The shellbound chambered nautilus is a champion of underwater jet propulsion. It can eke out efficiencies as high as 75%, far outclassing other jet-based swimmers like squid, salps, and jellyfish. That high efficiency is especially important for the nautilus, which spends a great deal of time at depths where the oxygen needed to fuel movement is in short supply. To get around, the nautilus draws water in through an enlarged orifice, then squirts it out little by little. Its this asymmetry between drawing in and expending that keeps efficiency high. By releasing a jet slower and at lower speeds, the nautilus is able to reduce wasteful losses to friction and thereby keep the efficiency high. The drawback is that the nautilus swims relatively slowly at an average of around 8 centimeters–less than one body length–per second. (Image credit: Simon and Simon Photography/University of Leeds; research credit: T. Neil and G. Askew; via NYTimes; submitted by Kam-Yung Soh)
Tag: swimming
Swimming Like a Balloon
For humans, swimming is relatively easy. Kick your legs, wheel your arms, and you’ll move forward. But for microswimmers, swimming can be more complicated. For them, the world is a viscous place, and the rules that we swim by can’t help them get around. In a highly viscous world, flows are reversible. Kick one limb down and you might move forward, but when you pull the limb up, you’ll be sucked right back to where you started. So microswimmers must use asymmetry in their swimming. In other words, their recovery stroke cannot be the mirror-image of their power stroke.
A new study suggests that simple elastic spheres could make good microswimmers through cyclic inflation and deflation. When the sphere deflates, it buckles, making a shape unlike its inflating one. This difference in shape change is enough to propel the sphere a little with each cycle. Right now the test system is a macroscale one, but the researchers hope to continue miniaturizing. (Image and research credit: A. Djellouli et al.; via APS Physics; submitted by Kam-Yung Soh)
Schooling Together
Since the 1970s, fluid dynamicists have chased the idea that fish swim in schools for hydrodynamic advantage. The original 2D conception of the idea placed fish in a diamond pattern so that their wakes would constructively interfere and improve swimming efficiency. In nature, that exact pattern is rarely seen, possibly due to 3D effects or the difficulty of maintaining the exact orientation. Fish do, however, show signs of grouping themselves for efficiency – especially when they’re forced to swim quickly.
A recent study found that tetras, a type of small fish often used as pets, prefer a staggered diamond configuration (left) when free-swimming at low speeds around one body length per second. At higher speeds, around four body lengths per second, groups of tetras preferred a side-by-side or “phalanx” configuration (right). Here the fish tended to synchronize their tail-beat frequency with their neighbors, essentially working together for a mutually beneficial wake structure. The researchers found that this configuration was much more efficient than a lone swimmer or uncoordinated group, implying that fish do school for energy-savings when they’re swimming fast. (Image and research credit: I. Ashraf et al., source; via Hakai; submitted by Kam-Yung Soh)
Bioluminescent Plankton
In nutrient-rich marine waters, dinoflagellates, a type of plankton, can flourish. At night, these tiny organisms are responsible for incredible blue light displays in the water. The dinoflagellates produce two chemicals – luciferase and luciferin – that, when combined, produce a distinctive blue glow. The plankton use this as a defense against predators, creating a flash of blue light when triggered by the shear stress of something swimming nearby. The dinoflagellates respond to any sudden application of shear stress this way, so they glow not only for predators, but for any disturbance – mobula rays (above), sea lions, boats, or even just a hand splashing in the water. In person, the experience feels downright magical. I had the opportunity to experience bioluminescence in the Galapagos last year. The light from the dinoflagellates is incredibly difficult to film because it can be so dim, but as the BBC demonstrates, it’s well worth the effort it takes to capture. (Image credit: BBC from Blue Planet II and Attenborough’s Life That Glows; video credit: BBC Earth)
Galapagos Week: Sea Turtles
It’s easy to imagine sea turtles as slow and awkward given our familiarity with their terrestrial cousins, tortoises, but this could hardly be further from the truth. There are currently seven living species of sea turtles and all use a mode of locomotion known as aquatic flight. As the name suggests, swimming sea turtles share a lot in common with birds and other fliers. They generate most of their propulsion by flapping their forelimbs. Like birds, they change the angle of attack of their flippers over the course of both their upstroke and downstroke.
Of course, a cruising sea turtle is more interested in thrust than lift, but the efficiency of flapping is far higher than that of a rowing motion. That holds true across a range of speeds and is probably why marine turtles, known for their vast migrations, predominantly use flapping. It’s also remarkable how fast they can move when they want to. The animations above show two species of sea turtles cruising casually at a speed where a snorkeler in fins could follow along. But when the turtles wanted to, they could take off at a clip no human could hope to match! (Image credit: N. Sharp; research credit: J. Davenport et al., J. Walker and M. Westneat, H. Prange, E. Dougherty et al.)
