Abstract
The Mocean wave energy converter consists of two sections, hinged at a central location, allowing the device to convert energy from the relative pitching motion of the sections. In a simplified form, the scattering problem for the device can be modelled as monochromatic waves incident upon a thin, inclined, surface-piercing plate of length L' in a finite depth d' of water. In this paper, the flow past such a plate is solved using a Boundary Element Method (BEM) and Computational Fluid Dynamics (CFD). While the BEM solution is based on linear potential flow theory, CFD directly solves the Navier–Stokes equations. Problems of this type are known to exhibit near-perfect reflection (indicated by a reflection coefficient |R|approx 1) of waves at specific wavenumbers k'. In this paper, we show that the resonant motion of the fluid induces large hydrodynamic forces on the plate. Furthermore, we argue that this low-frequency resonance resembles Helmholtz resonance, and that Mocean’s device being able to tune to these low frequencies does not act like an attenuator. For the case where the water is deep (d'>lambda '/2, where lambda '=2pi /k'), we find excellent agreement between our simulations and previous semi-analytical studies on the value of the resonant wave periods in deep water. We also find excellent agreement between the excitation forces on the plate computed using the BEM model, analytical results, and CFD for large inclination angles (alpha > 45^circ ). For alpha le 15^circ , both methods show the same trend, but the CFD predicts a significantly smaller peak in the excitation force compared with BEM, which we attribute to non-linear effects such as the non-linear Froude–Krylov force
Highlights
In recent years, there has been a renewed interest in the R&D of wave energy conversion technologies in the hope of commercialisation of the technology
The resulting fluid motion inside the wave channels produces a resonant peak in the water surface motion and the corresponding hydrodynamic forces, which is beneficial to power capture
There, we found that the peak in measured values of the hydrodynamic force peaked at lower values of k L than that predicted by numerical models that relied on linear potential flow theory
Summary
There has been a renewed interest in the R&D of wave energy conversion technologies in the hope of commercialisation of the technology. Sloshing-induced resonant behaviour has been observed to occur for a different class of WEC known as an oscillating wave surge converter (Renzi and Dias 2012) If they were present, we would expect sloshing modes to occur at incident wavelengths comparable to the characteristic width of the wave channel. For small (but non-zero) α, the reflection coefficient for the deep-water problem exhibits a ‘spiky’ behaviour (Parsons and Martin 1994); indicating that at certain frequencies, the plate completely reflects the incoming waves, and with an overall trend of increasing reflection as k L increases The authors attribute this effect to a so-called quasi-resonance between the wedge of fluid trapped above the plate and the incoming waves (Parsons and Martin 1994). The similarities are best understood in analogy to the wellstudied topic of resonance in harbours
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