Free-Surface Proximity Effects on Flow-Induced Vibrations of a Flexible Circular Cylinder*

This research investigated the effect of free surface proximity on the flow-induced vibration (FIV) response of a flexible cylinder. Using water tunnel experiments, the study analyzed fluid-structure-surface interactions by varying the cylinder's submerged height. Digital image correlation (DIC) was employed to capture the dynamic response, focusing on the spanwise amplitude and frequency of oscillations. The system's response was examined across a wide range of flow velocities. Results indicated that for a fully submerged cylinder, increasing flow velocity led to transitions from low to high oscillation modes and the occurrence of lock-in regions for each excited mode. However, near the free surface, the system's response—regarding the onset of oscillations, excited modes, amplitudes, and frequencies—varied significantly, especially at higher reduced velocities where mode transitions occurred. Reduced submerged heights shifted the onset of instabilities to higher flow velocities, decreased oscillation amplitudes, and reduced the number of observed mode shapes across the tested flow velocity range.