## 3 Results and Discussions
### Performance of New Omni-Directional Wind-Funnel Design
To achieve the optimum wind speed of 20 m/s at the venturi throat of the proposed omni-directional wind-funnel design, it is recommended to have the free-stream wind speed of 7.4 m/s collected at the inlet of the wind funnel. The speed ratio (SR) remains at 2.71 in this case. The velocity profile and pressure distribution results are shown in Figures 12 and 13. If the coupled bluff splitter body and PVEH plate are placed directly at a low free-stream wind speed of 7.4 m/s, it is noted from the electromechanical coupling FSI simulation that the maximum VIV amplitude at the tip of the PVEH plate achieves only 0.012 mm, and the generated voltage output is also insignificant, i.e., 0.22 V. Meanwhile, if the coupled bluff splitter body and PVEH plate are placed at the venturi throat of the proposed wind funnel, it is observed that the same low free-stream wind speed of 7.4 m/s could amplify the wind speed at the venturi throat to 20 m/s, and this is expected to produce the maximum VIV amplitude at the tip of the PVEH plate at 1.167 mm, which is equivalent to the open-circuit voltage output of 20.99 V at 80 Hz. The electromechanical coupling FSI simulation was repeated with a load-resistance condition. The value of the load resistance showed some effect on the vibrational characteristics of the PVEH, including the vibration amplitude and the resonant frequency [18]. The first resonant frequency has dropped to 76.79 Hz at adjusted optimal load resistance \(R_{opt\#76.79\mu Hz}=28\) k\(\Omega\). At 20 m/s wind speed, the vortex frequency of 76.79 Hz is created, and, thus, the maximum power output under this optimal load resistance is generated. It is noted that the VIV amplitude is recorded at 0.351 mm, and the maximum power output at wind speed of 20 m/s is 0.82 mW, with a voltage output of 4.96 V.  
Figure 11: Vortex frequency and lift force created under different wind speeds. It is noted that the 7.4 m/s wind speed is still considered high in Malaysia. By considering the wind condition in Malaysia, i.e., a maximum wind speed at 5.4 m/s, the coupled bluff splitter body and PVEH plate may need to be re-designed or fine-tuned to lower down its natural frequency, by either adding tip mass or adjusting the length of the PVEH plate, so it matches the vortex frequency created at the venturi throat at 14.45 m/s. Based on another electromechanical coupling FSI simulation conducted, when the natural frequency  
Figure 12: Velocity profile of the omni-directional wind funnel with free-stream wind speed of 7.4 m/s. Figure 13: Pressure distribution of the omni-directional wind funnel with free-stream wind speed of 7.4 m/s. of the coupled bluff splitter body and PVEH plate is lowered down to 56.73 Hz, by adding a tip mass of 1.25 g, it matches with the vortex frequency created at 56.71 Hz and, thus, produces the maximum VIV amplitude at the tip of the PVEH plate at 0.645 mm, equivalent to an open-circuit voltage output of 11.42 V. This shows a significant improvement of 323% by achieving resonance condition at 56.71 Hz through VIV, as compared with the original design of the coupled bluff splitter body and PVEH plate without a tip mass, which can only achieve approximately 0.15 mm of tip displacement and an open-circuit voltage output of 2.7 V, if it is placed under a 14.5 m/s wind speed, as shown in Figure 10.  
In summary, the maximum power output of the PVEH does not appear at the maximum vibrational displacement, as recorded during the open-circuit condition. The optimal load resistance value \(R_{opt}\) is computed based on the assumption that the vibration amplitude is independent of the externally connected load resistor. However, it is worth mentioning that under the electromechanical coupling FSI analysis, the load resistance value changes the fundamental resonant frequency and vibration amplitude, and this subsequently changes the optimal load resistance computation. Maximum power output could be obtained with the adjusted \(R_{opt}\) or through the impedance analysis.