Robotic Tuna Simulates Fish Design
Posted February 1, 1998
A dolphin frolics and leaps in the wake of a ship for miles. When the power needed for the dolphin to move at 20 knots is calculated, it is found that the dolphin
is too weak by a factor of seven to attain such speeds. Fishes and dolphins use the flow mechanisms around their bodies in the water to achieve, in some cases, more than 100 percent
efficiency.
Fishes use their streamlined bodies and principles of fluid mechanics to achieve extraordinary propulsion efficiencies, acceleration, and maneuverability. Yellowfin
tuna can swim at speeds of 40 knots. Other fishes can reverse direction without slowing down and with a turning radius of 10 to 30 percent of their body length. In contrast, ships must
decrease their speed by more than 50 percent before reversing. The turning radius of a ship is at least 10 times greater than that for a fish of corresponding size. Considering the huge
number of passengers and amount of cargo hauled by ships, an increase in the efficiency of the movement of ships would result in a huge savings in fuel, fewer accidents, and greater
safety for passengers and the environment. Other applications of greater maneuverability would enable robotic, free-swimming craft to explore deep oceans, and help maintain offshore
oil installations.
Using a replica of a bluefin tuna called Robotuna, biologists are gathering information about how fishes swim. Fishes recapture energy created in their own wake and
extract energy from waves and other turbulence in water. Fishes time the flapping of their tails to create vortices, small whirlpools in the water, to boost swimming efficiency. The
initial forceful flap of a fish's tail makes a large vortex. Another flap immediately following produces a counterrotating vortex. When the two vortices meet and combine, a jet is created
pushing away from the tail. The precise control, timing, and spacing of vortices is the primary control over the efficiency of swimming in fishes. Scientists hope to use the information
they are learning about swimming efficiency in fishes to design and construct more efficient ships.
References
Triantafyllou, Michael S. and George S. Trianatafyllou. "An Efficient Swimming Machine," Scientific American, March 1995, pp. 64-70.
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