Vortex-induced vibration of two circular cylinders in tandem near a free surface

This study numerically investigates the vortex-induced vibration of two rigidly coupled circular cylinders in tandem near a free surface. The effects of submergence depth, spacing distance, and Froude number on the vibration response and wake dynamics are systematically analyzed. Results show that proximity to the free surface significantly suppresses vibration amplitudes by up to 37% at a Froude number of 0.2 and a gap ratio of 0.5 compared to deeply submerged configurations. Shallow submergence also triggers earlier onset of lock-in and induces a distinct beating phenomenon, driven by spontaneous transitions between co-shedding and shear layer reattachment modes. At small spacing ratios, the system behaves like a single bluff body with weak hydrodynamic excitation, while larger spacing enables independent vortex shedding and downstream vortex merging, sustaining high-amplitude vibrations even beyond classical lock-in regimes. High Froude numbers induce significant free-surface distortions, including hydraulic jumps and wave breaking, which disrupt synchronization and give rise to dual-peak response profiles. A strong correlation is identified between structural vibrations and free-surface dynamics: near-field fluctuations are chaotic due to intense vortex interactions, while far-field waves evolve into organized periodic structures synchronized with the cylinder oscillation frequency.