Sharkskin actually increases drag

March 15, 2016
Sharkskin actually increases drag
Computer simulations unveil flow around sharkskin. Credit: A. Boomsma & F. Sotiropoulos/UMN

On an intuitive level, you'd expect a shark's skin to reduce drag. After all, the purpose of sharkskin-inspired riblets—the micro-grooved structures found in aircraft wings, wind turbine blades and Olympic-class swimsuits—is to do just that. Sharkskin's ability to reduce hydrodynamic drag, however, has been academically contested for the past 30 years.

To clarify this phenomenon, researchers at Stony Brook University and the University of Minnesota recently conducted simulations on the ability of the small, tooth-like denticles that make up sharkskin to modify hydrodynamic flow with an unprecedented level of resolution. Far from easing the glide through the water, they found, the structures can actually increase drag by up to 50 percent.

Fotis Sotiropoulos—whose previous work focused on developing computational tools to study the evolutionary impact of hydrodynamic factors on fish body shapes and swimming styles—and his Ph.D. student Aaron Boomsma discuss their work exploring the hydrodynamics of sharkskin this week in Physics of Fluids, from AIP Publishing.

"The work on sharkskin was a natural progression, especially after observing the commonalities between sharkskin and riblet films," said Sotiropoulos, dean of the College of Engineering and Applied Sciences at Stony Brook University and primary investigator of the project. "Our interest was piqued by the thought that sharkskin was capable of providing a hydrodynamic advantage to sharks."

Sotiropoulos and his colleagues used experimental data about the three-dimensional geometry of shortfin mako shark denticles provided by George Lauder, a professor of organismic and evolutionary biology at Harvard University, to create computational beds of sharkskin denticles in aligned and staggered configurations. They then applied numerical simulations based on immersed boundary concepts to study the details of turbulent water flow through and over the stationary denticle beds.

"Our simulations show conclusively, that for the tested configurations, sharkskin actually increases drag—as high as fifty percent," Sotiropoulos said.

The researchers also simulated the same flow over riblets, finding that they reduced drag by 5 percent.

This disparity arises due to differences between the objects' geometries: Riblets are able to confine viscous stress along their ridges because they're essentially two-dimensional, whereas the complex three-dimensional features of denticles generate turbulence and swirling flow patterns that complicate the confinement of viscous stress.

"This is a great example of how our attempts to get inspired by nature have led to something truly beneficial, even though the functionality of the original natural construct may not be as simple to explain or understand," said Sotiropoulos.

Future work for Sotiropoulos and his colleagues includes expanding their work to understand how sharkskin denticles perform under swimming conditions and its subsequent pressure forces. Sotiropoulos noted, however, that next-generation super computers are needed to gain a full understanding of this interplay.

Explore further: Manmade artificial shark skin boosts swimming

More information: "Direct numerical simulation of sharkskin denticles in turbulent channel flow," by Aaron Boomsma and Fotis Sotiropoulos, Physics of Fluids, March 15, 2016 (DOI: 10.1063/1.4942474

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Steelwolf
5 / 5 (1) Mar 15, 2016
In thinking about how the denticles would function under swimming conditions: I can see a possible mechanism for an aid to the shark's propulsion by the opening and closing of the gaps in the denticles as the skin and muscle underneath flexes. Recent article here stated that some fish appear to use the cavitation forces created by their bodies more than they do actually 'pushing' against the water, while there is pushing involved, it creates an opposite partial vacuum and the fish is able to use the vortex created by this motion to move them more economically than was previously thought. I can see how the denticles would add to the forward-moving curved surface area and creating a plethora of smaller vorticies that in flexing for the curl part of the swim which is where the vortical cavitation effect is most effective and the gaps close and also expels some of the water between them, adding to the forward thrust as well as riding the vortex wave for an efficient energy use.
Urgelt
5 / 5 (1) Mar 16, 2016
I agree, Steelwolf. We tend to want to find low-drag as advantageous, but what fish are doing for propulsion isn't much like what a submarine does, sticking a propeller in the back and forcing your way through the water. What fish need is to efficiently translate muscular contractions and relaxations all over their bodies into propulsion, and for that to work, there ought to be advantages to *higher* drag skins, to a point.

As we progress towards nanoscale engineering on industrial scales, it would be good to understand how fish swim, because we may be able to apply the lessons learned to all kinds of useful purposes. Here's hoping next-gen supercomputers will arrive soon.

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