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In 1996, Dieter Gürtler and his colleagues from the Mercedes Technology Center in Sindelfingen, Germany, were looking for a model for a holistically conceived bionic car, respecting at once physics, design, and aerodynamics. So he turned to Ronald Fricke, head of the ichthyology department at the Rosenstein Museum in Stuttgart, with its huge collection of preserved fish.
They were "thinking about a bionic car and ? fish models for it," Fricke recalls. The team's first instincts were to use streamlined sharks, penguins, dolphins, and tunas. But on a visit to the museum, it became clear that their "ideas of tunas and sharks were not useful to design cars, which contain a cubicle for passengers, while swiftly-swimming fishes are compressed," Fricke says. "It was our idea to choose a slowly but steadily swimming fish, and the boxfish was the first option."
The boxfish, found in coral reefs, has great structural strength but low mass. Despite the ungainly appearance of the fish, it also has extremely low flow resistance, with a drag coefficient of an incredible 0.06. For comparison, a penguin flying through water is considered extremely aerodynamic with a coefficient of 0.19. More streamlined, lower-drag fish do exist, but they are not as rigid or maneuverable as the boxfish, nor do they have a relatively huge cross section.
"Boxfishes interest biologists because they have very different morphologies from most flexible-bodied fish, yet they are highly maneuverable," according to Ian Bartol, an American boxfish researcher at Virginia's Old Dominion University. His work is funded by the Office of Naval Research, which is interested in designing more energy-efficient and stable underwater vehicles.
"From an applied standpoint," says Bartol, "locomotive studies on boxfish can be used by engineers and designers who are interested in biomimetic solutions to stability control, maneuverability, and even drag reduction." He points out the trade-off (usually found in both nature and engineering design) between stability and maneuverability. The boxfish displays both. The stability arises from the unique vortices generated by body shape and features, as does enhanced propulsion.
Fricke and his colleagues created a female Ostracion meleagris model, which they gave to DaimlerChrysler. "The female boxfish was chosen," according to Fricke, "because males have a protuberance on their head, not useful for a car, and besides, the females are stouter, which is good for a car shape." As the company began working on a prototype, "it became clear that the boxfish had an excellent aerodynamic shape, and even the car prototype was in the top range." Fricke notes, "Cars obviously have some [aerodynamic] disadvantages compared to fish. For example, they have to have windshield wipers and wheels. ? Also, the snout was reduced for aesthetic reasons ? and on the surface of the car, it wasn't possible to take advantage, for now, of the star-like structure of the scales, which add to aerodynamics and stability. No such surface structures are possible in the car."
The resulting concept car, introduced in June 2005 at a Washington, DC, conference, is "a complete transfer from nature to technology," Mercede's Thomas Weber says. The boxfish "has to move with as little energy consumption as possible ? withstand high pressures, and protect its body in collisions ? and move around in confined spaces ? [while] its rectangular anatomy is nearly identical to the cross-section of a car body." The car's drag coefficient is equal to that of a penguin. By comparison, a car with a coefficient of 0.26 is considered very well designed, indeed.
After the prototype was completed, the DaimlerChrysler team returned to the museum to see boxfish morphology in greater detail. "When we dissected a boxfish specimen," Fricke says, "we found an interesting heat-exchange mechanism close to the gill opening, apparently to cool down the blood temperature within the box of the boxfish." The engineers didn't use this exchange mechanism in the prototype, "but it's an important discovery for future applications, because the gill openings are very small compared to the cooling openings in cars, but still very effective."
"It's exciting that Mercedes-Benz has used the boxfish, but understandable, because it expresses so well the design constraints of a car: rigid, low drag, and a big cross-section," says Bartol, who adds that the concept car is "a good example of how some of nature's solutions together with human ingenuity and creativity can help advance technology." There was one element of the boxfish's anatomy that really impressed DaimlerChrysler: a three-pointed star on each of its scale plates, which just so happens to resemble Mercedes' famous logo. "However," Fricke laughs, "they didn't use that, some solutions are just too unfamiliar."