A comparative study of dual cylinders and triangle bluff bodies for piezoelectric energy harvesting

The flow patterns behind tandem bluff bodies can be used to generate electricity in piezoelectric energy harvesters. The vortices and wakes that form behind the bluff bodies create a pressure differential, which can be used to deform a piezoelectric film. In this study, we investigated the performance of dual triangle and dual cylinder bluff bodies in tandem at varying Reynolds numbers, Re, and spacing ratios, D. We compared the flow patterns behind the two types of bluff bodies. Sixteen hot wire anemometers were placed at different locations to measure the velocity developed behind the dual bluff bodies in tandem. The results showed that the velocities behind the cylinder bluff bodies were initially higher than those behind the triangle bluff bodies at lower Re. This is because the cylinder bluff bodies create a more turbulent flow, which results in higher velocities at lower Re. The best distance between the two bluff bodies was 3D and 5D, where the output velocities were maximized at more than 12ms−1. However, for dual triangle, the velocities eventually became higher than those behind the cylinder bluff bodies at higher Re and lower separation ratios (1D and 2D). 3D was the best distance for triangle to produce a higher velocity pattern, and this was best observed when Re = 10k, which is the lowest inlet velocity set. The results of the experiments are expected to show that the dual triangle bluff bodies produce higher velocities than the dual triangle bluff bodies, which will lead to a higher amount of energy being harvested. The results show that the amount of energy harvested were increase with increasing Re and decreasing D. The information enhancement can be done with turbulence analysis which could lead to the development of more efficient and versatile piezoelectric energy harvester.


Introduction
The world is currently facing an energy crisis, with finite and unsustainable energy sources causing environmental damage.To address this challenge, there is a need to explore new and sustainable energy solutions [1].Piezoelectric energy harvesters offer a promising technology that can help mitigate the energy crisis.These devices convert mechanical energy into electrical energy and can harness various energy sources like wind, vibrations, and pressure [2].
Piezoelectric energy harvesters have several advantages, such as their small size, lightweight nature, and affordability.They are also environmentally friendly, emitting no harmful emissions.These characteristics make them suitable for various applications, including remote sensing IOP Publishing doi:10.1088/1742-6596/2641/1/012015 2 [3], wearable electronics [4], and infrastructure monitoring [5].They can power remote sensors, enabling environmental data collection, facilitate the use of wearable devices for health monitoring, and enhance infrastructure safety by monitoring for signs of damage.
The objective of this study is to investigate the effects of multiple bluff bodies [6], specifically triangle and cylindrical shapes, arranged in a tandem configuration, on the efficiency of wind turbines.While previous studies have examined the impact of a single bluff body [7,8], the influence of multiple bluff bodies in a tandem arrangement remains unexplored.This paper aims to study the velocity formation behind two tandem triangle and cylindrical bluff bodies and analyze the differences between these two configurations.The study will compare the flow characteristics, including velocity and turbulence, behind the bluff bodies.Experimental measurements will be conducted in a wind tunnel, simulating real-world scenarios.The focus will be on observing the speed limit within both the triangle and cylindrical bluff body configurations placed tandem.
The previous study highlights the significance of optimizing the design of wind turbines to maximize their efficiency.Previous studies have shown the effectiveness of triangle and cylindrical bluff bodies in improving piezoelectric energy harvesting.Additionally, the wake region behind bluff bodies has been found to generate turbulence and pressure differences, crucial for energy harvesting.Factors such as bluff body shape, spacing, and Reynolds number impact the power output [7,8].
While existing study provides valuable insights, there are still gaps in knowledge, particularly regarding the effects of spacing, Re, and bluff body shapes on flow behavior.This research aims to fill these gaps by investigating the power output of dual triangle and cylindrical bluff bodies in tandem arrangement with different Reynolds numbers and spacing ratios.The selection of the optimal position for the piezoelectric film is also crucial to maximize system effectiveness [7,8,9].By addressing these research objectives, this study will providing insights into the flow characteristics and power performance of tandem bluff bodies.The findings will aid in the design and optimization of wind turbines, enhancing their efficiency and cost-effectiveness in harnessing wind energy [10].

