A hybrid experimental investigation of structural parameter effects on 2-DOF vortex-induced vibrations of a circular cylinder

This study develops a two-degree-of-freedom (2-DOF) virtual physical framework (VPF) to investigate structural parameter effects on 2-DOF vortex-induced vibrations (VIV) of a circular cylinder. The framework ensures strictly planar vibrations with equal mass in both directions, while allowing flexible modification of m ∗ , damping ratio ζ , and natural frequency ratio f n ∗ under constant-Reynolds-number conditions. Results show that 2-DOF VIV exhibits a super-upper branch where the cross-flow (CF) amplitude is amplified compared with the 1-DOF case, and this amplification weakens as the mass ratio increases. At the same time, the in-line (IL) amplitude, synchronization range, and dominant frequency are reduced with increasing mass ratio. The phase difference increases gradually with reduced velocity ( U R ) and reaches its maximum at the CF amplitude peak. This maximum phase difference decreases as the mass ratio increases, and when U R further enters the lower branch, the phase difference drops to about 10°. Correspondingly, the trajectory evolves from right-opening figure-of-eight shapes to nearly straight CF-dominated lines as the IL response diminishes. The natural frequency ratio further alters the vibration features. At f n ∗ = 0 . 5 , both CF and IL vibrations are suppressed, whereas at f n ∗ = 2 . 0 , dual resonance occurs with amplified IL response, reversed asymmetric trajectories, and a pronounced third-harmonic component in lift force. Structural damping reduces both CF and IL amplitudes but has little effect on the dominant frequency. These findings provide necessary corrections to conclusions drawn from 1-DOF VIV and offer structural parameter references for engineering design. • Developed a hybrid framework for 2-DOF VIV tests with tunable structural parameters. • Conducted tests revealing the effects of structural parameters on 2-DOF VIV behaviors. • Identified the “dual resonance” at f n ∗ = 2.0 featured by third-harmonic lift forces.