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Yfke van Bergen

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Watching ideal smoke rings in their laboratories, engineers with an interest in biology have often wondered whether squid use vortex ring propulsion to glide around their aquatic homes. But Mark Grosenbaugh and Erik Anderson of Woods Hole suspected that something quite different was going on, and set out to examine squid locomotion. It all began with an innocent equation for the ideal smoke ring: L/D=4, meaning that an ideal vortex ring results if the length of a fluid jet is four times the diameter of the pipe emitting the fluid. From his previous work, Anderson knew that this wasn't true for squid pushing water out of their 1 cm-wide jet nozzle. To settle the matter, Anderson and Grosenbaugh applied advanced strobe photography to the extremely tricky problem of visualising squid jet flow (p. 1125).

To see the flow of water behind steadily swimming squid, they placed microscopic silver-coated spheres into the water and illuminated them with a sheet of laser light. Pointing a digital camera at the light-reflecting spheres, they compared successive frames to see exactly how the spheres, and therefore the water, moved as squid swam through a flume. Anderson and Grosenbaugh saw that squid don't puff along on individual vortex rings, but instead eject a prolonged fluid jet that is normally 8 to 34 times the diameter of the squid's jet nozzle.

“But the `Aha!' moment really came when we realised that the background flow of water past the squid tends to discourage vortex ring propulsion in steady swimming,” Anderson says. He explains that most previous research examined jet flow issued into stationary water, but clearly, water flowing around the squid in its natural home influences jet structure. Ultimately, Anderson says, `all that's important is what the data tell us is really happening in nature. What may be ideal from an engineering standpoint may not be ideal for a particular niche. Our work illustrates the need to let biology speak for itself, especially in biomechanics.'