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Follow that pike - A Lycra skin could reveal how a fish cuts hydrodynamic drag

点击量:   时间:2017-05-17 07:02:12

By Mark Ward SHOALS of robotic pike may soon help to reveal why fish are so much more efficient at swimming than they should be. The first free-swimming robotic fish is nearing completion at the Massachusetts Institute of Technology. John Kumph and his colleagues at the department of ocean engineering have built an 80-centimetre-long model of the chain pickerel, a species of pike found in rivers throughout North America. He chose the pike because it is renowned for its speed and turning ability. In building the pike, Kumph drew on earlier work on a robotic tuna (This Week, 1 October 1994, p 22), which was tethered to a floating sledge and controlled through cables. But unlike the tuna, the pike will be free-swimming, controlled through a radio in its fibreglass nose cone. The nose cone is rigid and the pike’s skeleton is a spiral fibreglass spring. This makes the body strong and flexible enough to perform a strong swimming stroke. The main thrust comes from the side to side movement of the tail fin. This movement is generated by a servomotor pulling on tungsten cables attached to a spline along the backbone of the fish. Two small servomotors taken from model aircraft are used to move the pike’s pectoral fins and steer it. The fish is weighted so that it swims just below the surface. Tests last year in a 110 000-litre tank proved that the robopike could swim strongly and turn easily. All the servomotors are located in the rigid nose cone, because the constant flexing of the body could damage them and limit their effectiveness. The skin of the fish is made of Lycra stretched over a steel mesh. This is flexible enough to allow a powerful swimming stroke but strong enough to resist the pressure of water. Kumph says the tuna and pike were built to resolve Gray’s paradox—why fish can swim so fast. In theory fish should not be able to swim as fast as they do. “Fish do not seem to have the muscle power to swim at the speeds they do,” says Kumph. Although the robopike is a relatively crude model, Kumph says it is more efficient than it should be. Kumph is now working with Rodney Brooks, director of MIT’s artificial intelligence laboratory to refine the design of robopike and make it behave more like a real fish and to swim convincingly. Once these studies of the pike’s swimming technique are complete the researchers will begin looking at the flow of water over the Lycra skin, to see how the fish reduces its hydrodynamic drag. The results could help naval architects design much more efficient boats. “If we can reproduce that low drag without needing an undulating mechanism, we would be very rich,