Shark Skin, 3D Printing, and Aerodynamics

Sharks have very unique skin – it gives the animals some help in rapidly propelling themselves through the water thanks to tiny, teeth-like scales called denticles. Four years ago, scientists from Harvard University scanned sections of shark skin, then developed a 3D printable material from the data that replicated, to the best of its ability, the properties of real shark skin; the material was then used to give flexible paddles an astonishing 6.6% boost in swimming speed.

Now, a new team of Harvard engineers and evolutionary biologists, together with colleagues from the University of South Carolina, are building on this original 3D printing research to try to create more aerodynamic machines.

“The skin of sharks is covered by thousands and thousands of small scales, or denticles, which vary in shape and size around the body. We know a lot about the structure of these denticles — which are very similar to human teeth — but the function has been debated,” explained George Lauder, the Henry Bryant Bigelow Professor of Ichthyology and Professor of Biology in Harvard’s Department of Organismic and Evolutionary Biology and a member of the original research team.

While they may not seem very similar, airplanes and sharks are both designed to move efficiently through either air or water. They both decrease drag and generate lift using the shapes of their bodies. But sharks have been doing so for a lot longer than planes. Other ocean-inspired researchers have used 3D printing to improve airplanes, and there’s been a lot of research into the drag-reducing properties of shark denticles. But Lauder and the rest of the team decided to dig deeper.

According to Mehdi Saadat, a postdoctoral fellow at Harvard with a joint appointment in Mechanical Engineering at the University of South Carolina, “We asked, what if instead of mainly reducing drag, these particular shapes were actually better suited for increasing lift.”

Supported by the Office of Naval Research and the National Science Foundation, the researchers recently published a paper, titled “Shark skin-inspired designs that improve aerodynamic performance,” in the Journal of the Royal Society Interface; co-authors include Harvard PhD student August Domel; Saadat; James Weaver of Harvard’s Wyss Institute for Biologically Inspired Engineering; Hossein Haj-Hariri, the Dean of Engineering and Computing at the University of South Carolina; Katia Bertoldi, the William and Ami Kuan Danoff Professor of Applied Mechanics at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS); and Lauder.

The abstract reads, “Inspired by the drag-reducing properties of the tooth-like denticles that cover the skin of sharks, we describe here experimental and simulation-based investigations into the aerodynamic effects of novel denticle-inspired designs placed along the suction side of an aerofoil. Through parametric modelling to query a wide range of different designs, we discovered a set of denticle-inspired surface structures that achieve simultaneous drag reduction and lift generation on an aerofoil, resulting in lift-to-drag ratio improvements comparable to the best-reported for traditional low-profile vortex generators and even outperforming these existing designs at low angles of attack with improvements of up to 323%.”

The team tested their hypothesis and demonstrated a bio-inspired structure that could improve the aerodynamics of cars, drones, planes, and wind turbines. They used micro-CT scanning to image and 3D model the denticles of the shortfin mako, the world’s fastest shark; its denticles resemble a trident, due to their three raised ridges. The shapes were then 3D printed on the surface of an airfoil – a wing with a curved aerodynamic cross-section.

Domel, co-first author of the paper, said, “Airfoils are a primary component of all aerial devices. We wanted to test these structures on airfoils as a way of measuring their effect on lift and drag for applications in the design of various aerial devices such as drones, airplanes, and wind turbines.”

20 different configurations of denticle rows, sizes, and row positions were tested on airfoils placed inside a water flow tank. In addition to decreasing the drag, the 3D printed structures also increased the airfoils’ lift, essentially becoming low-profile, high-powered vortex generators.

Bertoldi said, “You can imagine these vortex generators being used on wind turbines or drones to increase the efficiency of the blades. The results open new avenues for improved, bioinspired aerodynamic designs.”

Now, if you’re like me, you’ve probably never heard the term vortex generator. But, also like me, it’s likely that you’ve seen one being used – both planes and cars employ these small, passive devices, which use a blade-like design to make a moving object more aerodynamic by altering the air flow over its surface.

“These shark-inspired vortex generators achieve lift-to-drag ratio improvements of up to 323 percent compared to an airfoil without vortex generators. With these proof of concept designs, we’ve demonstrated that these bioinspired vortex generators have the potential to outperform traditional designs,” said Domel.

The researchers have come a long way since using shark skin-inspired designs to improve the efficiency of boats. The team’s research introduces new ways to improve the aerodynamic design of vehicles that travel in the air and on land, and offers a new perspective on shark skin.

Lauder said, “This research not only outlines a novel shape for vortex generators but also provides insight into the role of complex and potentially multifunctional shark denticles.”

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[Source: Harvard]