The utilization of renewable energy is required immediately since emissions of carbon dioxide are being restricted. To this end, we are investigating the ocean wave energy converter, especially the floating OWC “Backward Bent Duct Buoy” (BBDB). The BBDB, proposed by Masuda in 1986, is a wave energy converter of the ‘moored floating oscillating water column’ type that is composed of an air chamber, an L-shaped bent duct, a buoyancy chamber, and a turbine. The BBDB has certain positive characteristics. Firstly, the primary conversion performance of the BBDB is better than other floating OWCs. Secondly, the length of the BBDB is shorter than other floating OWCs. Thirdly, as the BBDB advances in the incident wave direction with slow speed waves because of the negative wave drift force, the mooring cost can be reduced. In this research, experiments under a various wave periods were carried out to clarify the characteristics and cause of the generation of negative drift force acting on a BBDB in regular waves with a two-dimensional wave tank at Saga University. The length of the BBDB model is 85cm. To measure the wave drift force, the model is moored with horizontal wire-springs. The motions of the BBDB, such as surge, heave, and pitch, are measured by remotely using image processing. The fluid velocity around the BBDB is measured by using particle image velocimetry (PIV). Motion tests of the BBDB without mooring are also carried out to measure the horizontal velocity of the BBDB in waves. From the experimental results, the characteristics and causes of the generation of negative drift force acting on the BBDB in regular waves are discussed.

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