Low Reynolds number flow in and above asymmetric, triangular cavities

Beneficial aerodynamic effect of wing scales on the climbing flight of butterflies

It is hypothesized that butterfly wing scale geometry and surface patterning may function to improve aerodynamic efficiency. In order to investigate this hypothesis, a method to measure butterfly flapping kinematics optically over long uninhibited flapping sequences was developed. Statistical results for the climbing flight flapping kinematics of 11 butterflies, based on a total of 236 individual flights, both with and without their wing scales, are presented. Results show, that for each of the 11 butterflies, the mean climbing efficiency decreased after scales were removed. Data was reduced to a single set of differences of climbing efficiency using are paired t-test. Results show a mean decrease in climbing efficiency of 32.2% occurred with a 95% confidence interval of 45.6%-18.8%. Similar analysis showed that the flapping amplitude decreased by 7% while the flapping frequency did not show a significant difference. Results provide strong evidence that butterfly wing scale geometry and surface patterning improve butterfly climbing efficiency. The authors hypothesize that the wing scale's effect in measured climbing efficiency may be due to an improved aerodynamic efficiency of the butterfly and could similarly be used on flapping wing micro air vehicles to potentially achieve similar gains in efficiency.

Chaos-enhanced transport in cellular flows

Examples of chaos-enhanced transport in cellular (lows are presented. Illustrations span from boundary and jet induced motions within cavities to separated flows; chaotic adveetion is generated by time-modulation of walls, variation of angle of impingement of laminar jets, and naturally occurring oscillations in the velocity field as the Reynolds number is increased. Tools include manifold structure, the Melnikov technique, as well as eulerian and lagrangian descriptions of transport. A range of Peclet numbers is explored; the amount of transport enhancement over the nonchaotic case is exemplified in terms of heating of a fluid and the removal of a diffusive tracer; the enhancement over the non chaotic case can be as large as 40%.