21 fish and dolphins shouldn’t be able to swim as fast as they do,” he explains. Working in the 1930s, British zoologist Lord Gray calculated that a typical dolphin has the muscle to generate about half a horsepower. He found that according to the basic principles of fluid mechanics, that power should enable it to reach just 4 knots, but dolphins can swim at up to 30 knots. “The paradox is that dolphins and tuna have what looks like warp drive for underwater vehicles,” Dr Barrett says. A traditional AUV is a cylinder with a propeller on the back, and is severely limited by drag, which increases with the square of the speed. So most AUVs have speed, range and endurance limitations that restrict them to the continental shelves around the world, if launched from the coast. “Mike’s work at MIT was about how to break the range barrier, so we can explore the ocean, work on fisheries, solve global warming and do all the other things that undersea vehicles could be good at,” Dr Barrett says. “There are three ways to solve this. One is to build much better batteries, another is to make everything nuclear powered – but no nation would risk thousands of nuclear-powered mini-subs drifting around the ocean and washing up on beaches – and the third is to find a way to dramatically increase propulsive efficiency. And so we got into this by trying to solve Gray’s paradox. “In water, Gray’s paradox is like the sound barrier in air. Fish and cetaceans, particularly the large pelagic predators – sharks, tuna, dolphins and killer whales – are doing something that a conventional rigid-hulled vehicle is not doing that lets them have these enormous propulsive efficiencies. We are nowhere near fully solving the problem, but we have been able to drop the drag on a wiggly body down almost to zero,” he says. Wiggly drag reduction Wiggling, it turns out, is central to the natural secret that Prof Triantafyllou’s team figured out, because it plays a role in vortex-shedding in a manner that turns drag into thrust. Dr Barrett explains that the vortices, or ‘vorticity’, forms in the boundary layer between a moving body and the mass of stationary water surrounding it. “At the boundary is a viscid layer that is rolling out vorticity, and the vorticity will pop off the back end of the moving body,” Dr Barrett says. The direction in which the vorticity spins is crucial. In a rigid-hull vehicle moving from left to right in our imagined frame of reference, the vorticity on its port side will spin anticlockwise while that from its starboard side will spin clockwise. They continue to spin when they pop off the back of the vehicle, producing a net flow of water in the direction in which the vehicle is travelling, so that the vessel is effectively pulling the water along behind it. “Flow in this direction is drag, which increases with the square of your speed,” Dr Barrett says. “If you have a wiggly tail you can take the drag vortices fromone side of the body and drop themon the other side. You are turning your drag into thrust.” When a tuna or a shark beats its tail from side to side, each vortex it sheds is flicked to the opposite side of the fish’s line of symmetry, so those that form on the port side are shed to starboard and vice versa, although their direction of spin does not change. This causes them to produce a net water flow in the opposite direction to the fish’s movement, he says, so the fish is effectively pushing the water away behind it. In accordance with Newtonian mechanics, that action Dr David Barrett | In conversation If you have a wiggly tail you can take the drag vortices from one side of the body and drop them on the other side. You are turning your drag into thrust Uncrewed Systems Technology | June/July 2023 The overall shape of the robot tuna, coupled with an understanding of its hydrodynamics, harmonics and control, promise huge range and endurance improvements for AUVs
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