New findings on how sharks achieve drag reduction could inspire the design of ribs for more efficient planes and boats. In their study of the denticles of great white sharks, researchers at Tokyo Tech found that the height and distance of the ridges play a crucial role in reducing drag at different swimming speeds. Higher center ridges help sharks navigate efficiently at lower speeds, while lower side ridges become more important for reducing drag during high-speed hunting thrusts. The analysis also suggests that the speeds of an extinct basking shark, megalodon, may not be much different from those of the white shark.
The great white shark, one of the ocean’s most efficient predators, has small tooth-like structures on its skin called dermal denticles that potentially reduce frictional drag as it moves through the water. The reduction in air resistance would help them become fast hunters, capable of hunting speeds of up to 6.7 m/s, as well as long-distance swimmers, able to efficiently cover distances of up to 20,000 kilometers. Engineers have been inspired by these denticles to design ribs, or small unidirectional ridges, for airplanes and sailboats. However, the denticles in a shark’s body vary in shape, size and spacing, complicating understanding of how they collectively influence drag reduction.
From a study published in the Journal of the Royal Society Interfaceresearchers at the Tokyo Institute of Technology developed 3D models of white shark denticles. They analyzed the hydrodynamic size of high mid-ridges and low lateral edges of the denticles in relation to the shark’s swimming speed. The researchers found that these denticles are designed to reduce drag at a wide range of speeds, allowing sharks to gain bursts of speed for hunting and travel efficiently over long distances.
“Our calculations suggest that the combination of high and low edges of the denticles is the result of adaptation to both slow and fast swimming speeds, providing robustness to different swimming conditions,” said Associate Professor Hiroto Tanaka, the corresponding author of the study.
The researchers collected skin samples from 17 different locations on a white shark (Carcharodon carcharias) specimen, including the snout, dorsal fin, lateral body, ventral body, caudal fin, and both the dorsal and ventral sides of the pectoral fin (Figure 1).
These samples were scanned with a microfocus X-ray CT scanner to create detailed 3D models, which were then analyzed to measure the distance and height of the ridges. According to the previous fluid dynamics studies of ribs, the edges of denticles possibly reduce frictional drag by lifting turbulent vortices away from the shark skin surface. These vortices are larger and further away from the skin at lower cruising speeds, but shrink as the shark swims faster (Figure 2). Therefore, the distance and height of these ridges are crucial, as they determine how effectively the shark interacts with these vertebrae.
To study these aspects, researchers modeled the shark’s body as a flat plate of the same length and defined non-dimensional values ​​for the distance (S+) of the denticle edges. These values ​​normalize the physical dimensions of the ridges by the flow properties. Non-dimensional parameters were calculated for swimming speeds of 1 m/s, 2 m/s, 5 m/s and 10 m/s, and compared to a well-studied scalloped rib similar to shark teeth. It is known that the reference rib reduces friction resistance the most S+ of 17. Comparing this value allowed researchers to determine how the distance between the ridges of sharks’ denticles affects drag reduction at different speeds.
As a result, the researchers found that the S2+ of the upper middle ridges becomes about 17 at 2 m/s, which corresponds to the measured migration speed. At hunting speeds of 5 m/s, S1+ of the adjacent high and low ridges becomes about 17, indicating the maximum drag-reducing effect.
“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 can also be applied to other sharks, including extinct species,” says Tanaka . Based on available fossil data, the swimming speed of a megalodon, an extinct basking shark with similar denticle morphology, is calculated to be 2.7 m/s and 5.9 m/s, respectively. Despite its large body, the speed of the megalodon does not differ much from that of the white shark. By uncovering how shark denticles work, this study could lead to new high- and low-ridge rib designs in engineering and new analytical methods for the biological study of shark evolution.
