Fluid-structure interaction of a two-degree-of-freedom flat plate in the vicinity of the free surface*

Fluid-structure-surface interaction response of a flexibly mounted rigid flat plate near a free surface was studied through a set of water tunnel experiments combined with time-resolved volumetric quantitative flow measurements to understand the interaction between the flow, free surface and the structure. The plate had two degrees of freedom (DoF) in the pitching and plunging directions. The effects of proximity of the plate to the free surface of the water on the flow-induced instabilities of the plate were investigated for cases ranging from when the flat plate was fully submerged to when its centerline was parallel to the water's free surface. For fully submerged cases at both 1DoF in the pitching direction and 2DoF, the plate experienced divergence instability at lower reduced velocities that later transitioned to periodic limit cycle oscillations at higher reduced velocities. The limit cycle amplitudes of oscillations increased by increasing the flow velocity. As the submerged depth decreased, the limit cycle amplitudes of oscillations decreased, and the onset of oscillation was shifted towards higher reduced velocities. For the 2DOF system in the vicinity of the water surface the asymmetry in the flow field around the plate led to only divergence instability of the plate.