Vortex-induced vibration of a low-moment of inertia-ratio pivoted cylinder: Experiments and reduced-order modeling

Vortex-induced vibration of pivoted cylinders with swinging motion is dynamically distinct from the classical translational configuration, particularly in pendulum-type systems where rotational inertia, gravity, and buoyancy jointly affect the restoring characteristics. In this study, a low-moment of inertia-ratio pendulum-type pivoted cylinder is investigated through a combined experimental and analytical approach. A series of water-tunnel experiments are conducted to examine the effects of moment of inertia ratio, rotational stiffness, cylinder length, pendulum length, and added-mass position on the vibration response. A semi-empirical reduced-order rotational wake-oscillator model based on the Van der Pol formulation is established, and its empirical coefficients are identified from experimental data. An approximate analytical expression for the amplitude response is derived using the multiple-scales method. The calibrated model reproduces the main response trends observed in the experiments and is used to interpret the influence of the governing parameters. The results show that raising the position of the added mass broadens the lock-in region, increases the peak amplitude, and lowers the onset velocity of lock-in. Within the present model, these trends are consistent with a stronger softening tendency associated with the reduced restoring moment. In addition, approximate relationships are obtained between the Scruton number and the peak amplitude, and between the moment of inertia ratio and the lock-in bandwidth. These results provide an experimentally supported reduced-order model for describing the vortex-induced vibration of low-moment of inertia-ratio pivoted cylinders.