## 3 Results and Discussions
### Effect of Cylinder Longitudinal Spacing on the Energy Harvesting Performance  
Maintaining the trailing edge cylinder diameter (\(D=3.0\%\) of c) and transverse spacing (\(X=0.5\%\) of c) constant whilst varying the longitudinal spacings (\(Y\)) at 0%, 0.5%, 1.0%, 2.0%, 4.0%, and 6.0% of the chord's length provides valuable insights into the influence of these parameters on the energy harvesting performance. As illustrated in Figure 12a, an increase in \(Y\) results in a diminishing rate of decrease in the mean total power coefficient and efficiency of the oscillating foil. Even with \(Y=6.0\%\) of c, the trailing edge cylinders slightly enhance the energy harvesting performance, emphasizing their persistent positive impact. Figure 11: Distribution of the pressure coefficient on the surface 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**) \(t/T=0.75\). Figure 12: Comparison of \(C_{\text{Pm}}\) & \(\eta\), \(C_{P}\), \(C_{\text{Fy}}\), \(C_{\text{M}}\), and \(C_{\text{P\theta}}\) of the oscillating foil with/without a cylindrical spoiler structure at the trailing edge for different cylinder longitudinal spacings: (**a**) Average total power coefficient and efficiency; (**b**) instantaneous total power coefficient; (**c**) instantaneous lift coefficient; (**d**) instantaneous heaving power coefficients; (**e**) instantaneous torque coefficient; and (**f**) instantaneous pitching power coefficients. Figure 12b illustrates the instantaneous total power coefficient of the oscillating foil for various longitudinal spacings. In the absence of cylinders or with zero longitudinal spacing, the curves exhibit consistent shapes in both half-cycles. However, the introduction of cylinders along the foil's central axis with nonzero longitudinal spacing disrupts this symmetry within each cycle, particularly affecting the peak and trough values. The most significant deviation occurs at 2% of c, enhancing the performance in the first half-cycle but significantly reducing it in the second half-cycle. Notably, only at zero longitudinal spacing do both half-cycles display improved performance, resulting in the best overall outcome. Generally, for an oscillating foil with a cylinder at the trailing edge, the enhancement in the energy harvesting performance during the first half-cycle outweighs the decline in the second half-cycle, resulting in a higher combined performance compared to a configuration without a cylinder. However, this improvement diminishes with increasing longitudinal spacing. Figure 12c,d present the instantaneous lift coefficient and the instantaneous heaving power coefficient separately. In the first half-cycle, the trough values of the lift coefficient are lower than those without cylinders, while only the peak values at \(Y=0\%\) of c and \(0.5\%\) of c surpass those without cylinders in the second half-cycle. The synergy between instantaneous lift and heaving velocity near their peak and trough values is reflected in the relationship with the instantaneous heaving power coefficients. The presence of the cylinder at the trailing edge enhances the average total power coefficient in the first half-cycle, significantly boosting the energy harvesting performance. However, in the second half-cycle, only zero longitudinal spacing improves performance, while most other spacings lead to its reduction. Consequently, as longitudinal spacing increases, the overall performance improvement weakens gradually. For each operating condition, two peaks and two troughs appear in the torque coefficient curves, while three peaks and three troughs are evident in the pitching power coefficient curves, as shown in Figure 12e,f. The primary differences between the curves occur around the peaks and troughs. The torque coefficients with a cylindrical foil are larger than those without at a \(t/T\) of 0.2 but are only the same for \(Y=0\%\) of c and \(0.5\%\) of c at a \(t/T\) of 0.7.