Wave Attenuation by Submerged Oscillating Plates

Submerged oscillating plates are used as heaving plates to reduce the motion of floating objects, in wave energy devices to extract the wave energy, and as break waters to attenuate the wave field in shallow water. In this study ,we consider a horizontal, submerged plate in shallow water that is allowed to oscillate in the vertical direction due to the wave loads. The plate is attached to a linear spring and damper to control the oscillations. The focus of this study is on the transformation of the wave field by the submerged oscillating plate. To estimate the energy attenuation, wave reflection and transmission coefficients are determined from four wave gauges; two placed upwave and two placed downwave of the oscillating plate. The fluid is governed by the nonlinear Level I Green-Naghdi (GN)equations, coupled with the equations of vertical motion of the plate to determine its oscillations. Time series of water surface elevation recorded at gauges upwave and downwave of the plate, and the wave-induced plate oscillations, obtained by the GN model are compared with available laboratory experiments and other data, and very good agreement is observed. Wave reflection and transmission coefficients are then determined for a range of involved parameters, including wave condition (wave height and wave period), initial submergence depth of the plate, plate length, and the spring-damper system. It is found that a single submerged oscillating plate can have remarkable effect on the wave field, and that nonlinearity plays an important role in this wave-structure interaction problem. Discussion is provided on how the wave reflection and transmission vary with the wave condition, plate characteristic, initial submergence depth and spring-damper system.