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.rsif.royalsocietypublishing.org J. R. Soc. Interface 15: 20180473

Thriving artificial underwater drag-reduction materials inspired from aquatic animals: progresses and challenges

“…5 The reason for the passive bristling of shark denticles in reversing the ows has been described by Du Clos et al, who explained the downward backow between the denticles and the vortex above the denticles were associated with passive bristling by a high-speed camera. 50 Some adverse factors, such as backow, can be inhibited by the passive bristling of shark denticles. The study suggested that shark skin denticles may absorb the negative energy of backow by passive bristling and delay the onset of turbulence in the boundary layer.…”

Relationship Between Skin Scales and the Main Flow Field Around the Shortfin Mako Shark Isurus oxyrinchus

“…The first is that riblets can reduce momentum transfer and shear stress by preventing cross-flow over the scales ( Bechert et al, 2000b ). The second is that the scales are flexible and capable of bristling to stop reversed flow across the skin surface ( Lang et al, 2011 ; Du Clos et al, 2018 ; Santos et al, 2021 ). NASA Langley Research Center ( Walsh and Weinstein, 1978 ) has carried out research on the drag reduction performance of ribbed surfaces.…”

Dermal Denticle Diversity in Sharks: Novel Patterns on the Interbranchial Skin

“…One function that has been an important focus for research is the role of denticles in support of locomotion, as many extant shark species have denticles with morphologies that improve swimming performance. Fluid dynamic studies have revealed that denticles can improve swimming performance by enhancing thrust, reducing hydrodynamic drag, as well as changing the boundary layer characteristics of water flow over the body ( DuClos et al. 2018 ; Lang et al.…”