Multi-triggering modes in transverse cylinder VIVs under combined wave-current flows at low KC numbers

Previous investigations on transverse vortex-induced vibrations (VIVs) of cylindrical structures have predominantly focused on either steady-current or waves alone, despite their frequent coexistence in ocean environments. To address this gap, this study experimentally investigates the transverse VIVs of an elastically mounted circular cylinder under combined wave-current flows. Utilizing a low-structural-damping device at low KC numbers (0.14 ≤ KC ≤ 5.72), the study identifies five distinct vibration stages based on wake frequency evolution, i.e. the wave-dominated forced vibration stage, wave-current subharmonic-lock stage, wave-current transition stage, current-dominated pre-synchronization stage, and current-dominated resonance stage. Three triggering modes—Resonance (Mode I), Harmonic-Lock (Mode II), and Subharmonic-Lock (Mode III)—are elucidated from the interactions of the wave frequency, the Strouhal frequency, and the natural frequency of the cylinder. A key finding is that the critical reduced velocity remains constant at 4.6 in quasi-perturbed flow but increases with the KC number under other conditions, leading to an established empirical relationship for the three modes.