Does shark skin really reduce drag? Not so fast, study says

Many experts have long touted the benefits of covering surfaces in shark skin-like ridges called "riblets" because the technology has been purported to significantly reduce drag. But a new study in the journal Physics of Fluids finds that the denticles covering shark skin might actually increase drag in some situations – indicating that the premise of this technology might be fundamentally wrong.  

You might expect sharks, deadly fast as they are, to have smooth skin allowing them to slip through the water with ease and speed. But a shark's skin is actually covered in tooth-like features called denticles.

“It was a very rough texture — it’s unlike anything I have touched,” said study co-author Fotis Sotiropoulos, a fluid dynamicist at Stony Brook University, who felt one during a snorkeling trip on the coast of Belize.

Researchers have long thought that these denticles must manipulate the flow around sharks' bodies in ways that allowed them to reduce drag. In fact, the tiny features have inspired "riblets," patterns of ridges used to manipulate the flow at the surface. These structures, found in a variety of patterns, have been incorporated into wind turbines (to increase efficiency) and swimsuits (to raise swimming speed) and have even been developed for medical devices (to reduce hospital-acquired infections).

But for the last few decades, scientists have questioned whether denticles – the natural structures that inspired the manmade riblets – really do act to reduce drag. Studies have gone back and forth on the subject for about three decades, making the function of this ostensibly rough material an incredibly slippery subject.

“That has been at the center of controversy for a very long time,” Sotiropoulos said.

Part of the problem is that building a physical experiment to explore the complexities of the fluid flow at this scale is extremely difficult, Sotiropoulos said.

You have to get shark skin, you have to mount it properly and you have to expose it to the right flow conditions – and then you have to find a way to measure the tiny, turbulent patterns in that flow.

Instead, the researchers used data on the shortfin mako shark denticles from Harvard evolutionary biologist George Lauder, which perfectly described their shape. With the help of supercomputers, researchers developed a model that showed what would happen if you ran fluid over the denticles and what would happen if you ran it over the riblets.

The researchers found that while the riblets acted to reduce drag by 5.2%, the denticles actually increased drag by 44% (when rows of denticles were aligned) and 50% (when they were staggered).

“That was the surprising finding,” Sotiropoulos said.

Though the riblets may work, they’re not working in the same way that denticles might, the scientist said. That’s probably because riblets are essentially two-dimensional structures that create more organized flow patterns. But denticles are complex, three-dimensional structures, and so they generate far more complicated, turbulent flows.

“We cannot necessarily conclude from this study that shark skin does not reduce drag under any circumstances,” Sotiropoulos said, “because there are many other flow parameters that we can’t account for in our simulation right now.”

In other words, it’s possible that the denticles might work in very particular situations to reduce drag. In this experiment, the researchers were testing the riblets and denticles at about the equivalent of a shark’s cruising speed, Sotiropoulos said. So while the denticles didn’t reduce drag in the cruise phase, perhaps they would if the shark were "sprinting" through the water. It’s also possible that the denticles work best when on the undulating body of a living shark instead of attached to a (simulated) flat plate. Or perhaps they're there in large part for another purpose, serving as some kind of defense mechanism.

Those questions, however, will await further study, Sotiropoulos added.  

“Perhaps,” Sotiropoulos said, “the reason that sharks evolved to have this complex shark skin might be a bit more complicated and multifaceted than what we thought.”

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