<p>This study addresses the challenge by introducing a piezoelectric energy harvester based on vortex-induced vibration (VIV) and galloping interactions. Experiments on an elastically mounted circular cylinder equipped with two small square rods (SSR) in a DN100 pipe were conducted to examine how the circumferential angle of the SSR impacts the vibration response of cylinder, revealing distinct interaction modes (VIV-only and VIV-galloping interaction). The results show that placing the SSR toward the bluff body’s trailing edge accelerates the onset of galloping at lower velocities. In particular, as the SSR angle is in the range of <i>θ</i> = 160°–180°, the fluid-structure interaction behavior deviates from prior open-flow studies. This difference is attributed to the influence of the pipe wall and is analyzed using the shear layer interaction mode theory. The relationship between SSR placement angles and fluid-induced vibration (FIV) characteristics across various fluid velocities was also mapped, with dynamic influences assessed using the Strouhal number and stability parameter Δ<i>S</i>, helping to distinguish between interaction modes. Based on these findings, configurations with <i>θ</i> = 50°–70° and <i>θ</i> = 140°–150° are identified as preferable for enhanced power output, whereas <i>θ</i> = 170°–180° is better suited for optimizing efficiency and stability. These results provide good insights into the design and optimization of pipeline energy harvesting systems for industrial applications.</p>