  Aerodynamic force generation and mechanical power requirements of a dragonfly
(Aeschna juncea) in hovering flight are studied. The method of numerically
solving the Navier-Stokes equations in moving overset grids is used.
  There are two large vertical force peaks in one flapping cycle. One is in the
first half of the cycle, which is mainly due to the hindwings in their
downstroke; the other is in the second half of the cycle, which is mainly due
to the forewings in their downstroke. Hovering with a large stroke plane angle,
the dragonfly uses drag as a major source for its weight supporting force
(approximately 65% of the total vertical force is contributed by the drag and
35% by the lift of the wings). The vertical force coefficient of a wing is
twice as large as the quasi-steady value. The interaction between the fore- and
hindwings is not very strong and is detrimental to the vertical force
generation. Compared with the case of a single wing in the same motion, the
interaction effect reduces the vertical forces on the fore- and hindwings by
14% and 16% of that of the corresponding single wing, respectively. The large
vertical force is due to the unsteady flow effects. The mechanism of the
unsteady force is that in each downstroke of the hindwing or the forewing, a
new vortex ring containing downward momentum is generated, giving an upward
force. The body-mass-specific power is 37 W kg-1, which is mainly contributed
by the aerodynamic power.
