Nonlinear time-domain NWT simulations for two types of a backward bent duct buoy (BBDB) compared with 2D wave-tank experiments

A fully nonlinear time-domain NWT-simulation tool has been developed with nonlinear viscous-damping and pneumatic pressure terms. The tool was applied to the performance evaluation of fixed/floating sharp-corner and round-corner BBDB WECs. Shape-induced and motion-induced viscous damping for the BBDB system was considered in the potential-flow-based simulation with quadratic pneumatic pressure and fully nonlinear free-surface conditions. Systematic experiments were also conducted in a two-dimensional wave tank to verify the NWT-simulation results. The numerical results were further verified by checking the total power conservation of the entire system. The nonlinear simulations were also compared with linear ones. Since the round-corner BBDB has less energy loss caused by body shape and motion, the available power of the round-corner BBDB is generally bigger than that of the sharp-corner BBDB. The phenomenon of negative drift motion for only the sharp-corner BBDB for a certain wave-length range was also reproduced by the fully nonlinear BBDB simulations. This numerical simulation tool can be used for the reliable design of prototype systems for a given site and environmental conditions.