February 9, 2012
Shark’s ‘Denticles’ Help It Achieve Higher Speeds
Researchers have found that razor sharp tooth-like scales known as denticles found in a shark's skin actually help the fish swim faster.
The denticles are thought of to behave like the dimples on a golf ball, disturbing the flow of water over the surface to help reduce the drag and increase the thrust.
"What we found is that as the shark skin membrane moves, there is a separation of flow — the denticles create a low-pressure zone, called a leading-edge vortex, as the water moves over the skin," George Lauder, the Henry Bryant Bigelow Professor of Ichthyology at Harvard University, said in a press release.
"You can imagine this low-pressure area as sucking you forward. The denticles enhance this leading-edge vortex, so my hypothesis is that these structures that make up shark skin reduce drag, but I also believe them to be thrust enhancing."
The phenomenon was only found when the skin was attached to a flexible membrane, as opposed to a rigid structure.
To perform the experiment, Lauder went down to a fish market in Boston and bought several large makos to have fresh shark skin in the study.
He attached sections of the skin to both sides of a ridge aluminum foil, then immersed the foil in a flow tank to reproduce the swimming motion of a fish.
He then sanded off the denticles, and set the foil swimming again. However, he found that instead of slowing down without the denticles, the foil actually showed faster action.
"But then we remembered our premise that the sharks aren't rigid," Lauder wrote in the Journal of Experimental Biology.
He then glued two pieces of shark skin together to produce a flexible foil, and found that the intact skin foil swam 12.3 percent faster than the sanded skin.
Researchers have used studies like this to help create the latest technology for swimsuits, such as the Speedo Fastskin FS II fabric.
This particular swimsuit has a surface designed to mimic these denticles on a shark's skin. This technology has been thought of as being able to help a swimmer achieve higher speeds in the water. However, Lauder says this theory is completely misplaced.
"In fact, it's nothing like shark skin at all," Lauder said, of the swimsuit material. "What we have shown conclusively is that the surface properties themselves, which the manufacturer has in the past claimed to be bio-mimetic, don't do anything for propulsion."
He tested two shark skin swimsuit designs and found that a riblet surfaced one improved the flexible foil's swimming speed by 7.2 percent, but the Fastskin fabric had no effect at all.
However, Lauder did say it could improve the swimmer's performance in other ways.
"There are all sorts of effects at work that aren't due to the surface," Lauder said in a press release. "Swimmers who wear these suits are squeezed into them extremely tightly, so they are very streamlined. They're so tight could actually change your circulation, and increase the venous return to the body, and they are tailored to make it easier to maintain proper posture even when tired. I'm convinced they work, but it's not because of the surface."
He said the team of researchers will now try to image the flow as close to the surface as they can get, and will explore making artificial shark skin and manipulate it to see what effects that may have.
Image 2: Professor George Lauder has found that the rough surface of shark skin helps reduce drag and increase thrust as the animal swims. Interestingly, the research also tested the high-tech swimsuits and found that their surface (supposedly designed to mimic shark skin) has no effect on swimming speed. "I´m convinced they work, but it´s not because of the surface,” he said of the swimsuits. Credit: Kris Snibbe/Harvard Staff Photographer
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