Energy harvesting of flow induced vibration enhanced by bionic non-smooth surfaces

doi:10.21203/rs.3.rs-2695565/v1

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Energy harvesting of flow induced vibration enhanced by bionic non-smooth surfaces

https://doi.org/10.21203/rs.3.rs-2695565/v1

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Inspired by the shield scale on the shark surface, a D-type bionic fin with non-smooth surface is proposed and used in tandem cylinders piezoelectric energy harvesters (PEHs) for the utilization of wind energy on the roof of buildings. The repeating unit of D-type bionic fin is semicircle, and the corresponding center angle of each repeating unit is 7.2°. PEHs consist of a piezoelectric cantilever beam and a wind-interference cylinder connected to the beam tip. The influence of the spacing ratio on the amplitude of PEHs with D-type bionic fins added under elastic interference is studied through wind tunnel tests and three installation positions are designed: only installed upstream, only installed downstream, and not installed upstream and downstream (bare). It is found that the maximum amplitude response law of the upstream piezoelectric energy harvester (UPEH) is not affected by the D-type bionic fins, and the D-type bionic fins can make the downstream piezoelectric energy harvester (DPEH) realize the change of the maximum amplitude from small spacing ratio to large spacing ratio. In addition, the influence of the installation position of D-type bionic fins on the output voltage of upstream and downstream PEHs is also studied. The research shows that the addition of D-type bionic fins significantly changes the vibration behavior of PEHs. D-type bionic fins can enhance the energy harvest performance by coupling "coupled vortex-induced vibration" and wake induced galloping (WIG), and increasing the surface velocity of PEHs. D-type bionic fins can also expand the bandwidth of the voltage harvested by the PEHs. The analysis of the power under the experimental wind speed shows that the installation of D-type fins in PEHs can increase the output power of the upstream and downstream PEH by 392.28% and 13% respectively compared with the bare piezoelectric energy piezoelectric energy harvester (BARE-PEH). In addition, the computational fluid dynamics is used to analyze the flow pattern, wake structure and lift coefficient of PEHs, and the reason why the installation of D-type bionic fins in the upstream has an impact on the harvest performance of upstream and downstream PEHs at 1.5 spacing ratio is explained.

Bionic

Non-smooth surface

Piezoelectric energy harvester

Wind energy

Wake induced galloping

Wind tunnel experiment.

Wind power has been studied as a kind of green energy for decades [1], but there is still a lot of wind energy wasted on the urban roof. The small-scale wind energy on the urban roof can provide renewable alternative energy for aviation obstruction lights and wireless signal amplifiers, and realize the self-power supply of the device [2, 3]. Wind energy harvest on the roof is very important for warning lighting and communication signal facilities. There are three methods to convert wind energy into vibration energy and then convert it into electrical energy output: (1) piezoelectric type [4, 5] (2) electrostatic type [6] electromagnetic type [7], of which piezoelectric type has the advantages of high energy density, low cost, simple structure, compact, etc., and is more suitable for the small-scale wind power self-power supply device on the roof.

Flow-induced vibration (FIV) is mainly divided into vortex-induced vibration (VIV) [8, 9, 10], galloping [11, 12], flutter [13, 14] and wake-induced vibration (WIV) [15, 16]. The energy harvester based on VIV, galloping and WIV design has aroused the interest of scholars. Researchers have made a series of studies in order to use VIV to harvest energy. Modiri et al. [17] explored the effect of stiffness and Reynolds number (Re) on the performance of the VIV energy harvester through experiments. The experimental results show that the amplitude of the upper support of the VIV increases with the increase of the stiffness. The system with adjustable natural frequency can obtain the best energy harvest in the VIV-based ocean energy harvester by expanding the bandwidth. Akaydin et al. [18] established the theoretical model of vortex-induced vibration energy harvester (VIVEH) to predict the energy harvest performance. The results show that the performance of VIVEH can be changed by changing the characteristic size, shape and direction of the bluff body. Most VIVEH only work in the locking region, with a narrow range of working flow rates. However, energy harvester based on galloping need a larger excitation flow rate [19]. In order to improve the output power and environmental adaptability of VIV and galloping energy harvesters, Wang et al. [20] studied two types of wind energy harvesters based on VIV and galloping coupling. The blunt bodies of the two harvesters are butterfly and spindle respectively. CFD results show that small width ratio and vertical installation will lead to stronger aerodynamic force, thus improving the energy conversion efficiency of flow-induced vibration. Obviously, the energy harvested by the energy harvester based on the single blunt body model is limited.

