Three-dimensional effects on the aerodynamic performance of flapping wings in tandem configuration

Direct numerical simulations have been performed to analyze how three-dimensional effects influence the performance of wings in tandem configuration undergoing a two-dimensional optimal kinematics. This optimal motion is a combination of heaving and pitching of the airfoils in a uniform free-stream at a Reynolds number Re=1000 and Strouhal number Stc=0.7. Wings of two different aspect ratios, = 2 and 4, undergoing the 2D motion have been considered. It has been found that the interactions between the vortical structures of the fore- and the hind-wings are qualitatively similar to the two-dimensional case for both . However, the ratio between the mean thrust of the hind-wing and the fore-wing decreases from 80% in 2D to 70% in 3D, implying that the 3D effects are detrimental for the vortical interactions between the wings in terms of thrust production. Nonetheless, the propulsive efficiency remains constant both in 2D and 3D, for both . A more realistic flapping motion has also been analyzed and compared to the heaving motion. It has been found that the aerodynamic forces decrease when the wings are in flapping motion. This detrimental behavior has been linked to a sub-optimal motion of the inboard region of the wings. This sub-optimal region of the wings entails a decrease of the mean thrust and of the propulsive efficiency compared to the heaving case, which are more pronounced for the = 4 wings.