## 3 Results and Discussions
### Effect of Cylinder Transverse Spacing on the Energy Harvesting Performance
The presence of the cylinder introduces disruptions to the fluid flow, generating a heightened pressure region on the cylinder's surface, as depicted in Figure 10. The dynamic interaction between the oscillating foil and the cylinder induces variations in pressure distribution and flow field. To delve further into the detailed dynamics, when the oscillating foil executes a downward motion, a distinctive elevated pressure region emerges on the upper side of the cylinder's leading edge. Conversely, during the upward motion, the high-pressure region shifts in the opposite direction. For instance, in the case of the oscillating foil with a cylindrical spoiler structure at \(X\) of 0.5% of c and 1.5% of c during \(t/T=0.25\), the trailing edge coincides with the elevated high-pressure region at the front of the cylinder. This alignment induces a disturbance that significantly disrupts the pressure equilibrium at the trailing edge point of the cylinder, presenting a departure from the scenario in the  
Figure 10: Pressure coefficient cloud and streamline of the oscillating foil with/without a cylindrical spoiler structure at the trailing edge for different cylinder transverse spacings. (**a**) \(t/T=0.25\); (**b**) \(t/T=0.75\). absence of a cylinder. The consequence is a localized increase in the pressure coefficient. This localized pressure variation is most pronounced in the pressure coefficient distribution observed at \(X\) of 1.5% of c.  
As the distance between the cylinder and the trailing edge increases, the impact of localized pressure alterations and their influence on the intricate flow field diminishes. This diminishing influence becomes notably marginal at a transverse distance of 65.5% of the chord length. Consequently, the pressure and streamline distribution approach that of the original foil. Therefore, at a transverse distance of 65.5% of c, the energy harvesting performance becomes comparable to that of the original foil. The distribution of the pressure coefficient on the surface of the oscillating foil, with f corresponding to a different \(X\), is displayed in Figure 11. This depiction illuminates the evolving pressure dynamics and their impact on lift force and energy harvesting performance. When \(X=0.5\%\) of c, the envelope of the pressure coefficient distribution is at its maximum, resulting in the largest pressure difference and lift force, hence displaying optimal energy harvesting performance. As \(X=1.5\%\) of c, the difference between the upper surface and the lower surface decreases, compared to 0.5% of c, indicating a reduction in energy harvesting performance. At 9.5% of c, the direction of the pressure difference on the trailing edge indicates is opposite of the leading edge's opposing direction, and the energy harvesting performance is further weakened, being the lowest among the compared conditions. Finally, when \(X\) is 65.5% of c, the pressure coefficient distribution curves of the upper and the lower surfaces nearly overlap, resulting in the pressure being almost equal on both sides, and the influence of the trailing edge cylinder reduced to the minimum, displaying similar energy harvesting characteristics as those of the original foil without the trailing edge cylinder.