As shown in Fig. 1, when one or more cylinders are placed in the windward cylinder wake on the roof, the aerodynamic instability will lead to the WIV phenomenon. Kim et al. [21] studied the FIV characteristics of the test cylinder when the control cylinder is close to the rear of the test cylinder. The results show that when the control cylinder is close to the rear of the test cylinder, the divergent vibration in the test cylinder can be divided into three modes according to the amplitude characteristics. Li et al. [22] conducted wind tunnel experiments to measure the vortex shedding frequency of two finite height cylinders arranged in staggered configuration. It is found that the vortex shedding frequency of two cylinders depends largely on the spacing ratio between them. Fan et al. [23] conducted an experimental study on FIV of elastic multi-cylinder in series, side-by-side and staggered arrangement in a wind tunnel at different spacing ratios and incident angles. The vibration characteristics and transition characteristics of different configurations are determined. By discussing the response of each configuration, the FIV of series cylinder is divided into four states. Due to the complexity of upstream wake, the performance of WIV is usually studied by experiment and simulation. Inspired by WIV, some researchers proposed a combined piezoelectric energy harvests (PEHs) system to improve space utilization and output power performance [24]. Song et al. [25] studied a new enhanced energy harvest method through simulation modeling and experimental verification. Through simulation modeling and experimental verification, two PEHs are arranged in series in the direction of water flow to harvest the vibration caused by water flow. The experimental and simulation results show that downstream piezoelectric energy harvester (DPEH) has better energy harvest performance than upstream piezoelectric energy harvester (UPEH), especially in high-speed flow. The total output power of the two tandem piezoelectric energy harvester systems is significantly greater than that of the ordinary single PEH, which provides a good prospect for further exploration of the multiple PEHs system. In order to study the effect of different flow vibrations, Zhang et al. [26] tested two piezoelectric rigid cylinders connected by VIV and WIV. VIV test shows that the output voltage increases with the increase of load resistance. For the maximum output power, there is the best load resistance. WIV test shows that the vibration of the small cylinder is controlled by the vortex frequency of the large cylinder. The tandem cylinders have an optimal gap to obtain the maximum output voltage and power. For the same energy harvest device, WIV has higher power generation capacity. Inspired by the WIV of tandem cylinders, Yan et al. [27] proposed a piezoelectric wind energy harvester with upper and lower interference for highway wind resource development. The influence of upstream and downstream spacing on harvesting power was studied by wind tunnel experiments. The experimental results show that the upstream spacing is more important than the downstream spacing. The proposed series configuration of wake induced vibration greatly improves the performance of WIV energy harvest. Zhao et al. [28] proposed a new hydrodynamic PEHs using WIV. The mathematical models of the water kinetic energy harvester are established as inverted D-shaped, circular and D-shaped cylinders. Compared with the case without wake interference, the output power of the hydrodynamic PEHs using WIV is significantly improved. In short, the WIV-based energy harvester can harvest energy at high wind speed. However, its ability to harvest energy at low and medium wind speeds is weak [29]. Research shows that the aerodynamics of PEHs can be changed by changing its surface structure, and its vibration mode can be significantly affected, thus improving its performance [30]. Zhang et al. [31] studied the fish scale shape of the groove surface structure, designed its shape, and visually displayed the influence of the structure on turbulence. Yang et al. [32] studied the influence and control of non-smooth body structure on vehicle tail vortex. The method of combining computational fluid dynamics (CFD) numerical simulation with wind tunnel test is used to control the automobile tail vortex. Wang et al. [33] used convex columns, triangular prisms and wedges to change the surface roughness of PEHs. It is found that the convex cylinder PEH with diameter of 6 mm and height of 9 mm has the maximum vibration displacement and maximum voltage output.

