Flapping wings harvest kinetic energy from fluids by utilizing the coupled heave and pitch motions of an airfoil. Considerable research has been conducted on the energy-harvesting motion of flapping wings, primarily focusing on experimental and simulation studies that examine how the geometric and kinematic parameters affect the energy utilization efficiency. However, analyses of the flow structures and flow characteristics during the energy-harvesting process of flapping wings are relatively scarce. This study employs the proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) methods to thoroughly analyze the flow field in flapping wing energy harvesting. The primary focus is a comparative analysis of the wake flow characteristics between the original flapping wing and that equipped with leading- and trailing-edge flaps during their heaving and pitching motions. Through this comparison, the differences in flow structures and energy distributions between the flapping wing energy harvester with and without the control method are revealed, providing a theoretical basis for the development of more efficient energy capturing methods. The results demonstrate that lower-order POD modes can capture the main energy structures in the flow field, while the DMD method can effectively identify the frequency characteristics and the stability of unsteady structures. The application of active control to the flapping wings significantly improves their flow characteristics, reduces detrimental multi-frequency structures, and thereby optimizes the flow field and enhances the energy harvesting efficiency of the flapping wings.

The flow separation remarkbaly undermines wind turbines' aerodynamic performance thus requires. In order to develop flow control strategies, fundelmental research on the unsteady flow separation is abosutely necessary, and the Reduced-Order Model (ROM) methods offers a tool. In this paper, the unsteady flow fields around S809, a typical horizontal axis wind turbine airfoil, at deep and shallow stall angles of attack are obtained numerically and further analyzed by proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). Results show that POD and DMD can accurately capture the unsteady flow structures and dominant modes, but the former, based on magnitude of energy, ignores the flow structure with similar lift main frequency but less energy. DMD can accurately capture the flow field evolution (rate of increase, frequency, etc.) and develop targeted flow control strategies for the dominant frequency structure, offering the airfoil flow field conditions and aerodynamic performance.

In addition to reducing the consumption of fossil energy, improving the energy-harvesting efficiency is also one of the key factors to achieve the goal of "carbon peak and carbon neutrality" in China. Flapping wings are simple and environmentally friendly devices that can harvest energy from fluids. To improve the energy-harvesting efficiency of flapping wings, a novel flapping wing with trailing-edge flaps is proposed, whose performance is characterized by investigating the flow fields obtained by numerical simulations. By changing the deflection strategy of trailing-edge flaps, it is found that the continuous deflection in one motion cycle can achieve the most significant improvement in the energy-harvesting performance. Finally, a comparative analysis on the performance of active trailing-edge flaps and passive Gurney flaps is carried out. Results show that, depending on the reduced frequency, both types of flaps can achieve promising performance thus they are potential for future application. The Gurney flaps are applicable when the reduced frequency is in the range (f ^* = 0.08 ~ 0.14), with the maximum efficiency of 41.2% at f ^* = 0.12. By comparison, the trailing-edge flaps perform well in the higher frequency range (f ^* = 0.12 ~ 0.22), with the maximum efficiency of 46.3% at f ^* = 0.18.