James Browning and Arvind Santhanakrishnan, University of North Carolina, Chapel Hill
Effect of wing flexibility on flight in the smallest insects
Abstract:
The lift production in the flapping flight of insects from the scale
of fruit flies to hawk moths has been attributed to the generation of
a stable attached leading edge vortex (LEV), rotational circulation,
and ‘wake capture’ of the shear flow in between the upstroke and
downstroke cycles of wing motion (Dickinson, Lehmann & Sane, Science,
1999). Little is known about the flight of insects at biologically
intermediate Reynolds numbers (Re) in the range of 10 to 100 where the
role of inertial and viscous forces are non-negligible. These insects
are of wing length scales less than a millimeter, and are observed to
augment lift via adaptations in: the flight kinematics, wing
flexibility and morphology. Using a two-pronged approach of numerical
fluid-structure interaction simulations and experimental measurements
on dynamically scaled models, we explore the role of wing flexibility
on flapping flight at Re~10. In addition to a single wing rotation and
translation at a constant angle of attack, the case of two wings that
clap together at the end of the upstroke and peel apart at the start
of the downstroke is also considered. The results show that for a
rigid wing at Re < 30, the LEV is stably attached during each stroke
similar to what is observed in larger insects. However, the trailing
edge vortex (TEV) is not shed at the end of each stroke for these tiny
insects unlike for Re > 60, and this near vortical symmetry
restoration results in a decrease in lift forces and a significant
increase in drag forces which in turn lowers the aerodynamic
efficiency. Addition of wing flexibility appears to enhance the lift
force for a certain range of flexibilities. Furthermore, the inclusion
of flexibility in the case of two wings that clap and peel reduces the
maximum drag force and amplifies the asymmetry in the circulation
strengths of the LEV and TEV so as to compensate for the loss of
efficiency observed in the equivalent case of a single rigid wing.
Overall, we conclude that wing-wing interaction and flexion are
important factors that contribute to sustaining flight in the smallest
insects.
Mentor: Laura Miller (University of North Carolina)