To the best of our knowledge, there is no research to introduce the bionic non-smooth surface into the piezoelectric tandem cylinders energy harvester. Therefore, we have designed a bionic D-type fin that can improve the performance of PEHs under elastic interference, and introduced its design concept and experimental settings in Section 2. In the first of the Section 3, the optimal load resistance of the PEHs is discussed. Then, we discuss the influence of changing the spacing ratio on the amplitude of different PEHs. In the third of Section 3, the influence of the installation position and spacing ratio of D-type bionic fins on the voltage of the PEHs is studied. In order to better evaluate the performance of the PEHs and analyze the mechanism to improve its performance, the power analysis and eddy current dynamics analysis are carried out in the fourth and fifth of Section 3. Finally, summarizes the main findings of this study in Section 4.

2.1. Design concept

In the absence of boundary layer separation, the fluid resistance is not the sliding friction resistance between the object and the fluid, but the static friction, which is the resultant force of the shear stress on the surface of the blunt body and only related to the velocity gradient on the surface of the blunt body. Due to the viscous effect of the fluid, the velocity gradient formed between the far-field free flow and the object is not linear. The flow friction in the viscous fluid will affect the velocity gradient on the bluff surface and thus affect the resistance [34]. For the case of large Reynolds number, various vortex structures in the turbulent boundary layer will make the kinetic energy of the fluid dissipate into internal energy, resulting in insufficient utilization of the kinetic energy of the fluid. Non-smooth surface is a method based on bionic design to create a microstructure similar to the biological surface on the smooth surface to change the vortex structure near the body surface, reduce the fluid resistance, enhance the utilization rate of fluid kinetic energy and enhance energy harvest. Riblets are composed of small protrusions on the smooth surface and longitudinal microgrooves in the flow direction [35]. They have been proved to reduce the wall friction resistance of turbulence and have been well applied in the aviation and marine industries [36, 37]. The design of rib drag reduction is inspired by the toothed structure on the surface of shark scales, also known as shield scales. The structure can lift the fluid vortex near the body surface and reduce the resistance. Figure 2 (a) is the surface micro-morphology of a typical biological representative mackerel shark, which can move rapidly in water, taken by scanning electron microscope (SEM). It was observed that there are five ribs on each shield scale of Mackerel shark and the gap between the ribs is very small. The shield scales are densely arranged and almost have no gap. The size of the shield scales is generally 0.2 ~ 0.5mm, and the spacing is 30 ~ 100 µm [38]. Compared with other methods to enhance energy harvest, its effect only depends on the surface characteristics, without additional equipment, additional energy consumption and space occupation. It will not pollute the flow field, and even the cost of design and manufacturing is relatively low. Therefore, this paper designs a groove-shaped non-smooth surface based on the shark shield scale as shown in the figure. Based on the arrangement of the mackerel shark shield scale, a D-type bionic fin with a closely arranged groove surface structure with a diameter of d = 25mm and a thickness of s = 0.5mm is developed by 3D printing. As shown in Fig. 2 (b), the repeating unit of D-type bionic fin surface is a semicircle, which corresponds to the center angle of a single groove “α” is 7.2°, and the material is photosensitive resin. In this paper, the D-type fin is applied to the tandem cylinder energy harvester for the first time to explore its influence on the energy harvest after reducing the turbulent resistance.

