Active wave attenuation by a submerged heaving and pitching flat plate

Abstract

Protecting the coastline and structures in its vicinity are main tasks of coastal engineering. Here an environmental-friendly and cost-efficient active submerged flat plate (SFP) breakwater is proposed. It is investigated whether through introduction, phase and amplitude control of two motion modes, heaving and pitching, and in-situ adaptations of submergence depth and plate inclination in dependence on the incidence wave, wave attenuation can be improved and the bandwidth of appreciable performance can be widened. It is examined whether maximum wave attenuation results in energy loss of the system so that the energy can be harvested and used for motion control and structural adaptations. The wave/structure boundary value problem (BVP) is formulated in two dimensions (vertical) and covers monochromatic waves only, linear wave theory is applied; nonlinearities are introduced by applying the body-nonlinear method. Motion amplitude and incidence wave height restrictions are enforced to ensure that the assumptions for linear wave theory are not violated. Plate motion is introduced through prescribed forcing with incident wave frequency. The BVP is solved numerically using the radial basis function (RBF) collocation method. A novel lateral boundary condition is developed that allows wave incidence and radiation simultaneous. The RBF method proved to be able to model wave/structure interaction and plate motion; model calibration is required. Through the applied body-nonlinear method higher harmonics are introduced. Research results suggest that wave attenuation of the SFP is improved through above-stated measures and performance bandwidth widened. Motion phase angle control is paramount to ensure destructive interference between scattered and radiated wave on the lee side of the plate. Configurations with minimum wave transmission allow for energy harvesting.