Development of a nonlinear model for a latch-controlled point absorber system

A nonlinear model is developed to quantify the behavior of a latch-controlled point absorber (PA) system modulated by viscous forces in monochromatic waves. A scaled-down (1:2.2) heaving PA is selected as the base model to support the study with its output power being the estimated quantity of interest. The viscous effects are supplemented through the Morison drag force. A method to compute the damping coefficient from energy lost by the PA in free-decay test is suggested. The resultant hydrodynamic model is solved in the time domain using the fourth-order Runge-Kutta method. It is seen that the model can be operated in four modes: 1. Linear-Uncontrolled (L-UC) and 2. Nonlinear-Uncontrolled (NL-UC) 3. Linear-Controlled (L-C) and 4. Nonlinear-Controlled (NL-C). The optimization criterion for the nonlinear model is modified based on the frequency response from the NL-UC mode. Results indicate that linear-latching can over-predict the power capacity of the invested device by approximately 243%. Further, power efficiency calculations from NL-C imply that latching is effective only at low Keulegan-Carpenter (KC) numbers; however, the gain in power is substantial (seven times higher power) than the uncontrolled case, especially at off-resonant states.