2.2. Experimental device and process

The experimental device is shown in Fig. 2, and all experiments are conducted at the section size of 1.1×0.5×0.5m open wind tunnel. A high-precision metal probe type wind speed sensor is installed in the front of the center of the wind tunnel to measure the actual wind speed. The experimental wind speed is between 0.5–10 m/s, and the data is recorded every 0.5m/s, the recording time is 180s. The transparent acrylic plate is installed on three sides of the wind tunnel test section. The laser displacement sensor (KEYENCEIL600) is used to measure the displacement of PEHs, which has the characteristics of wide sampling frequency range and high measurement accuracy. Its main parameters are listed in Table 1. Use the wireless data acquisition instrument (DH5908) to measure the voltage generated by piezoelectric materials, and its parameters are listed in Table 2. It can continuously sample data up to 128KHz. Therefore, high-precision voltage measurement can be guaranteed. The cantilever beam elastic sheet made of 65Mn is used to fix the elastic tandem cylinders on the frame. The length L_b., width w and thickness t are 120mm, 16mm and 0.4mm respectively. The length of the transverse wind direction of the frame is long enough to ensure that the cylinder can vibrate in the transverse wind direction. The distance between the centers of two cylinders is L. Piezoelectric ceramic sheet (smart material corp. MFC-2807-P2) is tightly connected to the cantilever elastic sheet at the upstream and downstream. Its internal capacitance is 28nF and its size is 37×10×0.3mm³ piezoelectric ceramic material. The cylinder diameter D is 25mm, height H = 150mm, and the material is photosensitive resin. The spacing ratio is defined as L/D.

Table 1

Main parameters of laser displacement sensor
Parameter/unit	Value
Reference distance/mm	600
Measuring range/mm	200–1000
Repeatability/me	50
Resolving power/mm	50

By providing an initial displacement to a single PEH and allowing it to carry out free attenuation motion, the free attenuation curve and spectrum of the PEH can be obtained using Fast Fourier Transform (FFT), thus obtaining the natural frequency. As shown in the Fig. 3, the natural frequencies of bare cylinder and D-type bionic fin are 3.239Hz and 2.482Hz respectively. This paper focuses on the study of the influence of non-smooth riblet structure on the harvest ability of the PEH by changing the aerodynamics, and ignores the influence of D-type fins on the natural frequency of the PEH.

Table 2

Main parameters of the wireless data acquisition instrument.
Parameter/unit	Value
Continuous sampling speed/Kohs	128
Voltage range/mV	50–5000
Voltage minimum resolution/ova	50
Wireless communication range/m	200

In this paper, the influence of bionic non-smooth surface structure (D-type bionic fin) on the harvest performance of PEHs upstream and downstream of tandem cylinders under elastic interference is studied. Three groups of experiments were designed to discuss the influence of the spacing ratio and the installation position of D-type bionic fins on the performance of upstream and downstream PEHs. According to the research carried out by Hu et al. [39] on two elastically supported tandem cylinders in the wind tunnel, flow-induced vibration is divided into three typical forms according to the dimensionless spacing l: l ≤ 1.5, 1.5 ≤ l ≤ 3.0, 3.0 ≤ l ≤ 8.0. When two cylinders are arranged in tandem, there are shear layer interference and wake interference. The dimensionless spacing l corresponds to the spacing ratio L/D in this paper. Therefore, this paper selects three spacing ratios of 1.5, 3.0 and 5.0 for experimental research, and named them Mode 1, Mode 2 and Mode 3. The D-type bionic fin is installed at the upstream and downstream of the tandem cylinder PEHs, and the installation location nomenclature is shown in Table 3.

Table 3

Piezoelectric energy harvester nomenclature
Installation location	Without D type bionic fin	Upstream with D-type bionic fin	Downstream with D-type bionic fin
UPEH	BARE-UPEH	U-UPEH		D-UPEH
DPEH	BARE-DPEH	D-UPEH		D-DPEH

3.1. Load resistance analysis

In order to ensure that the PEHs in this paper can achieve the best energy harvest effect, we first explore the influence of load resistance on PEHs. Considering the impedance of vibrating piezoelectric ceramic, we analyzed the energy harvest effect of BARE-UPEH by changing the load resistance value. The experimental wind speed is 7.5m/s. In terms of spacing ratio, we chose three spacing ratios L/D: 1.5, 3.0 and 5.0, to explore the maximum voltage and harvested power of PEHs. It can be seen from Fig. 5 that the root mean square voltage (RMS Voltage) generated by BARE-UPEH at three spacing increases with the increase of load resistance, while the power reaches the peak at 1.8×10⁶MΩ. At the spacing ratio of 1.5, 3.0 and 5.0, the maximum power generated by BARE-UPEH is 0.74629mW, 0.42681mW and 0.52156mW respectively. It can be concluded that the optimal load resistance of PEHs in this paper is 1.8×10⁶ MΩ, both UPEH and DPEH circuits in the following experiments are conducted at this load resistance value.

