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Surfaces and interfaces

Surfaces and interfaces

The turbulent life of dolphins

18 May 2004

Japanese physicists have discovered why dolphins are able to swim so quickly and smoothly through water. Yoshimichi Hagiwara and colleagues at the Kyoto Institute of Technology found that the unusual skin of the dolphin -- which flakes off and is completely replaced every two hours -- plays a crucial role in helping reduce drag effects. The results could help engineers design energy-efficient boats, ships and submarines (H Nagamine et al. 2004 J. Turbulence 5 018)

Any body or object moving through water experiences both friction and so-called form drag due to the pressure of water against it. The streamlined shape of the dolphin obviously helps it to glide through water. However, until now it was not known if the way that the skin of the dolphin flakes off — a process known as desquamation — also helped to reduce drag.

To investigate this, Hagiwara and co-workers built computer models that simulated how a dolphin’s skin interacts with turbulent water flow and how it flakes off. They calculated that the undulating shape of the skin helps to reduce drag. More importantly, they found that the flakes of skin shed by the dolphin lower the drag by reducing the number of vortices that form around the dolphin as it moves through water. Unchecked, these vortices would reduce the dolphin’s speed.

Hagiwara and co-workers then built a model to check their simulations. They used waterproof glue to attach small squares of plastic film, measuring 1.5 by 0.8 millimetres, onto a wavy metal plate that represented the skin of the dolphin. The plastic squares gradually detached from the plate as the glue dissolved in the flow of water in a tank.

“This research is important because it gives us a greater insight into the mechanisms dolphins have evolved to cope with travelling through water,” said Hagiwara. “The results could help us build boats, ocean liners and submarines using technology based on these natural solutions.” The Japanese team now plans to improve its models and build more realistic test apparatus that will use soft silicon-rubber to represent dolphin skin.

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