Some species of sharks can swim at impressive speeds of 50 km/h (31 mph). Their skin is covered in bony scales called dermal denticles (literally ‘skin teeth’), generally 0.2-0.5 mm small, with fine regularly spaced (30–100 μm) longitudinal ridges aligned along the body axis. It has long been hypothesized that shark scales reduce drag by managing the water flow closest to the skin. In addition, shark denticles may help vortices (low-pressure regions of swirling water) stay attached to particular areas of the shark’s body, resulting in more suction and forward thrust. Thus, a shark’s denticles may increase swimming speeds by increasing thrust in addition to reducing drag.
Scale texture is just one of the factors that can influence shark skin hydrodynamics, however. Laboratory experiments have revealed that surfaces covered with shark skin, as well as synthetic replicas, experience faster swimming speeds (and presumably decreased drag) compared to surfaces with denticles removed. But this increase in speed only occurred when the textured surface was allowed to flex and bend (as a shark’s body would in the wild), and not when it was kept rigid. Why this difference exists is still under investigation. The shark scales’ ability to bristle in excess of 30-50˚ angles when the body bends may change the nature of fluid flow.
There is still debate regarding shark denticles’ effectiveness at reducing drag. A recent computational study found that shark skin experienced increased drag by 45-50% when compared to a flat plate without shark skin. These researchers speculate that the three-dimensional shape of a denticle interacts with local flow in such a way that it increases drag. However, it is important to note that this model made necessary simplifying assumptions, including a rigid surface on which the denticles are mounted and a static angle of bristling. Further research, including empirical studies, will be required to address these contrasting results.
Read more about experiments with shark skin in issue 8 of Zygote Quarterly:
“Although the denticles resemble riblets, both sharkskin arrangements increase total drag by 44%-50%, while the riblets reduce drag by 5%. Analysis of the simulated flow fields shows that the turbulent flow around denticles is highly three-dimensional and separated, with 25% of the total drag being form drag. The complex three-dimensional shape of the denticles gives rise to a mean flow dominated by strong secondary flows in sharp contrast with the mean flow generated by riblets, which is largely two-dimensional.” (Boomsma and Sotiropoulos 2016:035106-1)
Direct numerical simulation of sharkskin denticles in turbulent channel flowPhysics of FluidsMarch 15, 2016
Shark-skin surfaces for fluid-drag reduction in turbulent flow: a reviewPhilosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering SciencesSeptember 20, 2010
“… on both species [S. mokarran & C. limbatus] the scales were easily moveable to the touch to angles in excess of 30?. Experiments modeling an extreme angle of bristling for shortfin mako denticles confirmed the formation of embedded vortices within the inter-denticular cavities.” (Lang et al. 2008:7)
Bristled shark skin: a microgeometry for boundary layer control?Bioinspir. Biomim.October 15, 2008
“Mounting evidence indicates that drag reduction [via the role of dermal denticles] most likely occurs by reducing turbulent cross-flow near the scale surface, thereby reducing shear stress, and by control of flow separation around the body, which would reduce pressure drag.” (Motta et al. 2012:1096)
“We hypothesize that the erectable nature of the placoid scales along the flanks of the shortfin mako shark allows a reduction of flow reversal, backflow, and pressure drag as this fast-swimming shark maneuvers through the water column.” (Motta et al. 2012:1108)
Scale morphology and flexibility in the shortfin mako Isurus oxyrinchus and the blacktip shark Carcharhinus limbatusJournal of MorphologySeptember 11, 2012
“…shark denticles had no beneficial locomotor effect on the moving rigid shark skin foils, [but] denticles did improve swimming performance significantly (by an average of 12.3%) on flexible shark skin membrane foils compared with those in which the denticles had been removed… “ (Oeffner and and Lauder 2012:791)
“…The presence of denticles on the surface thus alters the flow environment near the flexing foil surface in such a manner that the LEV [leading edge vortex] adheres more closely to the foil surface…The lower the pressure on the foil surface and the closer the vortex core is to the foil surface, the higher the thrust force…This result suggests that one important effect of the skin denticles is to enhance thrust, and not simply to reduce drag.” (Oeffner and Lauder 2012:794)