Passive bristling of mako shark scales in reversing flows
Passive bristling of mako shark scales in reversing flows
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DOI：
10.1098/rsif.2018.0473
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发表时间：
2018-10-01
影响因子：
3.9
通讯作者：
Gemmell, Brad J.
中科院分区：
文献类型：
--
作者：
Du Clos, Kevin T.;Lang, Amy;Gemmell, Brad J.
鲨鱼皮已被证明可以减少湍流边界层流动中的阻力，但其流动控制机制尚不清楚。减阻通常归因于鳞片表面形态的静态效应，但最近已经认识到被动或主动鳞片致动或“刚毛”的可能减阻效应。在这里，我们提供了第一个直接的文件被动规模竖起由于逆转，湍流边界层流动。我们记录和分析了高速视频流的皮肤短finmako鲨鱼，尖吻鲭。这些视频显示快速规模的刚毛事件，平均持续时间约为2毫秒。被动刚毛发生在代表巡航游泳速度的流动条件下，并与两个流功能。第一个是向下的回流，推动了自下而上的规模扩大。第二个是在水垢上游形成的涡流，该涡流产生了负压区域，在不需要回流的情况下拉起了水垢。这两种流动条件都在比直回流所需的速度低的速度下开始竖起。这些结果进一步支持了鲨鱼鳞刚毛减阻的作用。
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 short finmako 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.