On Floating Breakwaters Efficiency - a 2DV Parametric Based Analysis

In long term evolution numerical models, the interactions of a floating barrier with the wave field is then deputed to some parametrized transfer functions, which mimic wave energy transmission and dissipation in the frequency domain. This thesis provide, as final result, two transfer functions (one for incident waves, one for reflected ones) for a particular class of compact shaped floating breakwaters. These functions are based on three main parameters, which have been derived on physical model results. The first one (x) is the ratio between the incoming wave frequency and an approximation of FB heave natural frequency, based on principal FB cross section dimensions. Wave steepness has been considered to be the second variable which helps in describing the amount of dissipated energy. An FB draft to water depth ratio has been identified. Available algorithms for the decomposition into incident and reflected waves of flume records are mostly Stokes-FFT based. Therefore they suffer some limitations for relatively high wave steepness (Ch. 4). Since the latter is considered as a crucial parameter, a lot of effort has been drawn in solving some conundrums of actual methods. Two algorithms are proposed. The first one (Ch. 5), based on empirical mode decomposition, did not give satisfactory results. The second one (Ch. 6) is based on linear waves superposition, but, getting rid of linear dispersion relation, detects automatically each phase celerity. The proposed algorithm appears to be effective for relatively shallow water waves, for which the phase modulation approach is more consistent than Stokes formulations. A Stokes 2nd order algorithm has also been implemented. In Ch. 7 the experimental set up is presented. A second order analysis of transmission and reflection processes is also introduced.Results are given (and discussed) in Ch. 8. Linear transmission and reflection transfer functions are derived, based on experimental data fitting. These are finally validated with irregular wave test measurements.It is found that the transmission process mainly depends on frequency (x) and on FB relative draft. The last parameter does not enter the reflection process, which basically described by x and wave steepness. In particular, steeper waves loose more energy, and are less reflected. For transmitted waves only, a significant amount of energy transfer from primary to secondary harmonics is observed

Formula to Predict Transmission for pi-Type Floating Breakwaters

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Experimentally Based Model to Size the Geometry of a New OWC Device, with Reference to the Mediterranean Sea Wave Environment

This note presents the Seabreath wave energy converter, basically a multi-chamber floating oscillating water column device, and the lumped model used to size its chambers, the ducts and the turbine. The model is based on extensive testing carried out in the wave flume of the University of Padova using fixed and floating models with a dummy power take off and indirect measurement of the produced power. A map with the available energy in the Mediterranean Sea is also proposed, showing possible ideal application sites. The Seabreath is finally dimensioned for a quarter scale test application in the Adriatic Sea, with a 3 kW turbine, and a capacity factor of 40%

Attenuating surface gravity waves with an array of submerged resonators: an experimental study

Metamaterials and photonic/phononic crystals have been successfully developed in recent years to achieve advanced wave manipulation and control, both in electromagnetism and mechanics. However, the underlying concepts are yet to be fully applied to the field of fluid dynamics and water waves. Here, we present an example of the interaction of surface gravity waves with a mechanical metamaterial, i.e., periodic underwater oscillating resonators. In particular, we study a device composed of an array of periodic submerged harmonic oscillators whose objective is to absorb wave energy and dissipate it inside the fluid in the form of heat. The study is performed using a state-of-the-art direct numerical simulation of the Navier–Stokes equation in its two-dimensional form with free boundary and moving bodies. We use a volume of fluid interface technique for tracking the surface and an immersed boundary method for the fluid–structure interaction. We first study the interaction of a monochromatic wave with a single oscillator and then add up to four resonators coupled only fluid-mechanically. We study the efficiency of the device in terms of the total energy dissipation and find that by adding resonators, the dissipation increases in a nontrivial way. As expected, a large energy attenuation is achieved when the wave and resonators are characterized by similar frequencies. As the number of resonators is increased, the range of attenuated frequencies also increases. The concept and results presented herein are of relevance for coastal protection applications

Copernicus Marine Service Ocean State Report, Issue 5

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