1. Introduction
In order to make full use of the vibrational energy and wind energy in the environment and solve the problem of self-supply of microelectronic devices such as low-power sensors, more and more scholars have begun to study environmental energy harvesting [
1]. The case for the use of microelectronic devices is involved in places such as vehicle driving, bridge monitoring, energy consumption monitoring, and structural health monitoring and airfoil vibration monitoring and other places [
2,
3,
4,
5]. The design of an energy harvester for wind energy harvesting and conversion to replace traditional forms of chemical batteries to supply power to microelectronic devices [
6] can effectively solve the places of limited use, high manufacturing costs, environmental pollution and other problems caused by conventional batteries [
7]. The energy harvester can be used in a variety of ways. Depending on the principle of operation, they can be divided into a piezoelectric energy harvester [
8,
9,
10], an electromagnetic energy harvester [
11], an electrostatic energy harvester [
12] and a frictional energy harvester [
13]. Among these, a piezoelectric energy harvester has the advantage of higher operating frequency range and higher energy density [
14]. Depending on the different mechanism of wind-induced vibration, a piezoelectric energy harvester mainly includes a vortex-induced energy harvester [
15,
16], a flutter energy harvester [
17,
18], a galloping energy harvester [
19] and a wake energy harvester [
20].
Vortex excitation usually occurs in cylindrical structures, where the air flow acts on the surface of the cylinder, resulting in alternating shedding of vortices on both sides of the cylinder to form alternating vortex excitation forces, which excite the cylinder to produce vortex vibration [
21,
22]. A classical vortex-induced energy harvester is generally composed of a terminal cylinder and a piezoelectric cantilever beam, which usually vibrates in a single degree of freedom [
23]. Wang [
24] studied the output characteristics of the double-beam vortex-induced vibration piezoelectric energy harvester through numerical analysis, and the results showed that the maximum output voltage was 8.42 V when the flow velocity was 5.6 m/s. Sun et al. [
25] studied a suspended piezoelectric energy harvester with the upper end of the piezoelectric cantilever fixed and the lower end connected to a cylindrical blunt body. After the water flows through the cylindrical blunt body, high- and low-pressure vortices are formed. The cylinder vibrates periodically under the action of hydrodynamic force. When the water velocity is 0.48 m/s, its maximum output power is 1.064 mW. Weinstein et al. [
26] designed a vortex-induced energy harvester consisting of a cylindrical harvester body and a single degree of freedom piezoelectric cantilever beam at the rear and installed a blade at the free end of the cantilever beam. When the wind velocity was 5 m/s, the output power was 3 mW, but it is difficult to miniaturize the energy harvester.
Flutter is a self-excited divergent vibration, which is suitable for higher flow rate conditions, characterized by self-excitation, nonlinearity, large amplitude and large deformation, but with low stability [
27]. A wind belt harvester is a taut belt with a high spreading ratio moving at low wind velocities. Fei et al. [
28] have proposed a similar energy harvester that converts mechanical vibrations into electrical energy via an electromagnetic transducer. Kwon [
29] designed a T-shaped double-crystal cantilever beam rigid flutter energy harvester, which has an output power of 4 mW when the wind velocity is 4 m/s. Dunnmon et al. [
30] designed an aeroelastic energy trap based on the nonlinear limit cycle oscillation phenomenon, and obtained an average output power of 2.5 mW at a wind velocity of 27 m/s. Bryant and Garcia [
31] proposed a flutter-based aeroelastic energy harvester that undergoes limit cycle oscillations at higher wind velocities above the critical wind velocity.
To broaden the energy harvest band and improve the energy harvest effect, the common methods are either to introduce magnetic nonlinearity or to combine two or more energy harvest mechanisms [
32,
33,
34]. Li et al. [
35] proposed a nonlinear magnetically coupled flutter energy harvester with a low wind velocity (about 1.0 m/s) and good energy harvesting characteristics in the low wind velocity range (1.0 m/s to 2.9 m/s). Rawnak Hamid et al. [
36] designed an energy harvester that combined the flutter energy harvest mechanism and electromagnetism into a single unit with a maximum output power of 0.550 μW. Mahmood et al. [
37] proposed a piezoelectric-electromagnetic composite energy harvester with a cylindrical blunt body fixed at the end of a piezoelectric cantilever beam, and a more compact and efficient energy harvest system by using the space inside the cylindrical blunt body to embed the magnetic coil. Wang et al. [
38] designed a new type of bluff body with rounded and folded corners to combine the vortex excitation and galloping vibration phases. The experimental results show that when the bluff body is half cuboid and half cylinder, its voltage output reaches 14.6 V. Shan et al. [
39] proposed a composite energy harvester that can capture vortex excitation and flutter energy at the same time. When the wind velocity is 14.48 m/s, its power density is 2.41 μW/cm
2. Ahmed B. Atrah et al. [
40] designed a composite electromagnetic energy harvester that uses cylindrical bluff body to induce wind belt vibration to capture energy, and experimentally studied the influence of Karman Vortex Street generated by different diameters and positions of cylindrical bluff body on wind belt vibration frequency and amplitude. The maximum output voltage of the energy harvester is 6 V.
