Fluid-Structure-Surface Interactions of a Plunging Plate Near a Free Surface*

This experimental study investigated fluid-structure-surface interactions of a flexibly mounted rigid flat plate oscillating in the plunging direction in axial flow. Using water tunnel experiments and time-resolved particle image velocimetry (PIV), the effects of proximity to the free surface on the plate's flow-induced vibration response were analyzed. Various Reynolds numbers and angles of attack were tested across different submerged heights. Results show that at deeper submergence, increasing the angle of attack intensifies fluid-plate interaction, leading to distinct leading-edge and trailing-edge vortex shedding (LEV and TEV). However, this interaction does not induce oscillations. Near the free surface, distinct flow regimes characterized by vortex shedding and the Coanda effect emerged, varying with angles of attack and flow velocities. Notably, limit-cycle oscillations (LCO) occurred within specific ranges of angles of attack and flow velocity due to the rebounded LEV from the free surface, causing surface deformation and interaction with the plate. The generated jet-like flow and immediate vortex coupling between LEV and free surface vortices result in a momentum surplus in the wake of the oscillating plate.