New FYFD video! Learn all about salps, vortex rings, and underwater robots. Thanasi Athanassiadis takes me inside his lab and his newly published research into how proximity affects the thrust two vortex rings can produce.
There are a ton of little things I love about how this video came out, especially the chalkboard animations. Check it the full video below and click through to the video description for lots more information about salps and vortex rings.
(Image and video credits: N. Sharp and A. Athanassiadis; Original salp images: A. Migotto and D. Altherr)
Unlike most seabirds, the frigatebird does not have waterproof feathers. Landing in the water during a transoceanic flight would quickly drown the bird, so instead they stay aloft. But until recently, scientists did not realize just how adept the birds are. Studying tagged frigatebirds in flight, researchers found that the birds could reach altitudes of 4000 meters and that they could soar without flapping for up to 64 kilometers! They achieve these heights by seeking out clouds, which mark strong atmospheric updrafts. The birds ride these thermals up to the cloud tops – well into freezing conditions – and then glide slowly back down.
Their bodies are impressively built for slow glides. Frigatebirds boast a low body weight for their large wing area. This ratio is known as wing loading, and it’s a fundamental characteristic of any flier, avian or otherwise. Low wing loading is key to gliding longer because it reduces the speed at which a glider loses altitude. (Image credit: D. Brossard; research credit: H. Weimarskirch et al.; via @skunkbear)
Every year newts move to the water in the springtime to mate before returning to land for the rest of the year. This annual aquatic relocation is accompanied by changes in the newt’s body. Flaps of skin grow from their upper jaw to their lower jaw, partially closing their mouths at the corners. This can be seen in the left column of the animation compared to the center and right.
Numerical simulation shows that this mouth change has a significant impact on the newt’s ability to hunt underwater. Newts are suction feeders, who open their jaws and expand their mouth cavity to suck in water and their prey. By closing off the corners of their mouths during their aquatic phase, the newts generate more suction, reaching peak flow velocities 10% to 50% higher than in their terrestrial form and enabling them to pull prey from 15% further away. When they leave the water, the newts lose the extra flaps so that their mouths can open wider for catching prey on land. (Image credit: S. Van Wassenbergh and E. Heiss, source)
Look closely enough at a shark’s skin, and you will find it is covered in tiny, anvil-shaped denticles (lower left). To try and discover how and why these denticles help sharks, researchers are 3D printing denticles in different patterns onto flexible sheets to create biomimetic shark skin (lower right).
They test the artificial shark skin in a water tunnel by moving it with prescribed motions and measuring different characteristics, like the swimming speed attained and the power required. When compared to a smooth but flexible control surface, one pattern came out ahead. The staggered-overlapped denticle pattern (shown in C of the lower right figure) achieved swimming speeds 20% higher than the smooth control despite having far more surface area due to the denticles. The cost of that speed was only 13% greater than the smooth case on average, and was about equal to the smooth case for small amplitude motion. This suggests that the patterning of a shark’s skin may help it swim faster with little to no additional cost in effort.
Tiny organisms live in a world dominated by viscosity. There’s no coasting or gliding. If a microorganism stops swimming, friction will bring it to a halt in less than the space of a hydrogen atom! To make matters worse, simply flapping an appendage forward and backward will get them nowhere. As we’ve seen before, these highly viscous laminar flows are reversible, meaning that a backward power stroke is simply undone by a mirrored forward recovery stroke. Instead, microorganisms like the paramecium swimming above are covered in tiny hairlike cilia which beat asymmetrically. They extend to their full length during the power stroke, but they stay bent during the forward recovery stroke. That asymmetry guarantees that they move more fluid backward than forward, thereby letting the paramecium make progress. (Image credit: C. Baroud, source)
Desert plants have evolved to efficiently collect and capture whatever water they can. Each leaf of the moss Syntrichia caninervis ends in a hairlike fiber called an awn (seen in white in the top image). Tiny as they are, awns are vital to the moss’s water collection, correlating to more than 20% of their dew collection. Extremely tiny grooves on the surface of the awn provide nucleation sites where dew condensed from fog collects. Once a droplet forms on the awn, it grows larger as more fog condenses (middle image). When the droplet grows large enough, the conical shape of the awn will cause surface tension to draw the droplets along the awn and toward the leaf (bottom image).
