22 In conversation | Dr David Barrett has an equal and opposite reaction, which is the thrust. In this context, Dr Barrett characterises thrust as negative drag. “I have built bodies that have massive amounts of negative drag,” he says. “Gray’s numbers are something like a 10 or 20to-1 multiplier, and I can get 2 or 3-to-1 here, but even so that means a 100 mile mission range could become 200 or 300 miles with the same battery.” The difference between Gray’s numbers, representing nature’s achievements in drag reduction, and Dr Barrett’s current best means there is a lot more to come. Sensing and harmonics One important source of future improvements is sensing. In common with most fish, sharks and tuna have a pair of lateral lines that run along the sides of the body that form an array of pressure sensors. “They run from the nose to the tail, and they can sense the pressure changes created by the vortices on both sides of the body so that they know when to release them,” he says. The sensors provide feedback to what is effectively a closed-loop tuned harmonic oscillator in fish, which he contrasts with the traditional bruteforce approach to propulsion, explaining that small forces applied in phase are much more energy-efficient. The body of a tuna, for example, has a mechanism that takes advantage of such harmonics. “The back end is a big cantilevered spring that oscillates at the same natural frequency that we need to shed the vortices to get the warp drive,” he says. “If you take a dead shark and tow it behind a boat, the back end will wiggle. Its mechanical design is set up to oscillate at the frequency it needs to propel itself, so it takes it very little energy.” Dr Barrett and his team have built 3D computer models of tuna musculature, starting by cutting several of the fish into half-inch thick sections and photographing them. “The muscles are attached to cartilage and bone in the front, and to a springy elastic tendon at the back. The tendon is very complex and has sub-tendons and muscles in it, and it is basically a variable stiffness spring.” The fish’s control system is decentralised, using a set of ganglia along the spine to run the oscillating motion, he explains. “It is a central pattern generator and, basically, the June/July 2023 | Uncrewed Systems Technology The robot tuna’s tails are segmented, and consist of pairs of linear Lorentz force electric actuators that exploit natural harmonics to minimise energy consumption and help turn drag into thrust Although we are nowhere near as sophisticated as Mother Nature, we are potentially far better than a traditional AUV in duration and speed
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