Assessing the efficiency of a distensible-tube wave attenuator at three different scales considering aero and hydro-elastic effects

Abstract

A water-filled floating distensible tube connected to a forward-bent oscillating water column is here considered in the context of harnessing energy from sea waves. Based on wave-tank experiments conducted at three different scales, under intermediate and deep-water incident regular waves of small and finite amplitudes, the present study addresses important aspects that determine the energy capture-width of the system. The comparison between the results obtained at the three scales reveals non-linear and aero-hydroelastic scale effects inherent in modelling the device. The following issues are also tackled: (i) the occurrence of various working modes of the floating tube and their contribution to its performance; (ii) the compressibility of the airflow through the orifice power take-off system; (iii) the viscous-effects associated with the Reynolds and Keulegan-Carpenter numbers. The parameters that govern the system's efficiency are the tube's length, the OWC natural period and the bulge wave tuning period. The system becomes very efficient not only at the OWC resonance, but also when the incident wavelength is equal to or twice the tube's length. Video frames further provide evidence of the relationship between the tube's working modes and its performance. An iterative algorithm based on Buckingham's formula, with an empirical expansibility factor, provides adequate estimates of the compressible flow through the power take-off. Viscous losses closely related to the Reynolds and Keulegan-Carpenter numbers are found to be important even for waves with moderate steepness. Finally, the experiments suggest that a single distensible-tube system in open sea has a maximum capture-width of no more than 2 tube-diameters.