During the optimization phase of a wave energy converter (WEC), it is essential to be able to rely on a model that is both fast and accurate. In this regard, Computational Fluid Dynamic (CFD) with Reynolds Averaged Navier–Stokes (RANS) approach is not suitable for optimization studies, given its computational cost, while methods based on potential theory are fast but not accurate enough. A good compromise can be found in boundary element methods (BEMs), based on potential theory, with the addition of non-linearities. This paper deals with the identification of viscous parameters to account for such non-linearities, based on CFD-Unsteady RANS (URANS) analysis. The work proposes two different methodologies to identify the viscous damping along the rotational degree of freedom (DOF) of pitch and roll: The first solely involves the outcomes of the CFD simulations, computing the viscous damping coefficients through the logarithmic decrement method, the second approach solves the Cummins’ equation of motion, via a Runge-Kutta scheme, selecting the damping coefficients that minimize the difference with CFD time series. The viscous damping is mostly linear for pitch and quadratic for roll, given the shape of the WEC analysed.

In this investigation, a comprehensive study was conducted on a U-shaped sloshing tank, based on reversing the classical treatment of such devices as motion stabilizers and using them instead to improve the performance of wave energy converters. The modeling encompasses a comparative analysis between a linear model and Computational Fluid Dynamics (CFD) simulations. The validation of the CFD methodology was rigorously executed via a series of experimental tests, subsequently enhancing the linear model. The refined linear model demonstrates a notable alignment with rigorously verified results, thus establishing itself as a reliable tool for advanced research, indicating promise for various applications. Furthermore, this novelty is addressed by simulating the integration of a U-tank device with a pitch-based wave energy converter, displaying a broadening of the operational bandwidth and a substantial performance improvement, raising the pitch motion of the floater to about 850% in correspondence with the new secondary peak over extended periods, effectively addressing previously identified limitations. This achievement contributes to the system’s practical relevance in marine energy conversion.

This paper investigates the dynamics of aU-shaped sloshing tank varying its geometrical properties. The U - tank is designed to be coupled and integrated within a floating Wave Energy Converter, generating a second peak in its dynamic and allowing a bandwidth widening or a response amplification. The geometrical and liquid-filling properties directly affect the dynamics according to analytic models until second-order effects arise, such as secondary sloshing in the free-surface place, for in-stance, due to broader reservoirs or the elbows becoming sharper for manufacturing purposes. In light of the above considerations, a high-fidelity simulation is performed to overcome all those effects that are not considered in the simplified models. The results show a performance decrease when the reservoirs become more extensive and the elbow sharper.

This paper describes the coupling of an oscillating U-shaped water tank, also called a U-tank, with a pitching floating Wave Energy Converter (WEC) to expand the response bandwidth. The performance of these energy converters strongly depends on their frequency response, and their resonance period is generally fixed once the geometric and inertial parameters of the system have been defined. The integration and appropriate control of dynamic inertial systems based on water ballast tanks enable slow tuning of the system's resonance with the incoming wave, maximizing energy extraction. The dynamic coupling of the hull with the water tank is then analyzed, and a passive control system is developed that acts on the air contained in the reservoirs of the U-tank by partitioning the volume within. Air expansion is then controlled by discretely adjusting the available, properly partitioned volume allowing the variation of the frequency response of the sloshing water tank and device. The resolution of the dynamics involves linear models based on the Boundary-Element-Method as far as hull hydrodynamics are concerned; a solution of the Euler equation describes the oscillating tank. Finally, the expansion and compression of the air contained in the reservoirs of the U-tank are assumed to be governed by a generic polytropic transformation law, and such a condition is linearized around the operating condition. The ISWEC device is adopted as a case study, and its energy harvesting working principle is based on the gyroscope technology. The results aim to confirm the ability to perform slow tuning of the device frequency response via regulation of the air volume of the sloshing water tanks.