(Credits: Syntrichia caninervis moss image – M. Lüth; videos and research – Z. Pan et al., Supplementary Videos 3 and 4; h/t to T. Truscott)
In some countries, there are still people using dynamite to catch fish. This practice is incredibly destructive, not just to adult fish but to the entire marine ecosystem. A blast wave traveling through air loses some its energy to the compression of the gas. Water, on the other hand, is incompressible, so the blast wave’s energy just keeps going, expanding its destructive radius. Many fish contain swim bladders, gas-filled organs the fish use to regulate their depth. When a shock wave passes through the fish, the gas in the swim bladder will expand and contract violently, much like the balloons shown underwater in the animation below. This typically ruptures the swim bladder and surrounding tissues.
Fish without swim bladders will often hemorrhage after being struck by a blast wave. The sudden changes in pressure create bubbles in the dissolved gases collected in their gills. Those bubbles tear apart the fish’s blood vessels.
Blasting is effective but entirely indiscriminate. It kills adults and juveniles of all species, not just the ones a fisherman can sell. Simultaneously, it destroys the slow-growing coral reefs that are key habitats for these populations. It’s an incredibly short-sighted practice that guarantees there will be no fish to catch in years to come. (Video credit: National Geographic; image credit: M. Rober, source; research credit: K. Dunlap, pdf)
Evolution often requires compromise between competing effects. Large-eared bats, for example, rely on the size of their ears to aid their echolocation, but such large ears can hurt them aerodynamically, thus limiting their flight. Results from a recent experiment, however, suggest that large ears are not a total loss aerodynamically speaking. Researchers used particle image velocimetry to study the wakes behind free-flying, large-eared bats and found significant downward flow behind the bats’ bodies. This indicates that the bats generate some lift with their ears, body, and/or tail. The position and tilt of the ears in flight is similar to forward swept wings, which the authors suggest could help contract the wake behind the ears and reduce its negative influence on flow over the wings. Although the evidence is not yet conclusive, the study does suggest that large ears may be more aerodynamically beneficial than they appear. (Image credit: L. Johansson et al./Lund University, source; via Jalopnik)
The next FYFD webcast will be this Saturday, May 21st at 1pm EDT. My guests will be Professor Jean Hertzberg of the University of Colorado at Boulder and Professor Kate Goodman of the University of Colorado at Denver. Dr. Hertzberg is the creator of the course Flow Visualization, an interdisciplinary course combining engineering, art, and fluid dynamics. It’s a class (and website) that’s been an inspiration for me and FYFD since the early days! Dr. Goodman, an expert in engineering education, earned her PhD studying the Flow Viz course and its impact. This will be wide-ranging discussion – with everything from experimental fluid dynamics and engineering education to art, photography, and hopefully even cardiac fluid dynamics!
The blue-outlined dolphins you see above get their glow from microorganisms called dinoflagellates. They are a type of bioluminescent plankton, shown in the lower image, that can be found in oceans around the world. Their glow comes from combining two chemicals: luciferase and luciferin. The dinoflagellates suspended in the ocean do this when they are disturbed–specifically, when the water around them transmits a shear stress above a certain threshold. Typically, this is caused by something larger–a potential predator–moving past, although it can also be stimulated by breaking waves. The higher the shear stress, the more intense the glow, but the dinoflagellates only use their bioluminescence sparingly. If you apply shear stress and keep applying it, their glow fades away without reactivating. After all, they can only produce so much chemical fuel. (Image credit: BBC from Attenborough’s Life That Glows; h/t to Gizmodo; research credit: E. Maldonado and M. Latz)
Birds do not always vocalize in order to make their songs. The male African broadbill, shown in the top video above, makes a very distinctive brreeeet in its flight displays, but as newly published research shows, the sound comes from its wings, not its voice. During the display, the broadbill spreads its primary feathers and sound is produced on the downstroke, when wingtip speeds reach about 16 m/s. By filming a broadbill wing with a high-speed camera in a wind tunnel at comparable air speeds, researchers could localize the sound production to the 6th and 7th primary feathers.
In the second video above, you can see these feathers twisting and fluttering in the breeze. This is an example of aeroelastic flutter, a phenomenon in which aerodynamic and structural forces couple to induce oscillations. The same phenomenon famously caused the collapse of the Tacoma Narrows Bridge in 1940. In the birds, however, the flutter is non-destructive and the vibration produces audible sound which the other feathers modulate into the calls we hear. Broadbills aren’t the only birds to use this trick; some species of hummingbirds use flutter in their tail feathers during mating displays. (Video, image, and research credits: C. Clark et al.; additional videos here)
