Energy-Harvesting Mechanism of a Heaving Airfoil in a Vortical Wake

Energy conversion from vortical flows using flapping wings is widely observed in nature flyers and has great potential for improving energy efficiency of micro aerial vehicles. Therefore, it has recently gained a significant research attention. The interactions between the incoming vortices and wings have previously been studied, but the mechanism inducing such interactions and its impact on energy-conversion efficiency has only marginally been discussed. To bridge this gap, this paper carries out a numerical study on the response of a two-dimensional heaving airfoil in a vortical wake generated by an upstream oscillating D-shaped cylinder. The study first emphasizes the influence of the leading- and trailing-edge vortices on energy conversion. The formation of these vortices is then linked to two major interaction modes observed between the airfoil and the incoming vortices: the suppressing mode and the reinforcing mode. Furthermore, a model of vortex-airfoil interactions is developed from potential theory of fluid dynamics to further understand the mechanism of inducing different interaction modes. The potential theory demonstrates that the topology of vortices surrounding the airfoil is crucial for triggering different interaction modes. The effects of the interaction modes on the energy-conversion efficiency are also demonstrated.