Eels have been observed to swim for thousands of kilometres without feeding during their migration period, and experiments with an eel robot may help explain how they do it. They suggest eels can swim in a more energy-efficient manner by changing the way they undulate their bodies.
at the Swiss Federal Institute of Technology Lausanne and his colleagues built a waterproof robot with an eel-like 鈥� or 鈥渁nguilliform鈥� 鈥� body shape. The robot, which they named 1-guilla, is 85 centimetres long and comprises a head that houses a battery and a computational unit, eight motorised segments and a flexible tail.
They programmed 1-guilla to undulate its long and slender body in several distinct ways, then studied how each undulation pattern affected the speed and efficiency of its swimming.
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First, the researchers programmed 1-guilla to repeatedly form an S shape, then stretch out straight, creating what is known as a 鈥渟tanding wave鈥� with its body. But this only made the robot thrash in place instead of swimming forward. To get it to move forward, the team had to programme 1-guilla to undulate so that the S-shaped curves seemed to migrate down the length of its body, creating so-called 鈥渢ravelling waves鈥�.
The team also found that 1-guilla could increase its speed by bending its body in a more extreme way to make more pronounced travelling waves. This seems to be because, when these waves reached 1-guilla鈥檚 tail, they swished it further from side to side and provided more forward propulsion.
However, this did not lead to efficient swimming. In fact, 1-guilla gained the most speed for the amount of power it drew from the battery 鈥� a measure of efficiency called cost of transport 鈥� when the motion of its body was the most unlike a standing wave and most like a travelling wave, and also when the swish of its tail was less pronounced. In some cases, the difference between the cost of transport of the two types of wavy motions was as much as 50 per cent.
Consequently, the researchers argue that the eels making long trips in nature are not achieving their fastest possible speeds but are performing these less power-hungry swimming motions.
The researchers鈥� video of 1-guilla in action was one of the winners of the competition at the annual meeting of the American Physical Society鈥檚 Division of Fluid Dynamics in Washington DC in November.
