This study numerically investigates two-dimensional incompressible flows over elastically mounted foils, undergoing semi-passive and fully passive motion. In our strongly coupled numerical models, we employ linear and cubic stiffness and damping terms in order to examine their highly nonlinear response. The undamped model of the fully passive system exhibits various responses from periodic to chaotic and then to flip-over for the reduced velocity, ranging from 1 to 10. However, introducing the cubic damping terms causes a significant decrease in the magnitude of plunging and pitching amplitudes without affecting the onset point of bifurcation. Also, plunging and pitching amplitudes are altered significantly after the point of onset. Furthermore, the performance metrics of each passive system are computed for power generation applications to demonstrate that semi-passive system attain efficiency up to 20% for a pitching amplitude of 50 with the excitation frequency in the narrow range of 0.15 to 0.20. On the other hand for a fully passive system, the efficiency of around 34% is obtained near the onset point of a bifurcation with a low mass ratio and linear damping terms. However, introducing cubic damping terms causes degradation in efficiency to bring it down to 14 - 20% for a wide range of reduced velocity.
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