Modulation of vortex-induced free vibration by external forced motion: experimental insights into coupled self-induced and forced responses

This study investigates how imposed transverse forced vibration modifies vortex-induced vibration of an elastically mounted cylinder, with a focus on uncovering the nonlinear interplay between forced and self-excited oscillations. Through carefully designed experiments spanning wide ranges of frequency and amplitude ratios under low and high mass ratios, three distinct response regimes are identified. In the dual-frequency regime, occurring at extreme frequency ratios, weak coupling allows coexistence of forced and natural frequencies, yielding alternating large/small amplitudes and wake transitions between two vortex pairs per cycle (2P) and two single vortices per cycle (2S) shedding. The frequency-switching regime, near resonance, features amplitude modulation and intermittent dominance of each excitation source, producing mixed-mode wakes. In the resonant regime, frequency and amplitude matching lead to complete synchronisation, sinusoidal motion and intensified, periodic 2P shedding with elongated shear layers. Crucially, comparative analysis reveals that the structural mass ratio fundamentally governs the regime boundaries and transient dynamics, noticeably compressing the transitional frequency-switching zone. Energy transfer and added mass coefficients reveal enhanced dissipation and inertial modifications near resonance. The observed nonlinear interactions challenge assumptions of linear superposition and offer new insight into coupled vibration control. These findings provide a foundation for designing structures that harness or mitigate flow-induced vibrations in marine, energy and fluid–structure systems.