Experimental and Low-Order Numerical Study of Flapping-Wing Turbine Performance and Leading-Edge Vortex Dynamics

Flapping-wing turbines (FWTs) – oscillating wings rather than radial rotors – represent an innovative clean renewable source and have shown competitive power extraction efficiencies relative to conventional rotary turbines, as demonstrated in literature with high-fidelity simulations and limited experiments on physical prototypes. Although interest in FWTs is growing, design optimization, adaptability and reliability have proven difficult for field implementation due to the high-order parameter design space that defines interactions between aerodynamics, wing motion, and energy extraction. Current engineering design approaches involve extensive computer simulations coupled with optimization solvers or trial-and-error experiments which are both time and resource intensive tools. The motivation for the current work was to provide insights into two key questions that an Engineer would need to consider for designing a FWT system: (i) Can high performance be achieved under attached flow conditions? and (ii) If separation and formation of a Leading Edge Vortex (LEV) can't be avoided, how does synchronization between the LEV and motion kinematics influence efficiency? Low order numerical simulations, along with a water channel experimental setup were used to investigate these questions. A discrete vortex method based on a Leading Edge Suction Parameter (LESP), developed previously in literature, was used to predict aerodynamic forces on the foil. Results from the LESP-Modulated Discrete-Vortex Method (LDVM) indicate that, if maintained, attached flow conditions achieve the highest predicted cycle average efficiency of 49%, outperforming cases with LEVs present. If separation and a LEV can't be avoided, experimental results, using a custom setup equipped with synchronized high precision force measurements along with streaked particle imaging for observing Leading Edge Vortex (LEV) evolution, highlight the importance of LEV synchronization. Instantaneous force trends were linked to the position of the LEV node, defined as the central point of the LEV core about which the flow circulates, relative to the foil geometry. Within a constant blockage parameter space, synchronization of LEV evolution led to a maximum efficiency of 29%. LEVs were found to have the biggest impact on both the pitch and heave degrees of freedom after crossing the pivot point at the quarter cycle, with overall performance most sensitive to the LEV shedding time.