Hydrodynamic Evaluation of Multi-Layer Wave Filters and Absorbers

Abstract

Wave filters and absorbers are important coastal structures. Their efficiency depends on wave reflection and transmission as well as on energy dissipation. Because of their nonlinear characteristics, wave filters and absorbers must be analyzed and optimized experimentally. In a more general approach, two universally valid coefficients are determined for each individual filter element. Based on these data, arbitrary, multi-layer wave absorbers can be evaluated. Filter hydrodynamics mainly depend on porosity and wave kinematics, i.e., the profiles of horizontal velocity and acceleration. With reference to an initial wave, the related near-field velocity and acceleration at the filter element are expressed as functions of a drag and an inertia term. The corresponding coefficients, which are universally applicable, in connection with the relevant horizontal velocity and acceleration at the filter area are used to calculate the reflection, transmission, energy dissipation, and wave forces on arbitrary filter or absorber components, elements, and systems. The numerical analysis is validated by an experimental program in regular and irregular seas, and transient wave trains. Transient wave trains are efficiently used for: 1) Separate measurements of the initial wave group, as well as its reflection and transmission. Due to the short duration, the incoming wave train is easily separated from the reflected and the transmitted wave. This allows an accurate analysis of the associated flow fields and energies. For a variety of wave filters, the paper presents the evaluation of the foregoing characteristic coefficients, based on experiments with representative transient wave groups. 2) Model tests in extreme wave conditions (freak waves). Tests with breaking transient wave packets prove that the obtained maximum wave force on a structure is much higher than the long-term model test results in irregular waves, for a given spectrum.