Experimental setup
The velocity formation behind two tandem bluff bodies, a cylinder and a triangle shape, was studied in the wind tunnel laboratory at UKM.The bluff bodies were constructed of laminated wood with specific dimensions: 0.1 m in diameter and 0.5 m in height for the cylinder, and two sides measuring 0.11 m and a bottom measuring 0.1 m with a height of 0.5 m for the triangle shape (see Figure 1).The D distance between the bluff bodies as shown in Figure 2 was varies from 1D, 2D, 3D, 4D, and 5D. Figure 2 also locate the hot wire measurement location behind the second bluff body was set to be 4x4 where four locations in stream-wise (x-axis) direction (0 mm, 40 mm, 80 mm, and 120 mm) and four span-wise (y-axis)direction (0, 12.5mm, 25mm, and 37.5mm).The hot wire anemometers were positioned downstream of the bluff bodies in the wake region.The input velocities were varied, and the spacing between the bluff bodies was adjusted according to the desired spacing ratio.The hot wire anemometers, mounted on a 2-axis robotic arm which could move in span-wise and stream-wise direction.In escalating the measurement the hot wire is set to move automatically in four positions in span wise direction, however the measurement location in stream wise is set manually.The velocities measured by the hot wire anemometers were recorded for 20 seconds where the velocity is let to flow for another 30 seconds to achieve stable air flow.This comprehensive data collection and analysis process aimed to provide insights into the differences in flow characteristics between the dual triangle and cylinder bluff bodies, specifically focusing on velocity, turbulence, and wind speed.The comparison between the graphs is then discussed in terms of the position with the highest velocity pattern.These results provide insights into the optimal location to place a piezoelectric film for energy harvesting.
The discussion of the triangle can be divided into two parts.First, the results for the triangle with 3D are presented.These results are shown in Figure 3 for Re from 10k to 60k.The flow pattern for the triangle can be seen to develop with low velocity at the center of the triangle.As the flow moves toward the fourth row in the y-axis, the velocity increases to a maximum.However, the flow pattern seem to flatten with the increase of Re.The highest velocity for dual triangles in tandem occurs when the separation distance is 3D.The maximum velocity is 6 ms −1 , as shown in Figure 3(a).At a separation distance of 3D, the highest velocity develops close to the triangular flat surface.However, when the distance between the triangles increases, the highest velocity occurs further behind, such as at (120 mm, 37.5 mm).Thus, the edges of the triangles are found to have concentrated and created higher velocity far from the triangular wake region.By increasing the Re, the flow circulation develops a longer tail at the downstream bluff body due to the distance between the dual bluff bodies (3D) for the triangular shape [11].The second part of the study investigated the velocity when the separation distance between the triangles was between 1D and 2D at the highest Re of 50k and 60k, as shown in Figure 4.With the same Re and D, the velocity was measured to be higher than the dual cylinder in tandem.However, instead of the higher velocity occurring close to the triangular flat surface, the position was shifted further behind the second triangle, in this case to (120 mm, 37.5 mm).
The tail of the air flow was extended further due to the increase in Re.Further investigation can be done on the vortex shedding area of the triangle, which may be caused by the tip shape, leading to a larger vortex area behind the second triangle bluff body.Figure 5 shows the result at 3D, the highest output velocities for cylinder .Figure 5(a) shows a velocity of 10 ms −1 at the first row in the y-axis.The velocity then increases to 13 ms −1 at the second and third rows.By increasing the Re to 20k resulted in maximum velocities of 12ms −1 for 3D (see Figure 5(b)).The velocity distribution for 3D is now higher in the middle of the cylinder and decreases to zero at the fourth row in the y-axis.The velocity pattern are repeated for Re equal to 30k and 40k but with lower velocity value of 6.7 ms −1 and 5.3 ms −1 respectively, as the Re increase.The velocity flow pattern is scattered when the Re is equal higher then 40k as shown in Figure 5(e) and Figure 5(f).
In figure 6 the velocities behind the cylinder bluff bodies at 5D spacing.The velocity pattern for 5D for all Re were generating the same pattern where the velocity value is low at the center and increase when moving to the side wall.however, the maximum velocity is found to decrease as the inlet velocity increase.Initially, the velocity decreased by half when the the Re increase to 20k as shown in Figure 6(b) and the value keep decreasing slowly as the Re increase as can be seen in Figures 6(c

Conclusion
The results of the study show that the velocity distributions behind cylinder and triangle tandem bluff bodies with different spacings are significantly different.The cylinder bluff bodies generally produce higher velocities than the triangle bluff bodies at lower Re (below 20k).However, as the Re increases (starting from 30k), the velocities behind the triangle bluff bodies become higher than those behind the cylinder bluff bodies.Compare to the study of [12], this result proved that the curved-surface bluff bodies are not fully performance best in all time.This paper also found that the difference between the cylinder and triangle tandem bluff bodies is more pronounced at lower Re.This is because the flow becomes more turbulent at higher Re, and the sharp edges of the triangle bluff bodies create more turbulence than the smooth surface of the cylinder bluff bodies.The turbulence in the flow behind the triangle bluff The study findings can be used to design tandem bluff bodies for specific applications.For example, if an application requires high velocities at low Re, then cylinder tandem bluff bodies with a spacing of 1D may be the best choice.However, if an application requires high velocities at high Re, then triangle tandem bluff bodies with 2D may be the best choice.Overall, the results suggest that the dual cylinders with 3D is more effective at generating output velocity at higher input velocities.However, the dual cylinder with 5D may be more effective at generating output velocities at lower input velocities.
To meet the optional configurations for minimizing interface and maximizing performance, dual cylinders in tandem with 3D and 5D may be the best position to place the piezoelectric film, while the for triangle it is more suitable to use at higher Re with low separation ratio.

8th
International Conference on Man Machine Systems 2023 Journal of Physics: Conference Series 2641 (2023) 012015

Figure 1 :
Figure 1: (a) Dual bluff bodies in tandem (b) Measurement area

2 Figure 4 :
Figure 4: Velocity profile of triangle dual bluff bodies Figure5shows the result at 3D, the highest output velocities for cylinder .Figure5(a)shows a velocity of 10 ms −1 at the first row in the y-axis.The velocity then increases to 13 ms −1 at the second and third rows.By increasing the Re to 20k resulted in maximum velocities of 12ms −1 for 3D (see Figure5(b)).The velocity distribution for 3D is now higher in the middle of the cylinder and decreases to zero at the fourth row in the y-axis.The velocity pattern are repeated for Re equal to 30k and 40k but with lower velocity value of 6.7 ms −1 and 5.3 ms −1 respectively, as the Re increase.The velocity flow pattern is scattered when the Re is equal higher then 40k as shown in Figure5(e) and Figure5(f).In figure6the velocities behind the cylinder bluff bodies at 5D spacing.The velocity pattern for 5D for all Re were generating the same pattern where the velocity value is low at the center and increase when moving to the side wall.however, the maximum velocity is found to decrease as the inlet velocity increase.Initially, the velocity decreased by half when the the Re increase to 20k as shown in Figure6(b) and the value keep decreasing slowly as the Re increase as can be seen in Figures 6(c),6(d), 6(e) and 6(f).