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 Stony¹ Brook² 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 unprecedented³ level of resolution. Far from easing the glide⁴ 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 evolutionary⁵ 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 Applied⁶ Sciences at Stony Brook University and primary investigator⁷ of the project. "Our interest was piqued⁸ 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 aligned⁹ and staggered configurations¹⁰. They then applied numerical simulations based on immersed boundary concepts to study the details of turbulent water flow through and over the stationary¹¹ denticle beds. 

 

"Our simulations show conclusively¹², that for the tested configurations, sharkskin actually increases drag -- as high as fifty percent," Sotiropoulos said.

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