3.2. Effect of spacing ratio on amplitude of UPEH and DPEH

As we all know, spacing ratio (L/D) is critical to the flow pattern and wake structure of two tandem PEHs in a cross flow. In other words, the aerodynamic coefficient depends on the spacing ratio [40]. As shown in the Fig. 6 (a) (b) (c), whether to install D-type bionic fins and change the installation position of D-type bionic fins, the maximum amplitude response of UPEH occurs at the spacing ratio of 1.5. This indicates that the D-type bionic fin may enhance the flow acceleration of fluid in the gap area between the two PEHs, thus generating the gap driving force and increasing the fluid force on UPEH [41]. As the spacing ratio increases, the gap driving force and flow acceleration in the gap area decrease, and the amplitude of the corresponding UPEH decreases. It shows that adding D-type bionic fins in tandem PEHs does not change this rule. It is worth mentioning that the maximum amplitude response of BARE-UPEH and U-UPEH occurs at 10 m/s, while D-UPEH occurs at about 5 m/s, indicating that a “lock-in zone” similar to vortex-induced vibration may occur near 5 m/s in D-DPEH, which will be analyzed in the voltage response.

As shown in the Fig. 6 (d) (e) (f), BARE-DPEH maximum amplitude is generated at 1.5 spacing ratio, which is consistent with the results of literature [42]. After installing D-type bionic fins upstream, the coupling effect of U-UPEH on downstream U-DPEH is enhanced, and the maximum amplitude produced by three-spacing U-DPEH under the experimental wind speed is close. For D-DPEH, at the spacing ratio of 5.0, due to the large spacing ratio, the vortex generated by D-UPEH has little impact on D-DPEH. D-DPEH is similar to the state of a single PEH. The amplitude response of D-DPEH is largely determined by the promotion of D-type bionic fin on vortex shedding. The maximum amplitude generation at the spacing ratio of 5.0 indicates that the enhanced intensity of D-type bionic fin on vortex shedding can break through the restriction of spacing ratio to amplitude response. An interesting phenomenon is found in this paper, from the perspective of the degree of influence of installing D-type bionic fins, on the amplitude of DPEH, the installation of D-type bionic fins directly downstream has a greater impact on the corresponding amplitude of DPEH than the installation of D-type fins upstream. To sum up, D-type bionic fin can make the maximum amplitude of DPEH change from small spacing to large spacing, and expand the application range of PEHs.

3.3. Output voltage analysis

In this paper, the voltage is divided into three regions: low velocity region 0 ~ 2.5m/s, medium velocity region 2.5 ~ 7.5m/s, and high velocity region 7.5 ~ 10m/s. As described in reference, the voltage trend is consistent with the amplitude trend, and the phenomenon is consistent [40]. This paper analyzes the three vibration modes and change trend based on voltage.

As shown in the Fig. 7 (a) (b), when the L/D = 1.5 (mode 1), BARE-UPEH and BARE-DPEH vibrate at the wind speed of 3 m/s and 1.5 m/s respectively, and then the voltage response slowly increases with the increase of wind speed, showing a typical wake induced galloping (WIG) phenomenon [43]. After adding D-type fins to the upstream PEH, the voltage of U-UPEH starts to respond at U = 3.5m/s, the voltage response increases first and then decreases, and the voltage response basically remains unchanged between 3.5 ~ 4.5m/s wind speed, similar to the situation of traditional VIV "locking zone". However, the difference is that the cut-in speed U_cut is different from the traditional vortex-induced vibration [44]. Traditional VIV occurs at a very low U_cut. Tang et al. [45] also observed a similar phenomenon and defined it as "coupled vortex-induced vibration". Interestingly, the voltage response of U-UPEH increases again when the wind speed is 7m /s, and the subsequent voltage response increases with the wind speed, showing the phenomenon of WIG. The RMS voltage generated at 10m/s reaches 84.15V, which is much higher than BARE-UPEH and D-UPEH. This may be caused by the D-type bionic fin increasing the velocity of surface fluid on both sides of U-UPEH. The fifth of Section 3 will further analyze the flow field to illustrate this phenomenon. The above phenomenon shows that adding D-type fins upstream of PEH can achieve complete coupling of the two vibration types of “coupled vortex-induced vibration” and WIG by controlling the wind speed, thus increasing the voltage harvested by UPEH. U-DPEH achieves better voltage response than BARE-DPEH in the "locked zone" of "coupled vortex-induced vibration", which shows the possibility that D-type bionic fin can cause greater voltage response with lower wind speed. The voltage of D-UPEH starts to respond at the speed of 3.5m/s, and also produces the phenomenon of "coupled vortex-induced vibration". The voltage response first increases and then decreases with the wind speed. At the same time, D-DPEH shows the phenomenon of WIG, and the voltage slowly rises with the wind speed.

