Vortex structures and local contribution of vortices to lift production.

<p>Vortex structures are identified by plotting an isosurface of the imaginary part of the complex eigenvalue of the local deformation tensor [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0132093#pone.0132093.ref031" target="_blank">31</a>]. Isosurfaces are shaded by the lift coefficient per unit volume (<i>cm</i>³ for the hawkmoth and <i>mm</i>³ for the fruit fly) contributed by local vorticity. Panels (a-c) and (d-e) show the three phases in the flapping of the hawkmoth and fruit fly respectively. The hawkmoth wing, which is operating at a higher Reynolds number, generates a number of distinct vortices including the LEV, a tip-vortex and a root-vortex. (a) the downstroke peak in VIL for the hawkmoth wing, the bulk of the lift-producing vorticity patches are associated with the LEV. (b) at <i>t</i>/<i>T</i> = 1.5, the end downstroke, there is a spiralling LEV and a strong tip-vortex but neither of them generates much lift. (c) at <i>t</i>/<i>T</i> = 1.9, at the peak in VIL during late upstroke, the LEV on the ventral surface of the wing is weak but the wing-tip vortex provides a noticeable contribution to lift. For the fruit fly wing, which operates at a lower Reynolds number, the LEV is <i>the</i> most dominant vortical structure in downstroke (d) and upstroke (f); vorticity at the end of downstroke (e) is negligible.