Feb 7 2018
Researchers are taking inspiration from an unlikely source, the ocean, to design more aerodynamic machines.
Environmental scanning electron microscope (ESEM) image of denticles, from the shortfin mako shark (scale bar: 200 mm) (Image credit: Harvard University)
A team of evolutionary biologists and engineers at Harvard University, in partnership with colleagues from the University of South Carolina, have provided insight into a decades-old mystery about sharkskin and, in the process, demonstrated a new, bio-inspired structure that could enhance the aerodynamic performance of planes, drones, wind turbines and cars.
Sharks and airplanes aren’t really all that different. Both are engineered to efficiently move through a substance (water and air), using the shape of their bodies to produce lift and decrease drag. The difference is that sharks have approximately a 400 million-year head start on the design process.
“The skin of sharks is covered by thousands and thousands of small scales, or denticles, which vary in shape and size around the body,” said George Lauder, the Henry Bryant Bigelow Professor of Ichthyology and Professor of Biology in the Department of Organismic and Evolutionary Biology, and co-author of the research. “We know a lot about the structure of these denticles — which are very similar to human teeth — but the function has been debated.”
A lot of research has concentrated on the drag reducing properties of denticles, but Lauder and his team speculated if there was more to the story.
“We asked, what if instead of mainly reducing drag, these particular shapes were actually better suited for increasing lift,” said Mehdi Saadat, a postdoctoral fellow at Harvard and the study’s co-first author. Saadat holds a joint appointment in Mechanical Engineering at the University of South Carolina.
To help investigate that hypothesis, the researchers partnered with a team of engineers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS). For inspiration, they studied the shortfin mako, the fastest shark in the world. The mako’s denticles have three raised ridges, like a trident. The team imaged and modeled the denticles in three dimensions using micro-CT scanning. Next, they 3D printed the shapes on the surface of a wing with a curved aerodynamic cross-section, called an airfoil.
“Airfoils are a primary component of all aerial devices,” said August Domel, a Ph.D. student at Harvard and co-first author of the paper. “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.”
The team tried out 20 different configurations of denticle sizes, rows and row positions on airfoils inside a water flow tank. They discovered that besides reducing drag, the denticle-shaped structures greatly increased lift, serving as high-powered, low-profile vortex generators.
Planes and cars are fitted with vortex generators, which are engineered to modify the air flow over the surface of a moving object to make it more aerodynamic. A majority of the current vortex generators in the field have a simple, blade-like design.
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 bio inspired vortex generators have the potential to outperform traditional designs.
August Domel, Ph.D. Student
“You can imagine these vortex generators being used on wind turbines or drones to increase the efficiency of the blades,” said Katia Bertoldi, William and Ami Kuan Danoff Professor of Applied Mechanics at SEAS and co-author of the study. “The results open new avenues for improved, bio-inspired aerodynamic designs.”
“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,” said Lauder.
The Harvard Office of Technology Development has protected the intellectual property relating to this research and is looking at commercialization opportunities.
The study was co-authored by James Weaver of the Wyss Institute for Biologically Inspired Engineering at Harvard, and Hossein Haj-Hariri, Dean of Engineering and Computing at the University of South Carolina. The Office of Naval Research and the National Science Foundation supported this research.
The study is published in the Journal of the Royal Society Interface.