As shown in the Fig. 7 (c) (d), when the L/D = 3.0 (mode 2), BARE-UPEH has almost no voltage response, indicating that BARE-UPEH does not vibrate at this time, which is consistent with the amplitude response mentioned above. The voltage response of U-UPEH and D-UPEH increases sharply in the high wind speed region, and the overall response is still at a low value, reaching the maximum value when the wind speed is 9m/s, and then the voltage response decreases with the increase of wind speed. U_cut of the U-DPEH is the largest, the voltage response rises slowly with the increase of wind speed and increases sharply when the wind speed is 6m/s. The overall phenomenon is WIG. The installation of D-type bionic fin makes the UPEH voltage response change from nothing to something in the high wind speed region. In the high wind speed region, the voltage response of U-DPEH is better than BARE-DPEH and D-DPEH, and the maximum RMS voltage can reach 35.10V. D-DPEH has the lowest U_cut and shows better voltage harvest capability in the medium and low wind speed areas. The voltage of D-DPEH increases first and then decreases in the high wind speed areas. The voltage response mode is chaotic. In this mode, the vortex shedding from the D-UPEH cannot be closed, and the fluid flow in this interval is the most complex [46].

As shown in the Fig. 7 (e) (f), when L/D = 5.0 (mode 3), the voltage response of BARE-UPEH and D-UPEH is relatively small within the experimental wind speed, the U_cut of U-UPEH is 8 m/s, and then WIG occurs. At the wind speed of 10 m/s, the RMS voltage is 6.169V. BARE-DPEH and D-DPEH show WIG phenomenon, but U_cut of D-DPEH is 4.5m/s, compared with BARE-DPEH and U-DPEH, the U_cut is lower, and the voltage response of D-DPEH is greater in the whole experimental wind speed range. This may be due to the installation of D-type bionic fins, which leads to the enhancement of D-DPEH vortex shedding [47]. U-DPEH also has the phenomenon of “coupled vortex-induced vibration”. The maximum RMS voltage generated in the “locking zone” is 35.77V, which is higher than BARE-DPEH at the same wind speed. Interestingly, installing D-type bionic fins will increase the natural frequency of the PEHs as it is shown in Fig. 4. Installing D-type bionic fins in the downstream energy harvester will theoretically increase the U_cut of D-DPEH, but D-DPEH has voltage response before D-UPEH and BARE-DPEH at the spacing ratio of 3.0 and 5.0, which indicates that installing D-type bionic fins in the middle and large spacing ratio may promote the vortex at the tail of D-DPEH to fall off ahead of time and reduce the U_cut. So as to expand the bandwidth of the voltage harvested by the PEH.

3.4. Evaluation of the output power enhancement of the proposed D-type bionic fin

Through various studies on the installation position and three modes of D-type bionic fins so far, we can determine that adding D-type bionic fins on upstream and downstream under the experimental wind speed can undoubtedly bring the maximum power increase, and can determine the single PEH with the best performance within the entire wind speed range considered in this study: U-UPEH, mode 1 (L/D = 1.5), wind speed 10m/s. As shown in Fig. 8 (a), the maximum power of U-UPEH is increased by 392.38%, and the maximum power of D-UPEH is increased by 116.19% compared with BARE-UPEH. Compared with mode 1 (L/D = 1.5), when mode 2 (L/D = 3.0) and mode 3 (L/D = 5.0), the power generated by UPEH is relatively small, and the output power of BARE-UPEH is almost zero. However, installing D-type bionic fins at upstream and downstream can still increase the power of U-UPEH.

