Energy harvesting from vortex-induced vibrations using a pendulum with near-wake interference

This study experimentally investigates the energy-harvesting performance of a low mass-damping pendulum-type cylinder undergoing flow-induced oscillations in the wake of a stationary upstream cylinder. The downstream cylinder pivots about its upper end and is subjected to a controllable electromagnetic braking torque, enabling direct measurement of the mechanical power extracted from the oscillatory motion. Experiments were conducted in a water channel and cover a wide parameter space, including variations in reduced velocities (in the range from around 4 to 12), centre-to-centre separations (2, 3, 3.5 and 4 diameters) and different levels of applied braking torque (from minimal to the maximum that allows response to take place). From an energy-harvesting perspective, the smallest separation investigated provides the most favourable performance, with power coefficients reaching values of approximately 0.12 over a broad range of reduced velocities. Increasing the separation distance progressively reduces both the peak power coefficient and the extent of the harvesting region. These findings highlight the potential of wake interactions to broaden the operational bandwidth of pendulum-based flow energy harvesters.