Energy harvesting from flow-induced vibration of a low-mass square cylinder with different incidence angles

This numerical study investigates the flow-induced vibration responses and energy harvesting characteristics of a low-mass square oscillator. We first test three typical incidence angles of α = 0°, 22.5°, and 45° with reduced velocities Ur ranging from 3.8 to 26. The most interesting phenomenon is that large-amplitude vibrations can be generated at high reduced velocities, regardless of the angle α. We show that this is because of the following mechanisms: (i) For α = 0°, galloping occurs, resulting in high-amplitude and low-frequency vibrations; (ii) for α = 45°, the cylinder undergoes vortex-induced vibrations (VIVs) without the high-amplitude galloping instability. The unsteady vortex shedding effects are enhanced by a very low mass ratio, leading to "VIV forever"in the tested range of Ur with high-level amplitudes; and (iii) for α = 22.5°, the oscillations in the high-Ur range include both VIV and galloping components. Thus, the large amplitude is caused by the galloping instability and enhanced vortex-shedding effects. Due to the existence of large-amplitude vibrations, the low-mass square cylinder demonstrates the potential and necessary robustness for energy harvesting applications. Overall, α = 45° is the most suitable arrangement for the conversion of power. To further improve the efficiency, we test a 45° cylinder under damping ratios ζ ranging from 0.01 to 0.7. The results indicate that the energy harvesting characteristics are sensitive to the damping ratio when ζ < 0.3. Of all the tested cases, ζ = 0.7 provides the highest average efficiency.