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Abstract
An innovative concept of wind turbines, the flapping foil power generator that exploits dynamic stall, is numerically studied at Reynolds number of 1100. The combination of the kinematic parameters and the coupling between the foil deformation and aerodynamic loads are investigated to uncover the physical mechanism for high performance.
Firstly, the discrete vortex method (DVM) is improved to capture flow separations at the leading and trailing edges of the foil. Its results compare well with those of immersed boundary-lattice Boltzmann method (IB-LBM) and experiments. Its computational cost is at least two orders of magnitude less than that of the IB-LBM.
Then, kinematic parameters are optimized using a multi-fidelity evolutionary algorithm implemented with a dynamic stall model and the improved DVM. The results show that despite the use of low fidelity models and limited budget of computational resources, the multi-fidelity strategy is capable of finding kinematic conditions suitable for high performance. In addition, detailed flow analysis using IB-LBM has revealed that high efficiency and power output are associated with the detachment of the leading edge vortex (LEV) near stroke reversal, resulting in a horseshoe-shaped vorticity wake with a width approximating the swept distance of foil behind the turbine plane. When the LEV detaches from the foil near mid stroke, both efficiency and power output suffer.
Finally, a flexible system consisting of a rigid foil and a passively actuated flat plate tail connected through a torsional spring to the trailing edge of the rigid foil is studied numerically using the IB-LBM for different mass densities and natural frequencies under different kinematic conditions. The results show that a tail with appropriate mass density and resonant frequency can improve the maximum efficiency by 7.24% compared to the rigid system. This is because the deflection of the tail reduces the low pressure region on the pressure surface caused by the LEV after the stroke reversal, resulting in a higher efficiency. In addition, a spring-connected tail with a low resonant frequency improves the performance significantly at high flapping frequencies.