Interaction of two fully passive flapping foils arranged in tandem and its influence on flow energy harvesting

We investigated the dynamics and energy harvesting performance of a novel flow-energy harvesting system that consists of two fully passive flapping foils arranged in tandem. Both experimental tests and numerical simulations were conducted to uncover the wake-foil and foil-foil interaction mechanism. The system was tested at a chord -based Reynolds number of 8.7 x 104 with various initial states and tandem distances. It was found that the aft foil was modulated and eventually locked by the wake of the flapping fore foil, leading to a stable phase dif-ference between the two foils that is independent of the foils' initial states and varies almost linearly with the tandem distance. Within the test range, the aft foil always exhibited larger heaving and pitch velocities, extracting in average 15.2% more power than the fore foil and the single foil. The best power extraction effi-ciency of 19.6% was achieved by the aft foil when the two foils are separated by only one chord length, while the worst efficiency of 15.9% was achieved by the fore foil when they are separated by two chord lengths. Collec-tively, the two foils can achieve the best efficiency of 36.8%, greater than the doubled value (i.e., 33.4%) of the single foil's efficiency.

Automated summary

We studied a novel flow-energy harvesting system consisting of two tandem fully passive flapping foils and investigated its dynamics and energy harvesting performance. Experimental tests and numerical simulations were conducted to understand the wake-foil and foil-foil interaction mechanism. It was found that the aft foil was modulated and locked by the wake of the fore foil, resulting in a stable phase difference between the two foils that varied linearly with the tandem distance. The aft foil consistently exhibited higher heaving and pitch velocities, extracting an average of 15.2% more power than the fore foil and the single foil. The best power extraction efficiency of 19.6% was achieved by the aft foil when the foils were separated by one chord length, while the fore foil had a worst efficiency of 15.9% when separated by two chord lengths. Overall, the two foils achieved a maximum efficiency of 36.8%, surpassing the doubled efficiency (33.4%) of a single foil.