UNDERWATER ROBOTS DO a lot of neat things—take photos of underwater volcanoes, track leopard sharks, and explore shipwrecks—but they could still learn a few things from fish. Especially the rocket-fast, insanely agile tuna.
(From Wired / by Menaka Wilhelm) — Tuna are built to cruise across oceans, usually at around 2 mph. But they can crank up to 45 mph at the drop of a snack (Michael Phelps races at around 5 or 6 mph, for comparison). And tuna are agile, too, able to whip after fast-turning squids or sardines. They owe their agility, in part, to a newfound hydraulic system that allows them to raise and lower some specialized fins. These sickle-shaped fins on the top and bottom of its body aren’t for thrust. At full extension, they stabilize the fish’s body at high speeds. When the fins are lowered, a tuna can turn on a dime. And as researchers report in a study published today in Science, the fact that these fins are controlled with the help of hydraulics—and not muscles alone—could teach them a lot about bringing underwater robots up to speed.
In the early days of bioinspired robotics, engineers came up with the robotuna, a machine intended to cruise as well as its scaly role model. Robots have gotten better at being in the ocean since then, but aquatic life still beats them in the power category. Fish don’t run out of batteries. They’re also never held back by fiber-optic tethers. Your average wild tuna, with help from a myriad of specialized features, maneuvers and cruises continuously for more than a decade. But roboticists may be in luck—biologists are still discovering new ways that fish function underwater.
Finding a totally new piece of the tuna’s anatomy intrigued Barbara Block, who has been studying fish at Stanford’s Hopkins Marine Station for decades. A biomimetics researcher in her lab, Vadim Pavlov, was studying tuna fins to develop tracking tags when he dissected his way to terra incognita. “We found this strange system of channels, muscles and bones,” says Pavlov. “They looked very strange, and they looked like disconnected pieces of a big puzzle. We had no idea what they did.”
Like any good dissector, he injected a dyed silicone gel through the network, mapping the channels in bright blue. The dye highlighted a large chamber at the base of the sickle-shaped median fins, and smaller channels nestled between the fish’s back muscles and the fin also turned blue. Color spread into the fin, too, darkening separate channels around the bony rays that hold the fin up like the poles of a tent. It looked like the tuna could contract muscles at the base of the chamber to pressurize it, squeezing fluid out into the smaller channels and elevating its fin.
There was only one way to be sure, though. Pavlov targeted a spot just behind the fin—probably the least leaky route to the chamber beneath—and used a syringe of saline solution to artificially pressurize the main chamber. Bingo. The fin popped right up.
That was exciting, but it still left lots of questions about how the hydraulic system actually aids tuna maneuvers. Proving something on a dead fish doesn’t quite cut it.
So Block and her team analyzed high speed video of tuna schools in 20,000 gallon research tanks, looking to see if swimming tuna indeed raise and lower their fins at the angles that Pavlov experimentally observed. Pavlov translated those measurements into a computerized fluid dynamics model. By looking at the lift and drag on a fish in 120 simulations of swimming, he found that raising those median fins stabilizes a tuna like a yacht keel—preventing roll-over. In quick turns, backing off on stability sped up maneuvering.
Read the full story here: https://www.wired.com/story/tuna-fish-school-human-engineers-in-hydraulics/