‘Jet Propulsion’ of Squid Could Enhance Design of Underwater Vehicles

A Caribbean reef squid. (“Betty Wills (Atsme), Wikimedia Commons, License CC-BY-SA 4.0)

(CN) — Scientists in Scotland, the United States and China are hard at work studying the mechanics behind the swimming techniques of squids and other cephalopods, with the ultimate goal of applying the research to underwater technology.

In a study published Tuesday in the journal Physics of Fluids, authors Yang Luo, Qing Xiao, Qiang Zhu, and Guang Pan discuss their breakthrough findings on these impressive swimmers.

New technologies have taken many lessons from nature into design consideration, from tentacle-like robotic arms that hold and observe jellyfish to drones with flapping-wing flight styles capable of pollinating plants. Now, scientists are looking to squids and their form of jet propulsion swimming for inspiration for future underwater vehicles.

When swimming, squids will often use the fin on their heads to maneuver through the water, but for faster motion they will use jet propulsion. To move themselves forward, they will fill their respiratory mantle cavity with water and quickly contract those muscles, releasing the water in a single stream to propel themselves away. They can do this several times in quick succession, only needing to refill their water supply before blasting off again, granting them a quick escape from a predator or a swift attack to catch their prey.

However, there are still a great deal of unknown variables that affect this efficient swimming technique, including variations to adapt to turbulent flow in the water. In the new study, the authors recreated cephalopod jet propulsion in a model to analyze this locomotion under turbulent conditions and gain insights for underwater machinery.

The researchers designed a 2-D squid-like model capable of performing jet propulsion underwater. It was equipped with a flexible mantle body, a pressure chamber within, and a nozzle meant to simulate the flow of water expelling from the body. In this recreation, the researchers put pressure on the body, like the squid’s constricting mantle muscles.

“As a result, the internal volume of the body decreases and water inside the chamber is ejected to form a jet flow,” said co-author Yang Luo, a research assistant at the University of Strathclyde in Glasgow, Scotland. “The squid is propelled forward by the strong jet in the opposite direction, then the mantle inflates automatically as a result of stored elastic energy. During inflation of the mantle, water is sucked into the chamber and gets ejected during the next mantle deflation.”

Luo explained that cephalopod jet propulsion is actually more effective in turbulent water than in constant streams of water, known as laminar flow. They found that turbulent water resulted in opposing flows colliding around the squids, creating forceful vortices.

“This may help provide a better understanding of why burst-and-coast swimming is used by juvenile and adult squids that operate within turbulent flows more frequently compared with squid hatchlings that operate within laminar flows,” Luo said.

The researchers believe that when the squid moves through turbulent waters and multiple opposing flows collide, a vortex and a subsequent phenomenon called symmetry breaking are created. This determines which direction the squid will be thrusted in, but with jet propulsion, the cephalopods are able to avoid these unstable vortices by darting out of the way.

“The findings of our work about the mechanism of symmetry-breaking instability provides guidance for the design of squid-inspired underwater robots and vehicles,” Luo said. “Continuous jet propulsion may not be favorable, and specific measures are needed to mitigate the effect of this instability during the design of jet propulsion-inspired underwater vehicles or propulsors via active control of body deformation to change the evolution of the internal vortices pattern.”

Moving forward, the researchers will work on how to incorporate this information into their designs for bio-inspired underwater technology. Underwater robots provide an abundance of advantages, as they are able to traverse depths unreachable by people and contribute to scientific research, but they also face many challenges. Unmanageable pressure, corrosion from ocean materials and damages from the ocean environment can all hinder their progress. Taking notes from the ocean’s current inhabitants could be key to securing the success of these robots.

“It’s difficult to determine at this point,” Luo said when asked about the possibility of jet propulsion-powered submarines in the near future. “But as a relatively less extensively studied form of underwater propulsion, it is advantageous in terms of a straightforward mechanism for effective instantaneous escape and high maneuverability. This makes it promising for integrating with typical thruster propulsion to achieve on-demand maneuverability.”

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