of 0.93. Durability test demonstrates that the AVM generates stable output frequency in 120,000 cycles. A prototype and its controller are fabricated. Wind tunnel tests of this prototype show that it can give alarm when the gas velocity goes above the upper threshold or below the lower threshold. The proposed AVM could be a good solution for simple and effective coal mine ventilation alarm. [ABSTRACT FROM AUTHOR]

In many special agricultural environments, many wireless sensors have a problem of power supply selection. Energy harvesting in the agricultural environment based on vortex-induced vibration (VIV) has the potential to solve the problem. In this paper, an energy harvester based on the VIV is designed in an agricultural environment. Relevant parameters of the harvester are studied with wind tunnel experiment to improve the efficiency of energy conversation. The results show that: (i) For large mass ratio, m* » 1, and the same mass ratio m*, the smaller the damping ratio ζ, the larger the normalized amplitude A*, the larger the maximum efficiency η of VIV energy harvesting; (ii) m* ζ 1, and under a certain range of Reynolds numbers, the smaller the mass-damping parameter m*Z, the larger the normalized amplitude A*, the larger the maximum and average efficiency η of VIV energy harvesting. (iii) m* » 1, the larger the mass ratio m*, the larger the range of resonance; the normalized frequency f* ≃ 1, the stable VIV locked state appears. The research results can provide references for the design of VIV energy harvesters in agricultural environments. [ABSTRACT FROM AUTHOR]

A novel vortex-induced piezoelectric energy converter (VIPEC) is presented in this paper to harvest ocean kinetic energy in the underwater environment. The converter consists of a circular cylinder, a pivoted plate attached to the tail of the cylinder, several piezoelectric patches and a storage circuit. Vortex-induced pressure difference acts on the plate and drives the plate to squeeze piezo patches to convert fluid dynamic energy into electric energy. The output voltage is derived from the piezoelectric constitutive equation with fluid forces. In order to evaluate the performance of the VIPEC, two-dimensional computational fluid dynamics (CFD) simulations based on the Reynolds averaged Navier-Stokes (RANS) equation and the shear stress transport (SST) k-w turbulence model are conducted. The CFD method is firstly verified for different grid resolutions and time steps, and then validated using simulation and experimental data. The influences of the plate length and flow velocity on the wake structure, the driving force and the performance of the VIPEC are investigated. The results reveal that different parameters reach their peaks at different plate lengths, and the converter has a maximal output voltage of 2.3 mV in a specified condition and the maximal power density reaches 0.035μW/m3 with a resistance load of 10 MW. The influence of the simulated subcritical Reynolds number on the driving force is not noticeable. The simulation results also demonstrate the feasibility of this device. [ABSTRACT FROM AUTHOR]

This study is an effort to encapsulate the fundamentals and major findings in the area of fluid-solid interaction, particularly the flow-induced vibrations (FIV). Periodic flow separation and vortex shedding stretching downstream induce dynamic fluid forces on the bluff body and results in oscillatory motion of the body. The motion is generally referred to as flow-induced vibrations. FIV is a dynamic phenomenon as the motion, or the vibration of the body is subjected to the continuously changing fluid forces. Sometimes FIV is modeled as forced vibrations to mimic the vibration response due to the fluid forces. FIV is a deep concern of engineers for the design of modern heat exchangers, particularly the shell-and-tube type, as it is the major cause for the tube failures. Effect of important parameters such as Reynolds number, spacing ratio, damping coefficient, mass ratio and reduced velocity on the vibration characteristics (such as Strouhal number, vortex shedding, vibration frequency and amplitude, etc.) is summarized. Flow over a bluff body with wakes developed has been studied widely in the past decades. Several review articles are available in the literature on the area of vortex shedding and FIV. None of them, however, discusses the cases of FIV with heat transfer. In particular systems, FIV is often coupled to heat transfer, e.g., in nuclear power plants, FIV causes wear and tear to heat exchangers, which can eventually lead to catastrophic failure. As the circular shape is the most common shape for tubes and pipes encountered in practice, this review will only focus on the FIV of circular cylinders. In this attempt, FIV of single and multiple cylinders in staggered arrangement, including tandem and side-by-side arrangement is summarized for heated and unheated cylinder(s) in the one- and two-degree of freedom. The review also synthesizes the effect of fouling on heat transfer and flow characteristics. Finally, research prospects for heated circular cylinders are also stated. [ABSTRACT FROM AUTHOR]

In this study, magnetic force is introduced to the design of a bi-directional U-shaped piezoelectric energy harvester for vortex-induced vibrations. The theoretical model of the beam structure is derived based on the Euler–Bernoulli beam theory. The vortex-induced vibration and the non-linear magnetic force are modeled according to the Rayleigh oscillator and the charge model, respectively. A prototype is fabricated and tested in two orthogonal directions under vortex-induced vibrations in a wind tunnel. Up and down wind-speed sweeps are carried out to investigate the non-linear responses of the harvester. The distance between the magnets and the length of the side beams are adjusted to examine the influence of the magnetic force on the lock-in region and voltage output of the harvester. Overall, the harvester shows strong non-linearity in the horizontal excitations. After adding magnets to the system, significant improvement of the lock-in region and the peak voltage is noticed in the horizontal mode under both up and down sweeps. [ABSTRACT FROM AUTHOR]

