In this paper, a two-dimensional numerical method in time domain to estimate drift force acting on a floating OWC-type wave energy converter "Backward Bent Duct Buoy (BBDB)" moored loosely by elastic lines is proposed. Primary conversion efficiency of this device is also calculated. The turbine load is replaced for the orifice load in order to simplify the problem. In analysis of the wave motion, vortex method which can consider fluid viscosity is used. Vorticity layer model and Core-Spreading method are used in order to calculate vorticity creation from body surface and diffusion of the vorticity into the fluid, respectively. For air flow in air chamber, an equation of state and conservation of mass and energy with assumption of air being the perfect gas are used. From these equations, motions of the floating body, water surface elevation and air pressure in the air chamber, primary conversion efficiency, mooring force and drift force etc. are calculated. Wave tank tests on BBDB in regular waves are also carried out and numerical results are compared with experimental results.
Recently, renewable energy has become of major interest in concerning depletion of petroleum and global warming in future. The utilization of the wave energy which is one of the renewable energy is also advanced in all over the world. Authors have been investigating a floating type wave energy converter (WEC) "Backward Bend Duct Buoy (BBDB)" proposed by Masuda (1986) (Nagata et al., 2006, Nagata et al., 2007, Toyota et al., 2008). The BBDB is an oscillating water column (OWC) type wave energy converter. Fig.1 shows the concept of the BBDB. The BBDB consists of an air chamber, an L- shaped duct, a buoyancy module, a turbine and a generator. This device has some advantages, that is, i) the primary conversion efficiency is higher than other floating OWCs, ii) as the wavelength in which primary conversion efficiency is maximum is about four times of the length of the BBDB, a long floating structure is not required, iii) as BBDB slowly advances against wave propagation direction in particular wave frequency band because of horizontal negative drift force, the mooring force and mooring cost are reduced in irregular waves. Authors carries out 2D-tank tests for the drift force acting on the BBDB in regular waves and tried to clarify characteristics and mechanism of generation of horizontal negative drift force. However, we did not come to clarify the cause of generation (Imai et al., 2009).
