Wake reorganization and energy exchange in strongly nonlinear vortex-induced vibrations

The combination of strong structural nonlinearity and flow-induced vibration can enhance energy harvesting efficiency while potentially triggering system instability; however, its specific mechanism remains obscure. Motivated by this, a sliding-type electromagnetic vortex-induced vibration harvester, designed for durable operation in flowing water by integrating nonlinear stiffness, frictional and fluid damping, and nonlinear electromechanical coupling, is investigated. A wake-oscillator model is used to capture the nonlinear features of the system and enable parametric studies; analyses based on coupled computational fluid dynamics/computational structural dynamics/electrical simulation elucidate the wake-mode reorganization and energy-transfer pathways, with water-tunnel tests with particle image velocimetry validating the simulations and revealing key flow features. Differences among linear, nonlinear hardening, and bistable configurations are observed and elucidated in terms of frequency lock-in, amplitude changes, and power output. The nonlinear hardening case exhibits widened bandwidth and increased overall output. In the bistable configuration, a barrier-crossing amplitude reset-and-build-up (BC-ARB) phenomenon is identified. The BC-ARB phenomenon features a delayed and stretched roll-up of shear layer, which reverses the lift-velocity phase relative to structural velocity, creating a negative-work interval that rapidly resets the vibration amplitude; the wake then shifts to a 2S (two single vortices per cycle) mode alongside a synchrony phase return and amplitude rebuilding within the potential well. The phenomenon is found to be sensitive to the mass ratio and, alongside chaos, reduces the harvested power in the bistable configuration. This work elucidates how structural nonlinearity reshapes wake modes and energy transfer, informing parameter optimization for hydrodynamic energy harvester design and vibration control.