Dragonflies Utilize Flapping Wings Phasing and Spanwise Characteristics   to Achieve Aerodynamic Performance
  While dragonflies are highly agile flyers, some key aerodynamic mechanisms responsible for their flight performance remain inadequately understood. Based on forward flight conditions, we investigate dragonfliess spanwise aerodynamic behaviors associated with flapping wings phasing relationship. Overall, the leading edge vortex (LEV) on the forewing forms without the influence of the hindwing. For hindwing, the wing root region prominently displays a trailing edge vortex (TEV). In the inner span region, the vortical flow structures around the hindwing is influenced by the forewings LEV when both wings are in close proximity and move in opposite directions. In the mid-span region, downwash following the forewing suppresses LEV formation on the hindwing. Finally the outer span region of the hindwing develops its LEV by wake capture at the end of a stroke cycle. In the inner region, the timing of shedding on both fore- and hind-wings is synchronized, which is not the case elsewhere. These varied flow structures suggest that the fore- and hind-wings, along their spanwise directions, play different roles in force generation. 
Csaba Hefler, Huihe Qiu, Wei Shyy
physics.flu-dyn