Dataset Information

Passive bristling of mako shark scales in reversing flows.

ABSTRACT: 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.

SUBMITTER: Du Clos KT

PROVIDER: S-EPMC6228499 | biostudies-literature | 2018 Oct

REPOSITORIES: biostudies-literature

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Passive bristling of mako shark scales in reversing flows.

Du Clos Kevin T KT Lang Amy A Devey Sean S Motta Philip J PJ Habegger Maria Laura ML Gemmell Brad J BJ

Journal of the Royal Society, Interface 20181024 147

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 r ...[more]

PMID: 30355806

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Project description:The aim of this study was to reveal potential relationship between the main flow field around a shortfin mako shark and the surface morphology of shark skin. Firstly, a numerical simulation using the large eddy simulation (LES) method was conducted to obtain the main flow field around a smooth shark model. Then, the surface morphology characteristics of a shark (Isurus oxyrinchus) at different positions were characterized by scanning electron microscope (SEM), which showed that the morphology, riblet size, and density of scales at different positions on the shark were significantly different. At positions where the surfaces face into the water flow direction (i.e., nose and leading edge of fins), the scales were flat and round, with a lower density, and the pressure or wall shear stress (WSS) was greater. Scales with three longitudinal riblets ending in three tips were found on the middle and trailing edges of the first dorsal fin and caudal fin, where water flow states progress from transitional to turbulent. The ranges of the ratio of riblet depth to spacing (RD/RS) in the anterior zone, middle zone and posterior zone of the shark were 0.05-0.17, 0.08-0.23, and 0.32-0.33, respectively. The riblet angle generally followed the flow direction, but it varied across different areas of the body. The turbulence intensity increased gradually across the first dorsal fin, pectoral fin, caudal fin, and the shark body overall. In summary, it was found that the microstructure riblets on the shark skin surface, generally thought to be drag reduction structures, were only located in transitional and turbulent regions at the middle and trailing edge of the shark body and fin surfaces, and there were almost no microstructural grooves in the laminar flow regions along the leading edge. These findings can provide design guidance for engineering applications of bionic riblet surfaces. Riblets placed in transitional and fully turbulent regions can be used to effectively reduce drag. The riblet direction should be consistent with the direction of flow.

Dataset Information

Passive bristling of mako shark scales in reversing flows.

Publications

Passive bristling of mako shark scales in reversing flows.

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