Enhancing energy-harvesting performance of a flapping foil using a leading-edge detached small structure

This paper proposes a novel stationary small-structure flow-control method, which entails placing a small structure, including a cylinder, an airfoil, or a flat plate at a specific distance from the leading edge of a flapping foil, to improve its energy-harvesting performance at low wind speeds. The influence of two critical factors, namely, the diameter of the detached small cylinder and its distance from the leading edge of the foil, on the energy-acquisition efficiency of the flapping foil is first examined numerically. The optimal position and diameter of the small cylinder are then determined based on the analysis of the lifting energy gain mechanism through the force curve and the flow field characteristics. Furthermore, the effects of other small structure shapes, including a tiny flat plate and airfoil, on the aerodynamic characteristics of the flapping foil have also been investigated. The present findings indicate that, although the small airfoil positioned at the foil's leading edge best sustains a large-scale leading-edge vortex and delivers an 11.45% efficiency gain at the reduced frequency of 0.14, the small cylinder can extend the efficiency improvement across a relatively broader range of reduced frequencies, making it the most promising for practical applications.