FYFD

Tag: science

  • Featured Video Play Icon

    Pigeon Flutter

    Birds are well-known for their vocalizations, but this isn’t their only way to produce noise. A new study on crested pigeons finds that the birds’ wings produce distinctive high and low notes during take-off. A low note takes place during each upstroke, and a high note is heard during the downstroke. A major source of the noise is the highly modified P8 feather. When airflow over the feather is fast enough, it sets off twisting and torsion in the feather through aeroelastic flutter. It’s this vibration that causes the noise. By playing back the notes at different speeds, researchers found that the crested pigeons use the notes’ timing as an alarm. When the cycle of high and low repeats in quick succession, they respond by taking off to escape the perceived danger.

    Other bird species are also known to use aeroelastic flutter to make noise. Check out these hummingbirds, which use flutter in their mating displays.   (Video credit: Science; research credit: T. Murray et al.)

  • Oceans of Clouds

    Oceans of Clouds

    One of the most amazing things about fluid dynamics, in my opinion, is that the same rules apply across an incredible array of situations. The equations of motion are the same whether your fluid is water, air, or honey. Your flier can be a Cessna airplane or a fruit fly; again, the equations are the same. This is part of the reason that patterns in flows are repeated whether in the laboratory or out in nature – and it’s the reason why a timelapse of fog clouds can look just like ocean waves. Ultimately, the physics is the same; clouds just move slower than ocean waves! (Image credit: L. Leber, source; via James H.)

  • Emulsions By Condensation

    Emulsions By Condensation

    Oil and water are hard to mix, as any salad dressing aficionado will attest. Technically, the two fluids are immiscible – they won’t mix with one another – but one way around this is to emulsify them by distributing droplets of one in the other. This is usually accomplished by shaking or using sound waves to vibrate the mixture, but the results are typically short-lived. The larger a droplet is, the more gravity affects it, causing the buoyant oil to rise and separate from the water.

    The key to making an emulsion last is creating tiny droplets, which a new study accomplishes energy efficiently through condensation. Instead of mixing the oil and water immediately, the researchers used a surface covered in a mixture of oil and surfactant and cooled it in a humid chamber. As the temperature dropped, water condensed onto the oil and became encapsulated, creating nanoscale emulsion droplets. At such a tiny scale, buoyant forces are unable to overcome surface tension, so the emulsion remains stable for months. (Image credit: MIT, source; research credit: I. Guha et al.; via MIT News)

  • Oil Splatters

    Oil Splatters

    Most cooks have experienced the unpleasantness of getting splattered with hot oil while cooking. Here’s a closer look at what’s actually going on. The pan is covered by a thin layer of hot olive oil. Whenever a water drop gets added – from, say, those freshly washed greens you’re trying to saute – it sinks through the oil due to its greater density. Surrounded by hot oil and/or pan, the water heats up and vaporizes with a sudden expansion. This throws the overlying oil upward, creating long jets of hot oil that break into flying droplets. These are what actually hit you. This is a small-scale demonstration, but it gets at the heart of why you don’t throw water on an oil fire. (Image credit: C. Kalelkar and S. Paul, source)

  • Symmetric Wakes

    Symmetric Wakes

    Nature is full of remarkable patterns and moments of symmetry. This image shows the wake behind two rotating cylinders. Half of the cylinders are visible at the far left. The flow moves left to right. The cylinders are rotating at the same rate but in opposite directions, clockwise for the cylinder on top and counter-clockwise for the bottom one. At this speed relative to the freestream, there is a beautiful symmetry to the vortices in the wake, but the researchers found that even a slight deviation from this condition quickly destroyed the pattern. The flow is visualized here by introducing tiny hydrogen bubbles via electrolysis. The bubbles are small enough that their buoyancy has no appreciable effect. (Image credit: S. Kumar and B. Gonzalez)

  • Schooling Together

    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)

  • Convection Without Heat

    Convection Without Heat

    Glycerol is a sweet, highly viscous fluid that’s very good at absorbing moisture from the ambient air. That’s why a drop of pure glycerol in laboratory conditions quickly develops convection cells – even when upside-down, as shown above. This is not the picture of Bénard-Marangoni convection we’re used to. There’s no temperature or density change involved; in fact, there’s no buoyancy involved at all! This convection is driven entirely by surface tension. As glycerol at the surface absorbs moisture, its surface tension decreases. This generates flow from the center of a cell toward its exterior, where the surface tension is higher. Conservation of mass, also known as continuity, requires that fresh, undiluted glycerol get pulled up in the wake of this flow. It, too, absorbs moisture and the process continues. (Image credit: S. Shin et al., pdf)

  • Bubbles Sliding

    Bubbles Sliding

    Two-phase flows involve more than one state of matter – in this case, both gas and liquid phases. Flows like this are common, especially in applications involving heat transfer. In some heat exchangers, bubbles will rise and then slide along an inclined surface, as shown above. The motion of these bubbles is a complicated interplay between the surface, the bubble, and the surrounding fluid. The bubble’s wake, visualized here using schlieren imaging, is unsteady and turbulent. Although the bubble oscillates in its path, the wake spreads even wider, and its turbulence stirs up the liquid nearby, increasing the heat transfer. (Image and research credit: R. O’Reilly Meehan et al., source)

  • Featured Video Play Icon

    Build Your Own Fluidized Bed

    Previously, we featured some GIFs of bubbling, fluidized sand (below). Inspired by the same video, Dianna from Physics Girl decided to build her own set-up, discovering along the way that it’s a little tougher than you might think. To work well, you’ll need very fine, dry particles and a good way to uniformly distribute the air so it doesn’t simply bubble up in one spot. And if you accidentally apply too much air pressure, you may get a face full of sand. The final results are very fun, though, and hopefully Dianna’s lessons learned will help any other DIYers interested in trying this experiment at home. For a little more on the physics here and in related topics, check out some of our previous posts on fluidization, soil liquefaction, quicksand, and dam failures. (Video credit: Physics Girl; image credit: R. Cheng, source)

  • Bioluminescent Plankton

    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)