As shown in Fig. 8 (b), the maximum power of U-DPEH is less than BARE-DPEH when L/D = 1.5 within the experimental wind speed range. However, considering that the excitation wind speed U_cut=7.5m/s of the maximum power generated by U-DPEH is lower than the excitation wind speed U_cut=10m/s of BARE-DPEH, and this maximum power is generated under "coupled vortex-induced vibration", the above voltage analysis also confirms this view, so U-DPEH also shows the potential of harvesting energy at lower wind speed. The maximum power of U-DPEH and D-DPEH at mode 2 (L/D = 3.0) and mode3 (L/D = 5.0) is also improved compared with BARE-DPEH, which is 13% and 8.8%. If we consider a single PEH, we recommend the U-UPEH in Mode 1 and if two PEHs harvest energy at the same time, we recommend the U-UPEH and U-DPEH in Mode 2.

3.5. Flow analysis

An interesting phenomenon observed in this paper is shown in Fig. 7(a), (b), in the case of mode 1 (L/D = 1.5), when the wind speed is U = 10m/s, U-UPEH shows a much higher energy harvest capability than BARE-UPEH, while the energy harvest capability of D-DPEH is lower than BARE-DPEH. Therefore, we carried out two-dimensional unsteady numerical simulation at this wind speed to further understand the physical differences between UPEH and DPEH in the process of flow-induced vibration when installing D-type bionic fins upstream under mode 1 (L/D = 1.5).

The length and width of the simulated flow field are 60D and 40D respectively. The PEHs are placed at 15D away from the entrance and middle position in the width direction. Unstructured triangular mesh is introduced to divide the calculation area. In order to optimize the calculation, the calculation area is divided into three areas according to the distance from the PEHs, and the grid is further refined near the wall of the PEHs to improve the simulation accuracy. The maximum grid size of these three areas is 0.12D, 0.08D and 0.04D respectively. The total number of meshes is about 600000, and the calculation time step is 0.001s. The inlet and outlet boundary conditions are 10m/s uniform wind speed and pressure outlet flow. The horizontal boundary of the fluid field and the boundary of the two PEHs are assumed to be sliding walls and non-sliding walls, respectively. Since the Reynolds number Re = 3.6×10⁵, there is a turbulent wake behind the DPEH and the flow area behind the DPEH is an intermediate subcritical region with periodic shedding vortex, although the Reynolds average Navier-Stokes (RANS) method can greatly reduce the computational cost, it ignores the turbulence pulsation information, which makes the computational data in the turbulent wake distorted. Compared with the RANS method, large eddy simulation (LES) can capture the periodic shedding of the entire eddy by directly solving the large-scale eddy. After modeling, it can describe the complex small-scale turbulent eddy at different locations in time and space in detail. Considering the calculation accuracy and cost, LES is introduced to calculate the turbulent fluid flow around the proposed PEHs by CFD.

As shown in Fig. 9, when the incoming flow meets PEHs, a relatively low speed zone will appear in front of the UPEH, and then the air flow will flow along both sides of the UPEH and form a high speed zone on both sides of the UPEH and continue to flow between the UPEH and the DPEH to form a low speed zone, so then the high speed zone will continue to form on both sides of the DPEH, and the vortex shedding is caused by the speed difference, so then the vortex will be generated alternately in the S shape at the tail of the DPEH.

As shown in Fig. 9 and Fig. 10, the U-UPEH has higher flow velocity and greater pressure difference on both sides compared with BARE-UPEH, which indicates that the lateral excitation force generated by the pressure difference on both sides of U-UPEH is greater, so it is a good explanation for the amplitude response ability and voltage harvest ability of U-UPEH shown in the experiment. This also verifies the effect of manufacturing a microstructure similar to the bionic non-smooth surface structure mentioned in the design of D-type bionic fins to enhance the fluid vortex near the body surface, thus reducing the fluid resistance and increasing the flow rate. Compared with U-DPEH, the intensity of vortex generated by the wake of BARE-DPEH is larger, and the frequency of vortex shedding is faster. After BARE-DPEH, it can maintain a longer vortex area and a higher vortex intensity. At the same time, there is a greater velocity difference and pressure difference on both sides of BARE-DPEH, which fully verifies that Fig. 7 (b) BARE-DPEH harvests voltage greater than U-DPEH at the wind speed of 10m/s.

