Numerical study on the performance of a flapping foil power generator with a passively flapping flat plate

In this study, the flow characteristics of a flapping foil power generator with a passively flapping flat plate have been investigated using the lattice Boltzmann method (LBM) and immersed boundary method at Reynolds number of 1100 based on the chord length. The aim is to explore potential benefits of flow induced passive actuation of the flat plate tail to the power output and efficiency. The problem consists of a rigid NACA0015 foil undergoing prescribed pitch and plunge motions and a tail hinged to the rigid foil using a torsion spring. In this study, kinematic parameters of the flapping foil power generator including pitch and plunge amplitudes, reduced frequency, phase angle between the plunge and pitch motions and pivot location were chosen to be the optimal values of the rigid case suggested by the previous literature and the contours of efficiency computed by the LBM. Apart from the rigid tail case, different tail flexibilities, represented by the frequency ratio of the natural frequency of the structure to the flapping frequency, were considered. Three cases including the rigid tail are explored in detail through histories of lift force, moment and power coefficients and contours of pressure and vorticity. The performance of the flapping foil power generator with a very flexible tail (natural frequency < flapping frequency) degrades due to poor synchronization between aerodynamic loads and prescribed motions. However, an appropriate frequency ratio can enhance the average power coefficient and efficiency. The study provides illustrates about how the passive actuation of the tail affects the formation and transition of the trailing edge vortex and enhances the lift force near stroke reversal.