FYFD

Tag: biology

  • Denticles and Sharkskin

    Denticles and Sharkskin

    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.

    For more on shark hydrodynamics, check out my previous posts on the topic, and if you want even more shark science, check out these great videos. (Image credit: R. Espanto; J. Oeffner and G. Lauder; L. Wen et al.; research credit: L. Wen et al., 1, 2)

  • Swimming at Microscale

    Swimming at Microscale

    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)

  • Drawing Up Dew

    Drawing Up Dew

    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)

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    Why Fishing with Dynamite is So Harmful

    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)

  • Flying with Large Ears

    Flying with Large Ears

    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!

    (Original images: P. Davis et al.; B. Moore; L. Swift et al.)

  • Bioluminescent Plankton

    Bioluminescent Plankton

    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)

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    Fluttering Feathers

    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)

  • Bumblebees in Turbulence

    Bumblebees in Turbulence

    Bumblebees are small all-weather foragers, capable of flying despite tough conditions. Given the trouble that micro air vehicles have when flying in gusty winds, bumblebees can help engineers to understand how nature successfully deals with turbulence. Under smooth laminar conditions like those shown in the animation above, bumblebees stay aloft by beating their wings forward and backward in a figure-8-like motion. On both the forward downstroke and the backward upstroke, you’ll notice a blue bulge near the front of the bee’s wing. This is a leading-edge vortex, which provides much of the bee’s lift.

    Researchers were curious how adding turbulence would affect their virtual bee’s flight. The still image above shows the bee in moderate freestream turbulence (shown in cyan). Surprisingly, this outside turbulence has very little effect on the flow generated by the bee, shown in pink. In fact, the researchers found that the bees could fly through turbulence without a significant increase in power. Too much turbulence does make it hard for the bee to control its flight, though. The bee’s shape makes it prone to rolling, and the researchers estimated, based on a bee’s 20 ms reaction time, that bumblebees can probably only correct that roll and maintain controlled flight at turbulence intensities less than 63% of the mean wind speed. (Image credits: T. Engels et al., source; via Physics Focus)

  • Mushrooms Make Their Own Breeze

    Mushrooms Make Their Own Breeze

    Plants and other non-motile organisms have developed some clever methods to disperse their seeds and spores for reproduction. Some plants use vortex rings for dispersal; others make their seeds aerodynamic. Low ground-dwellers like mushrooms must contend with a lack of wind to lift their spores and carry them away. Instead, they use evaporative cooling to generate their own air currents.

    Mushroom caps contain a lot of water and, as that water evaporates, it cools air near the mushroom, just as sweat evaporating off your skin cools you. That cooler, denser air tends to spread, carrying the spores outward. At the same time, the freshly evaporated water vapor is less dense than the surrounding air, so it rises. This combination of rising and spreading is capable of carrying spores tens of centimeters into the air, where the wind is stronger and able to carry spores further.  (Image credit: New Atlantis, source; research credit: E. Dressaire et al.)

  • Hovering Hummingbirds

    Hovering Hummingbirds

    Hummingbirds are incredible flyers, especially when it comes to hovering. To hover stationary and stable enough to feed, the hummingbird’s flapping pattern not only has to generate enough lift, or vertical force, to counteract their weight, but the bird must balance any forward or backward forces generated during flapping.

    As you can see in the animations above, when hovering the hummingbird’s wings move forward and back rather than up and down. When slowed down even further, the figure-8 motion of the wings becomes apparent. This careful motion is key to the hover; it allows the bird to generate about 70% of its lift on the downstroke when the wings move forward and creates the remainder of the lift needed on the upstroke. For much more high-speed footage of hummingbirds, check out the full BBC Earth Unplugged video, but be warned: you may experience a cuteness overdose! (Image credit: BBC Earth Unplugged, source)