Studying the skin of the great white shark could help reduce drag in aircraft

The great white shark (Carcharodon carcharias) is a swift and mighty hunter, capable of reaching speeds as high as 6.7 m/s when breaching, although it prefers to swim at slower speeds for migration and while waiting for prey. A team of Japanese researchers has studied the structure of the great white's skin to learn more about how these creatures adapt so well to a wide range of speeds. Their findings could lead to more efficient aircraft and boats with greatly reduced drag, according to a recent paper published in the Journal of the Royal Society Interface.

As previously reported, anyone who has touched a shark knows the skin feels smooth if you stroke from nose to tail. Reverse the direction, however, and it feels like sandpaper. That's because of tiny translucent scales, roughly 0.2 millimeters in size, called "denticles" (because they strongly resemble teeth) all over the shark's body, especially concentrated in the animal's flanks and fins. It's like a suit of armor for sharks and it also serves as a means of reducing drag in the water while swimming.

Pressure drag is the result of flow separation around an object, like an aircraft or the body of a mako shark as it moves through water; the magnitude of pressure drag is determined by the shape of the object. It's what happens when the fluid flow separates from the surface of an object, forming eddies and vortices that impede the object's movement. Since the shark's body is constantly undulating as it swims, it needs something to help keep the flow attached around that body to reduce that drag. Denticles serve that purpose.

There is also friction drag arising from the shear force between the fluid medium and a moving object's surface. Basically, when an object moves through a fluid, like air or water, the fluid closest to the object's surface—known as the boundary layer—gets dragged along with it, exerting a force on the object opposite to the direction of motion.  The greater the distance from the surface, the greater the velocity of the flow speed.

For instance, mako sharks can swim as fast as 70 to 80 mph, earning them the moniker "cheetahs of the ocean." Back in 2019, scientists at the University of Alabama determined one major factor in how mako sharks are able to move so fast: the unique structure of their skin, especially the denticles around the flank and fin regions of their bodies. Mako sharks have evolved a distinct passive "bristling" aspect on some of their scales to swim faster. Near regions like the nose, the scales aren't especially flexible, more like molars embedded in the skin. But near the flanks and fins, the scales are much more flexible.

That has a profound effect on the degree of pressure drag the mako shark encounters as it swims. The denticles of the mako shark can flex at angles more than 40 degrees from its body—but only in the direction of reversing flow (i.e., from tail to nose). This controls the degree of flow separation, similar to the dimples on a golf ball. The dimpling, or scales in the case of the mako shark, help maintain attached flow around the body, reducing the size of the wake.

Winning combination

The authors of this latest paper focused on how the great white shark's denticles might reduce friction drag. They collected 17 skin samples from different locations on the body of a great white shark in the collection of the National Museum of Nature and Science in Tsukuba, Japan, including the snout, dorsal fin, lateral and ventral body, caudal fin, and the ventral sides of the pectoral fin. (The shark was already dead when it was found by local fishermen in 2014.) The samples were wrapped in wet tissue paper and placed in a plastic tube to keep them from drying out and then CT scanned.

Prior studies had shown that denticle ridges reduced friction drag by lifting turbulence vortices away from the shark skin surface, and such vortices are larger and further away from the skin at slower speeds, shrinking as the shark swims faster. So the spacing and height of those ridges would be crucial to how the shark's body interacts with those vortices and how those aspects influence drag reduction across different speeds. The CT scanning data was used to create 3D surface models of denticles using CAD software so the researchers could measure ridge spacing and height. The team next ran multiple fluid dynamics calculations to estimate the swimming speed at which the ridges on the different ventricles would reduce speed.

"Our calculations suggest that the combination of high and low ridges of the denticles results from adapting to both slow and high swimming speeds, thereby offering robustness to various swimming conditions,” said co-author Hiroto Tanaka of the Tokyo Institute of Technology in Japan. “High ridges likely reduce drag at low swimming speeds, and high-low alternating ridges reduce drag at high swimming speeds, covering the full range of swimming speeds. Our calculation method also can be applied to other sharks, including extinct species."

Their fluid dynamics calculation method also enabled them to estimate the hunting and migration speeds of other shark species based on the ridge spacing of the denticles, such as mako sharks, the extinct megalodon, and the scalloped hammerhead shark, all of which have similar denticles to great white sharks. Tanaka et al. found that the hunting and migrating speeds of the mako shark were about 10.5 m/s and 4.9 m/s, respectively, while those for the megalodon were about 5.9 m/s and 2.7 m/s, respectively.

This kind of research could one day lead to new designs capable of reducing drag on aircraft or helicopters, among other potential applications—possibly even high-tech swimsuits for professional athletes. Engineers have long been trying to find ways to apply riblets in the transportation industry, per Tanaka et al., citing 3M's production of riblet films with triangular ridges for a competition sailboat that won the America's Cup back in 1987. Later attempts to apply the films to aircraft in the 1990s were less successful, although SwissAir has been operating riblet-equipped aircraft since 2022, with Lufthansa following suit last year. The authors hope that their discovery might inspire new riblet designs with high and low ribs capable of adapting to a wider range of speeds.

DOI: Journal of the Royal Society Interface, 2024. 10.1098/rsif.2024.0063  (About DOIs).