Today’s post wraps up Galapagos Week here at FYFD, but there’s plenty more Galapagos-relevant fluid dynamics to go around. Here are some previous, related posts: how frigatebirds cruise the seas without getting wet; aerodynamics of flying fish; hydrodynamics of humpback whales; incredible bioluminescent plankton; and leaping mobula rays.
Galapagos Week: Marine Iguanas
One of the most unique inhabitants of the Galapagos Islands is the marine iguana. These reptiles live in colonies of thousands and subsist entirely on marine algae. Smaller iguanas are intertidal feeders, grazing on green and red algae when it is exposed near low tide. But the largest iguanas feed near midday by swimming out and diving to feed on richer pastures.
The iguanas are surprisingly good swimmers, even though marine iguanas exhibit little extra specialization for it compared to other iguana species. They swim both at the surface and underwater with an undulatory motion driven by their tails. The iguana also streamlines its body somewhat by tucking its legs along its sides. Although the marine iguana is a much slower and less efficient swimmer than a bony fish of equal size, swimming is still a good choice for getting around. The marine iguana expends only 75% as much energy per distance swimming as it does walking. The big challenge is staying warm in the cold Galapagos waters. Small iguanas are both less efficient swimmers and lose body heat faster. This is why you’ll only see the biggest iguanas feeding underwater. (Image credits: N. Sharp; research credit: K. Trillmich and F. Trillmich; J. Videler and B. Nolet; G. Bartholomew)
This is the first post of Galapagos Week here on FYFD. Check back every day for new Galapagos-themed posts!
Optimal Swimming
What do trout, sharks, and whales have in common? All are fast swimmers and share remarkable similarities in their swimming dynamics despite different sizes, shapes, and environments. A new study analyzing aquatic locomotion examines the characteristics of these swimmers. The researchers found that a typical parameter for studying swimming fish – the Strouhal number, which relates swimming speed, body length, and tail-beat frequency – only tells part of the story. When cruising at minimum power input, a fish cannot choose its Strouhal number – that characteristic is completely determined by the fish’s shape, which determines its drag.
Instead, researchers found that a second additional number – the ratio of the tail-beat amplitude to the body length – was also needed to describe optimal swimming. Taken together, their model predicts that optimal swimming performance lies within a narrow range of the two numbers. And when the researchers examined cruising behaviors of a diverse variety of fish and whales, they found that they did indeed swim in the ranges predicted by the model. Now that we better understand characteristics of efficient swimming, engineers can use the model to guide designs of new biologically-inspired robot swimmers. (Image credit: N. Sharp, source; research credit: M. Saadat et al.)
Chains of Salps
Salps are small, jellyfish-like marine invertebrates that swim by ejecting a pulsatile jet. They are unusual creatures whose lives have two major stages: one in which salps swim individually and one in which they link together and swim in large chains. In the chain, salps don’t synchronize their jetting; each salp jets with its own phase and frequency. A new study suggests that, in spite of this lack of synchronicity, the salp chain’s swimming reduces the animals’ drag. There are several factors that contribute to this result. One is that drag is generally lower on a body moving at constant speed compared to one moving in bursts. When linked together and firing randomly, all the individual jets tend to average out into one continuous swimming speed. There’s even a benefit to being out of sync: previous work showed that synchronized jets lose some of their thrust when they are too close together. Salps avoid that loss by keeping to their own beat. (Image and research credit: K. Sutherland and D. Weihs, source; via Gizmodo)
The Hydraulics Behind a Tuna’s Turns
Tuna are remarkably agile for their size. Many species reach lengths exceeding the height of a human adult, yet they can still make tight turns, especially when hunting. A recent study described one mechanism that aids the fish – a built-in hydraulic system for raising its second dorsal and anal fins. The tuna use fluid from their lymphatic system – which produces and transports white blood cells in both humans and tuna – to pressurize chambers at the base of some fins, causing the fin to rise. The extra support puts the fin in a hydrodynamically advantageous position and helps stabilize the fish when turning quickly, allowing them to change direction without slowing. (Video credit: Science; research credit: V. Pavlov et al.)
The Japanese Pufferfish
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If you’ve ever dived or snorkeled over a sandy lake or ocean bottom, you’ve probably seen some neat patterns there. But it’s hard to compete with the Japanese pufferfish for pure artistry. This small fish creates enormous and elaborate designs in the sand in order to attract a mate. The male fish moves the sand into place by flapping his fins very close to the surface. Above a critical flapping velocity, his fins generate vortices capable of picking up sand, as seen below. With repeated passes, the fish is able to excavate the trough that is key to his creation. It’s a constant fight against the current, though.
Puffers aren’t the only ones who flap their fins to move the sands. Rays and flounders use this technique to bury themselves and hide (Video credit: BBC Earth; image credit: A. Sauret, source; research credit: A. Sauret et al.)
