Shark's bristling scales make it faster, and may help make planes faster too

Capable of swimming at speeds of up to 74 km/h (46 mph), the shortfin mako is the world's fastest species of shark. Scientists now have a new understanding of how it's able to reach such speeds, and they believe that their findings could be applied to improving human technology.

As is the case with other sharks, the skin of the shortfin mako is covered with tiny scales known as denticles. These are angled back from the front of the fish, so the skin feels smooth if you run your hand along it from nose to tail, but rough if you go "against the grain" in the other direction.

Led by astronautical engineer Amy Lang, a team from the University of Alabama discovered that on key parts of the shortfin mako, those 0.2-mm-long scales (pictured below) are capable of flexing up to an angle of 40 degrees out from the body. They do so in response to reverse water flow, which occurs due to a phenomenon known as flow separation.

In a nutshell, flow separation happens when fluid passes around the front of an object that's travelling through a liquid environment (or that's held in moving liquid), forming eddies alongside its back end. Those eddies create drag, slowing the object down – this effect can hamper both watercraft and aircraft, as it also occurs in the air.

Thanks to the mako's pop-out denticles, though, flow separation is greatly minimized along the length of its body. The protruding scales help prevent the formation of eddies, allowing the fish to move faster through the water. This beneficial effect was observed when the researchers placed samples of shortfin mako flank skin in a water-flow tunnel.

"We set up an experiment in the tunnel with a measured amount of flow separation induced on a smooth surface. Then we replaced the smooth surface with shark skin and requantified the flow separation," says Lang. "In all cases with the flank skin, we saw the size of the separated flow region reduced significantly by the presence of the skin."

It is now hoped that the research could lead to new materials for use on fixed-wing airplanes and the rotor blades of helicopters, making them more aerodynamic.

Source: American Physical Society via EurekAlert