Passive Separation Control of Shortfin Mako Shark Skin in a Turbulent Boundary Layer

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.

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Passive bristling of mako shark scales in reversing flows

Journal of The Royal Society Interface ◽

10.1098/rsif.2018.0473 ◽

2018 ◽

Vol 15 (147) ◽

pp. 20180473 ◽

Cited By ~ 5

Author(s):

Kevin T. Du Clos ◽

Amy Lang ◽

Sean Devey ◽

Philip J. Motta ◽

Maria Laura Habegger ◽

...

Keyword(s):

Boundary Layer ◽

Turbulent Boundary Layer ◽

Drag Reduction ◽

High Speed ◽

Scale Up ◽

Control Mechanisms ◽

Flow Conditions ◽

Boundary Layer Flows ◽

Mako Shark ◽

Shortfin Mako

Shark skin has been shown to reduce drag in turbulent boundary layer flows, but the flow control mechanisms by which it does so are not well understood. Drag reduction has generally been attributed to static effects of scale surface morphology, but possible drag reduction effects of passive or active scale actuation, or ‘bristling’, have been recognized more recently. Here, we provide the first direct documentation of passive scale bristling due to reversing, turbulent boundary layer flows. We recorded and analysed high-speed videos of flow over the skin of a shortfin mako shark, Isurus oxyrinchus . These videos revealed rapid scale bristling events with mean durations of approximately 2 ms. Passive bristling occurred under flow conditions representative of cruise swimming speeds and was associated with two flow features. The first was a downward backflow that pushed a scale-up from below. The second was a vortex just upstream of the scale that created a negative pressure region, which pulled up a scale without requiring backflow. Both flow conditions initiated bristling at lower velocities than those required for a straight backflow. These results provide further support for the role of shark scale bristling in drag reduction.

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