Shark skin and owl feathers could inspire quieter underwater sonar

Sharks and owls are evolutionarily optimized in surprisingly similar ways. When it comes to the ocean’s apex predator, their skin’s textured patterns, known as riblets, help cut down on drag. With owls, their tiny feather ridges called serrations allow them to fly silently while hunting prey.

Although the naturally-occurring aids have inspired biomimicry-based aeronautic designs in the past, a collaborative team of researchers from the University of California, Berkeley and MIT Lincoln Laboratory recently investigated if these same principles could also apply to underwater tools. Their findings, published in a new study in Extreme Mechanics Letters, indicate the designs could be adapted to improve the towed sonar arrays (TSAs) utilized by ships and submarines.

TSAs are vital for marine vessels engaged in underwater security or exploration projects. But if ships start cruising at decent speeds, the ensuing drag around the equipment can generate extra noise that interferes with sonar capabilities.

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Utilizing computational modeling, researchers tested various riblet shapes and patterns interacting with simulated water environments. From calm currents to the more commonly unpredictable flows seen in oceans, the team observed how smooth, triangular, trapezoidal, and scalloped riblets might affect fluid dynamics and acoustics.

Of these variations, the rectangular form showed the most promising results in choppy water—reducing noise by over 14-percent alongside a roughly 5 percent reduction in drag. When the riblets were finer and closer to one another, drag could be reduced by as much as an additional 25 percent.

These simulations not only showcased potential riblet patterns for sonar casings, but also illuminated new fluid dynamics that underpin noise reduction during turbulent water flows. In a process researchers call “vortex lifting,” flows are elevated and redirected away from the textured surfaces while also lowering their rotational strength.

“This elevation is key to reducing the intense pressure fluctuations that are generated by the interaction between the water flow and the array wall, leading to noise production,” Zixiao Wei, a mechanical engineering graduate student and study first author, said in a recent statement.

The team also noted that adding the animal-inspired textures to TSAs and other underwater vehicles wouldn’t just help humans—it could improve habitat conditions for marine wildlife, as well. Systems reliant on riblet patterns could make for quieter operating, thereby reducing the chances of artificially disturbing their surrounding ecosystems.

That said, it’s one thing to simulate shark skin—actually replicating it has proven extremely difficult. But with additional testing and deployment, Wei believes the new designs will showcase “the vast potential of biomimicry in advancing engineering and technology.”