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The WEPTOS is a well-known wave energy converter (www.weptos.com), formed by several rotors, with a shape that draws upon the reputable Salters Duck geometry. The WEPTOS has a large efficiency under waves of one particular frequency, i.e. when resonance conditions occur. In order to extend the range of resonance conditions, the possible use of a negative stiffness in the rotor system is analysed. This note presents some considerations on the effectiveness of negative stiffness based on physical model experiments and on the numerical simulations used to interpret the test results. More specifically, the aim of the work is to evaluate how the WEPTOS performance is affected by the presence of a spring into the system that enhances the rotor oscillations applying a negative (un-stabilizing) torque proportional to the degree of rotation. Experiments were carried out in the deepwater wave basin of Aalborg University, on a 1:30 scale model of one rotor of the WEPTOS. Three different configurations were analysed, providing different values of the negative stiffness. A set of 16 regular and 5 irregular wave conditions were tested, with maximum heights up to approximately 6 m at prototype scale, periods ranging from 5 to 9 s. The incident wave characteristics, the device rotations and the power dissipated by a dummy power converter were accurately measured. The effect of the negative stiffness was partly hindered by the presence of friction, so that no conclusive evidence could be drawn. A simple numerical model, where the rotor was interpreted as an oscillating mass-damper- spring system, was set up and calibrated to the experimental results. Reasonably good agreement between predictions and measurements were found at model scale. The numerical simulations at prototype dimensions, where friction effects have a relatively lower importance, revealed that the springs responsible of the negative stiffness increase in fact the efficiency of the device.

This paper describes 2-D numerical simulations of velocity and pressure fields generated by non-breaking waves on a vertical breakwater with a recurved parapet wall. The influence of the geometrical characteristics of the parapet is investigated. An impulsive pressure force is identified and discussed with respect to the pure vertical wall case. This force is generated by the seaward flow confinement induced by the surging wave crest. We refer to this impulsive impact as “confined-crest impact”. A large part of the vertical wall is affected by an impulsive increase in pressure caused by pulsating wave, compared to the case where the parapet is completely vertical. The maximum values of the impulsive pressures are localized under the recurved parapet. The total force increase on the entire structure may be significant when compared to the pure vertical wall case.

The general aim of this paper is to examine the effect of floating breakwater layouts on wave transmission, loads along moorings and connectors, under oblique waves. The specific contribution of this work is to provide novel and accurate experimental results for a configuration that is widely adopted in existing prototypes. Tests were carried out in the wave basin (3.8 m×20.6 m×0.8 m) of the Maritime Laboratory of the University of Padova, Italy. Two layouts, characterised by different degrees of complexity (I- and J-shaped), and three obliquities (0∘,30∘,60∘) were examined. With increased wave obliquity, the floating breakwater becomes more efficient thanks to the decrease of wave transmission and of the forces along moorings and connectors. Under perpendicular waves, with increasing layout complexity, no significant effect on wave transmission is observed, whereas mooring and intermodule connector forces significantly change, particularly in the case of frequent overtopping.

One of the greatest challenges of coastal engineering today is the need for coastal protection in the changing climate scenario. Places which are nowadays protected will demand upgraded defences and more sites will require security; in all cases a large amount of resources will be needed to ensure beach maintenance and coastal safety. This may be an opportunity for the multi-purpose use of Wave Energy Converters (WECs) if the foreseen increase of energy demand in coastal areas is also considered. In this paper a group of WECs based on different operating concepts is numerically tested in front of two beaches, i.e. the Bay of Santander in Spain and Las Glorias beach in Mexico, representing two different case studies where the long-shore sediment transport is dominant. The hydrodynamics induced by these devices is represented by means of a 2D elliptic modified mild-slope model that is calibrated against new experimental results. The wave field is then used as input for the analytical calculation of the long-shore sediment transport and the coastline trend is estimated by applying the continuity of sediment equation. The characteristics of the selected numerical models give this work a first approach level. All the devices were found to produce a positive trend (accretion) at least in small areas. Recommendations are given to facilitate the selection of the device and the design of the farm layout for shore protection purpose.