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Sharks, dolphins and butterflies swim and fly in different flow regimes, yet the structure of their surfaces interacting with the surrounding fluid all appear to contain very important microscopic features that lead to reduced drag and increased flying or swimming efficiency. Sharks have moveable scales (approximately 200 microns in size) that act as a passive, flow-actuated dynamic roughness for separation control. Water tunnel experiments with real shortfin mako shark skin samples mounted to models have shown significant control of flow separation in both laminar and turbulent boundary layer scenarios. Dolphins have sinusoidal-shaped millimeter-sized transverse grooves covering a large percentage of their body. Experiments show that similar geometries embedded in a turbulent boundary layer can lead to separation control at the slight expense of increased friction drag. Alternatively, butterfly scales (100 microns in size covering the wings in a roof shingle pattern) appear to fundamentally alter the local skin friction drag depending on flow orientation for what is dominantly a laminar boundary layer interacting with the wings. However, in this case the surface may also slow the growth and formation of the leading-edge vortex and these effects shown in experiments may help explain a mean decrease in climbing efficiency (joules per flap) of 37.8% for live butterflies once their scales were removed. An overview of these results is discussed for these three cases, bringing out the importance of finding solutions in nature for essential engineering problems.

The effects of longitudinal square grooves on the growth of the leading edge vortex (LEV) produced by an impulsively-started flat plate are investigated experimentally. The Reynolds numbers are 1416, 2833 and 5667, and the maximum vortex formation time is 2.8. The square grooves are inspired by the surface patterning (scales) on butterfly wings. It is hypothesized that the grooves play a role in the LEV formation and development by creating a stronger secondary vortex near the LE that impedes the growth of the LEV. To evaluate this hypothesis, circulation curves of the total positive vorticity field and LEV of a smooth plate and grooved plate are compared. Also, the secondary vorticity generated by the LEV interaction with the patterned surface is studied, as well as the subsequent effect on the LEV's growth rate and peak circulation. The vortex development process varies for each Reynolds number. At the lowest number, the effects of the grooves are minimal; however, they are most significant at the highest Reynolds number where the unsteady interactions between the secondary vortex and the leading edge shear layer become stronger for the grooved plate causing an earlier separation of the LEV. Therefore, contrary to the hypothesis, the data shows that stronger secondary vortex causes a stronger LEV with an earlier separation. The stronger secondary vortex interacts more rigorously with the LE shear layer causing the latter to feed more concentrated vorticity to the LEV.

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