Sharkskin actually increases drag

March 15, 2016, American Institute of Physics
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

Related Stories

Manmade artificial shark skin boosts swimming

May 14, 2014

People have thought for decades that the rough skin of sharks may give them a swimming boost and now scientists from Harvard University have made the first ever realistic simulated shark skin. They also measured that the ...

Amazing skin gives sharks a push

February 9, 2012

Shark skin has long been known to improve the fish's swimming performance by reducing drag, but now George Lauder and Johannes Oeffner from Harvard University show that in addition, the skin generates thrust, giving the fish ...

The math of shark skin

July 3, 2015

"Sharks are almost perfectly evolved animals. We can learn a lot from studying them," says Emory mathematician Alessandro Veneziani.

Enhancing the efficiency of wind turbines

November 22, 2010

A milestone in the history of renewable energy occurred in the year 2008 when more new wind-turbine power generation capacity was added in the U.S. than new coal-fired power generation. The costs of producing power with wind ...

Recommended for you

ATLAS experiment observes light scattering off light

March 20, 2019

Light-by-light scattering is a very rare phenomenon in which two photons interact, producing another pair of photons. This process was among the earliest predictions of quantum electrodynamics (QED), the quantum theory of ...

How heavy elements come about in the universe

March 19, 2019

Heavy elements are produced during stellar explosion or on the surfaces of neutron stars through the capture of hydrogen nuclei (protons). This occurs at extremely high temperatures, but at relatively low energies. An international ...

Trembling aspen leaves could save future Mars rovers

March 18, 2019

Researchers at the University of Warwick have been inspired by the unique movement of trembling aspen leaves, to devise an energy harvesting mechanism that could power weather sensors in hostile environments and could even ...

Quantum sensing method measures minuscule magnetic fields

March 15, 2019

A new way of measuring atomic-scale magnetic fields with great precision, not only up and down but sideways as well, has been developed by researchers at MIT. The new tool could be useful in applications as diverse as mapping ...


Adjust slider to filter visible comments by rank

Display comments: newest first

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.
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.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.