  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.
