Interactions of the wakes of two flapping wings in hover

This paper deals with interactions of the wakes of two flapping wings on aerodynamic characteristics in hover. With dynamically scaled-up robotic wings installed in a water tank, the effects of both the shape of motion profiles and the mean flapping angle of the wings (the average location of the wings with respect to pivots) were investigated. For the zero mean flapping angle case, the interaction commonly reduced aerodynamic force production regardless of the shape of motion profiles. A leading-edge vortex (LEV) near the wing base was slightly deconcentrated, and the effective angle of attack was less than that in the single wing cases. Further investigation on the mean flapping angle revealed that regardless of the shape of motion profiles, the interaction of the two wakes gradually intensified at the dorsal part where the wings came closer, resulting in a periodic imbalanced force production for every half wingbeat cycle. A particle image velocimetry showed a deconcentrated LEV during the upstroke. Deeply inclined downward fluxes with a fair wind brought on the lower effective angle of attack on the wings in the upstroke. During the downstroke, on the other hand, the primary vortex of LEV clearly intensified with a massive headwind. This was clearly observed near each wing base. The minor vortex along the leading edge was noticeably more concentrated than that in the single wing case. These periodically appeared with a combination of back-and-forth wingbeat motions, preserving such imbalanced production while keeping hovering flight.