Quantitative analysis of energy harvesting characteristics for 3D side-by-side double cylinders based on entropy production theory

Vortex-induced vibration energy harvesting from low-velocity water flows is typically achieved using systems composed of two or more cylinders. This study performed three-dimensional numerical simulations on side-by-side dual cylinders undergoing transverse vibration at Reynolds numbers ranging from 2.79 × 104 to 1.68 × 105. The vortex-induced vibration responses were investigated for four side-by-side configurations with spacing ratio (S/D) of 2, 2.5, 3, and 4, a mass ratio (m∗) of 2.4, and a mass-damping ratio (m∗ζ) of 0.013, across a reduced velocity (Ur) range of 2–12. The results demonstrate that, within the spacing ratios S = 2 and 3D and Ur = 4–6, side-by-side dual cylinders exhibit significantly enhanced energy harvesting from water flows compared to a single cylinder. Notably, at Ur = 6, the maximum harvested power and efficiency reach 1.154 and 1.44 times those of a single cylinder, respectively. Under side-by-side configurations, it was observed that anti-phase correlation during the lock-in regime can markedly improve both the power output and efficiency of the energy-harvesting cylinders. A correspondence between distinct wake modes and energy capture performance was established by comparing the evolution of vortex dynamics and the energy harvesting results across different spacing ratios. Furthermore, the relationship between vortex evolution and entropy generation was examined, revealing that the entropy production rate caused by turbulence dissipation is the primary source of entropy production, consistent with the quantitative analysis. The local entropy production rate caused by wall shear stress is larger in vortex formation and smaller in the process of vortex shedding; this variation reflects the intensity of fluid–structure interaction. Finally, the collision situation in the side-by-side arrangement with a small spacing ratio is discussed.