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.