Vortex-excited flutter is currently a hot research topic for energy harvest using wind energy, but there are still problems of complex structure and low efficiency. The nonlinear energy harvester of Cylindrical blunt vortex excitation was investigated by Hou et al. [
41]. It was found that a maximum unrectified power of 0.21 mW could be obtained when the flow velocity was approximately 1.6 m/s. A hybrid energy harvesters containing two airfoil components have been investigated by Li et al. [
27,
42]. Based on our previous works, the further investigation of a vortex-excited and flutter composite energy harvester is carried out. In this paper, an energy harvester with a cylindrical bluff body and two symmetrically distributed airfoil components attached to the end of a cantilever beam is proposed, which can simultaneously harvest energy for both vortex-induced vibration and flutter. The coupling effect between the two is investigated by conducting several groups of wind tunnel experiments to improve the output performance of the composite energy harvester.
2. Design of the VFPEH
In this paper, a vortex-induced and flutter coupled excitation piezoelectric energy harvester (VFPEH) with symmetrical airfoil is proposed, which is composed of a piezoelectric cantilever beam module, a cylindrical bluff body (hollow structure), a long shaft, two bearings (Nedel XUDZ, Yokohama, Japan) and two airfoil components, as shown in
Figure 1. The piezoelectric cantilever beam module is composed of a metal substrate and a piezoelectric ceramic sheet. The piezoelectric ceramic sheet is fixed at a position 10 mm away from the fixed end of the substrate, the cylindrical bluff body is fixed at the free end of the cantilever beam, and the two airfoil parts are symmetrically fixed at both ends of the long axis placed on the same axis with the cylinder. The standard airfoil used is NACA 0012; the interface pattern is symmetrical to the airfoil chord. The airfoil is connected to the long axis at the airfoil chord and at the maximum thickness of the airfoil, according to the relevant definition of the airfoil parameters, the position of the maximum thickness is at 30% of the chord length from the leading edge.
When the harvester system VFPEH is placed in the incoming airflow, the cylindrical bluff body and airfoil take experience a vortex-induced vibration and flutter, respectively. When the wind velocity is lower than the flutter onset of velocity, a vortex-induced vibration in the VFPEH takes place. On the contrary, it produces flutter and drives the cylindrical bluff body to swing with a certain amplitude and increases the deformation of the piezoelectric cantilever beam. In this process, the piezoelectric cantilever beam not only experiences a bending deformation, but also a torsional deformation due to the existence of airfoil, which increases the deformation of the cantilever and improves the energy harvesting performance.
3. Experimental Setup
In order to investigate the aerodynamic response and output of a symmetrical airfoil vortex-induced flutter composite piezoelectric energy harvester, a simple wind tunnel experimental system was built. As shown in
Figure 2, the experimental system consists of an air duct, a blower, a frequency converter, a wind velocity tachometer, an oscilloscope (Tektronix), a computer and an energy harvester system.
The cylindrical bluff body and airfoil structure of the VFPEH adopt 3D printing technology, and the material is degradable lactic acid PLA. The material of choice for the piezoelectric sheet is the lead zirconate titanate piezoelectric ceramic PZT-5H (Shenzhen Electronics). Referring to the dimensions of other papers [
27], and taking into account the dimensions of the wind tunnel test system used in this paper, the material parameters and dimensional parameters for each part of the VFPEH have been tentatively determined as shown in
Table 1. Based on our previous works [
27,
42], the mathematical model of the hybrid energy harvesters was derived. As shown in
Table 1, fixed cantilever beam structural parameters are used and the effect of the cantilever beam structural parameters on the energy capture device is not considered in this paper.
The airfoil component is a standard airfoil type NACA0012 manufactured using 3D printing technology. The airfoil is connected to the shaft using an interference fit. NACA0012 airfoil dimension parameters are shown in
Table 2.
5. Conclusions
In this paper, a vortex-induced flutter composite piezoelectric energy harvester is designed. The structure is a symmetrical arrangement of airfoils on both sides of the cylindrical bluff body, which can simultaneously realize both vortex-induced vibration energy harvest and flutter energy harvest. An experimental study on the influence of key parameters on its output performance was carried out, coupled with experimental research on the effect of vortex-induced vibration and flutter energy harvest. The main results are as follows:
(1) The piezoelectric energy harvester VFPEH-30 with a symmetrical arrangement of airfoils and cylindrical blunt body can play a role in reducing the starting wind velocity, which is 4.8 m/s, much lower than the classical cylindrical blunt body vortex excited piezoelectric energy harvester VEH-30 and the airfoil flutter piezoelectric energy harvester FEH, by 32.40% and 43.53%, respectively.
(2) The symmetrical arrangement of the airfoils on both sides of the cylindrical blunt body enables the cantilever beam of the energy harvester to produce bending and torsional compound vibrations, realizing energy harvest in both bending and torsional directions. It effectively increases its output voltage and output power. The effect of energy harvest varies from one cylindrical blunt body to another, between 20–35 mm. The energy harvest is better when the blunt body diameter is 30 mm, the output voltage of the VFPEH-30 first increases and then decreases with increasing air velocity. The results show that the maximum output power is 6.47 mW at a wind speed of 9.0 m/s and an optimum load of 140 k, which is much higher than that of the VEH-30 and FEH.
(3) Compared with the classical vortex-induced piezoelectric energy harvester and the flutter piezoelectric energy harvester, the vortex-induced and flutter composite piezoelectric energy harvester has a higher power output at a lower wind speed. This work provides an effective experimental basis for a combined vortex-flutter energy harvester with two mechanisms.