This paper proposes a novel and efficient energy harvester (EH) system, for capturing simultaneously flutter and vortex-induced vibration. There exists a coupling effect between flexible spring energy harvester (FSEH) and cantilever beam energy harvester (CBEH) in aerodynamic response and output characteristic. Many prototypes of the harvester were manufactured to explore the coupling effect in a wind tunnel. The experimental results demonstrate that FSEH is mainly subjected to flutter-induced vibration and CBEH undergoes vortex-induced vibration. Disturbance of FSEH first takes place, a limited oscillation cycle then occurs, and chaos ultimately happens as airflow velocity increase. Root mean square voltages are more than 11 V for FSEH at beyond 10.52 m/s, which shows the better output performance over the existing harvesters. Vibration response and output voltage of various harvesters are mutually enhanced with each other. An enhancing ratio for FSEH-130-25 is up to 69.6% over FSEH-130-0, while the enhancing ratio for CBEH-130-30 is 198.3% compared to CBEH-0-30. Field application testing manifests that discharging time to power the pedometer is almost twice as long as the charging one for FSEH-130-25 at 14.48 m/s. The current research offers a suggestive guidance for promoting future practical application in micro airfoil aircrafts. [ABSTRACT FROM AUTHOR]

Vortex-induced oscillations offer a potential means to harness hydrokinetic energy even at low current speeds. In this study, we consider a novel converter where a cylinder undergoes angular oscillations with respect to a pivot point, in contrast to most previous configurations, where the cylinder undergoes flow-induced oscillations transversely to the incident free stream. We formulate a theoretical model to deal with the coupling of the hydrodynamics and the structural dynamics, and we numerically solve the resulting nonlinear equation of cylinder motion in order to assess the performance of the energy converter. The hydrodynamical model utilizes a novel approach where the fluid forces acting on the oscillating cylinder are split into components acting along and normal to the instantaneous relative velocity between the moving cylinder and the free stream. Contour plots illustrate the effects of the main design parameters (in dimensionless form) on the angular response of the cylinder and the energy efficiency of the converter. Peak efficiencies of approximately 20% can be attained by optimal selection of the main design parameters. Guidelines on the sizing of actual converters are discussed. [ABSTRACT FROM AUTHOR]

An energy-harvesting device for a riser based on vortex-induced vibration is proposed to overcome the power supply problem for a marine deep-water riser-monitoring device. To estimate the upper limit of its energy-capture efficiency, as well as the weight and size of the device designed, a discrete model of the riser was configured. With the experimental settings of Stappenbelt and Blevins, vortex-induced vibrations of the discrete cylinder with two degrees of freedom were simulated, and the parameters affecting the energy-acquisition efficiency of the riser were analyzed. The analysis of the dimensionless amplitude ratio showed that this ratio for the system decreased with increasing mass ratio and damping ratio. An analysis showed that the influences of the damping ratio on the energy-capture efficiency were different under medium and low-mass-ratio conditions. A maximum value of 38.44% was achieved when the mass ratio was 2.36 and the damping ratio was 0.05. [ABSTRACT FROM AUTHOR]

This work reports a comparative study on piezoelectric energy harvesting from vortex-induced vibration (VIV) with multi-stability characteristics by introducing the nonlinear magnetic forces. A lumped-parameter model for the piezoelectric cantilever-cylinder structure is considered for the sake of qualitative investigation. Firstly, the buckling displacement of harvester in monostable and bistable configurations is evaluated by virtue of a static analysis. Then, the coupled frequency and damping of the harvester varying with the electrical load resistance are determined for different values of the spacing distance between magnets. Subsequently, the dynamic behaviors and generated voltage of the harvester in two configurations are elaborately investigated, showing that varying the spacing distance is followed by a shift of lock-in region which is significant for performance optimization according to ambient wind conditions. In addition, the results show the harvester in monostable configuration displays a hardening behavior while a softening behavior takes place in bistable configuration, both of the harvester in two configurations can widen the synchronization region. [ABSTRACT FROM AUTHOR]

A vortex-induced vibration (VIV) experiment on a standing variable-tension deepsea riser was conducted to investigate the applicability and sensitivity of Bare Fiber Bragg Grating (BFBG) sensor technology for testing deepsea riser vibrations. The dominant frequencies, dimensionless displacements, in-line and cross-flow couplings of the riser VIV under different top tensions were observed through wavelet transform and modal decomposition. The result indicated that, excited by the same external flow velocities, the cross-flow and in-line dominant frequencies of the riser both decreased with increasing top tension. In terms of displacement responses, increasing top tension caused the root mean square (RMS) displacement to decrease and the vibration amplitude to reduce. In terms of cross-flow and in-line coupling, the closer a location is to the ends of the riser, the smaller the trajectory is and the more standard the "8" becomes. During top tension increases, there exists a "lag" in the time when the riser's vibration trajectory becomes an "8". The Slalom Surround Installation approach can effectively prevent the local breakage of the optical fiber string. BFBG sensor technology can give an accurate presentation of the displacement time history, vibration amplitude and frequency of the riser, provides a clear picture of how the riser's mode and VIV evolve as a function of flow velocity. [ABSTRACT FROM AUTHOR]