By integrating the pressure distribution around the UPEH and DPEH cross sections, the lift coefficient that changes with time is calculated. As shown in Fig. 11, U-UPEH and BARE-DPEH show higher lift coefficients than BARE-UPEH and U-DPEH, which also verifies that the amplitude response and voltage response discussed above are consistent with the experimental data. Since the vibration of the PEHs is determined by the lift coefficient and the drag coefficient, this paper only discusses how to improve the performance of the lift coefficient by adding D-type bionic fins.

In this study, based on bionic design, a novel non-smooth surface structure-D type bionic fin is proposed to develop the best PEH for roof applications. The possibility of improving the efficiency of energy harvest by installing D-type fins on the upstream and downstream of tandem cylinders is theoretically discussed. Through the wind tunnel test, the decoupling test is carried out on the spacing ratio of the tandem cylinders and the installation position of the D-type bionic fins. Through the computational fluid dynamics simulation, the PEHs are analyzed from the angle of the flow field, and the flow pattern, wake structure and lift coefficient of UPEH and DPEH with the optimal spacing ratio are discussed. The research results show that:

(1) For maximum output power, PEHs have the best load resistance, which is 1.8×10⁶MΩ.

(2) Changing the spacing ratio has a significant impact on the energy harvest effect of the PEHs. Compared with the BARE-PEH, the proposed D-type bionic fin shows a relatively high vibration response and a rapid increase in the experimental wind speed range.

(3) By changing the placement of D-type bionic fins, the PEHs with D-type bionic fins installed in the range of experimental wind speed is almost improved compared with BARE-PEH, UPEH can obtain 392.38% of the maximum power increase rate, and DPEH can obtain 13% of the maximum power increase. D-type bionic fins can also expand the bandwidth of the voltage harvested by the PEHs.

(4) Through CFD simulation, it was found that the non-smooth surface structure changed the flow field structure and vortex shedding rate, coupled with "coupled vortex-induced vibration" and WIG phenomenon to improve the maximum PEHs power. The simulation results showed that the velocity difference and pressure difference on both sides of U-UPEH and BARE-DPEH were greater, indicating that the installation of D-type fins changed the eddy current generated by the upstream and downstream cylinders, which made the fluid flow around the upstream and downstream PEHs more complex, It also enhances the oscillating aerodynamic force of the PEHs, thus increasing the harvest capacity of the upstream and downstream PEHs, and further explains the reason why D-type bionic fins enhance the harvest of flow-induced vibration energy.

Authors‘ Contribution

Jiawei Wang: Methodology, Investigation, Formal analysis, Writing – original draft. Bowen Tang: Methodology, Investigation, Formal analysis. Hewei Wang: Writing – review & editing. Wei Tan: Resources, Supervision, Writing – review & editing.

Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Materials and Data Availability

The data that has been used is confidential.

Acknowledgments

The research work presented in this paper is financially supported by the National Natural Science Foundation of China (21978202).

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Reviewers invited by journal
06 Jul, 2023
Reviewers agreed at journal
29 Mar, 2023
Editor invited by journal
24 Mar, 2023
Editor assigned by journal
15 Mar, 2023
First submitted to journal
15 Mar, 2023

You are reading this latest preprint version

Energy harvesting of flow induced vibration enhanced by bionic non-smooth surfaces

Status:

Version 1

Abstract

Figures

1. Introduction

2. Design Concept And Experimental

2.1. Design concept

2.2. Experimental device and process

3. Results And Discussion

3.1. Load resistance analysis

3.2. Effect of spacing ratio on amplitude of UPEH and DPEH

3.3. Output voltage analysis

3.4. Evaluation of the output power enhancement of the proposed D-type bionic fin

3.5. Flow analysis

4. Conclusion

Declarations

References

Status:

Version 1