FYFD

Category: Research

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    Non-Newtonian Splashes

    What happens when a stream of liquid falls through a screen? As the above video shows, water creates a beautiful flower-like burst of fluid when it hits a screen. Adding a little polymer to the water makes it non-Newtonian and more viscous. When hitting the screen, this slows it down but doesn’t prevent the fluid from flowing.

    Add enough polymer, though, and the fluid becomes what’s known as a yield-stress fluid. These fluids behave much like a solid–they don’t flow–until you apply a certain amount of stress. Then they’ll flow. If you’ve ever tried to get ketchup out of a glass bottle, then you’re familiar with how these yield-stress fluids act. When dropped onto a screen, the yield-stress fluid just forms a pile–unless the impact speed is high enough to create the necessary force to get the fluid to flow! (Video credit: B. Blackwell et al.)

  • Plesiosaur Swimming

    Plesiosaur Swimming

    Plesiosaurs are marine reptiles that thrived during the Jurassic period and went extinct some 66 million years ago. Since the first discoveries of plesiosaur fossils centuries ago, scientists have debated how the four-limbed creature would have swam. One approach to answering this question is to examine the efficiency of different strokes. Researchers have done this computationally by building a digital plesiosaur with biologically realistic joint motions. They then couple the model plesiosaur’s body motions with the movement of fluid around the body. With this computational model, they then simulate many different methods for moving the plesiosaur’s limbs and search for the most efficient one.

    What they found is that the plesiosaur’s propulsion is dominated by its forelimbs, which likely moved with a flight stroke similar to that of a penguin or sea turtle. Despite their size, the hindlimbs were able to produce very little thrust, suggesting that they were primarily used for stability and maneuverability. (Image credits: S. Liu et al., GIF source)

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    Clogging, In Hourglasses and Crowds

    Hourglasses are pretty common, but you’ve probably never given much thought to the way they flow. An hourglass designer has to carefully select the sizing of the neck and the grains. Choosing a neck that’s too small relative to the grain size will result in frequent clogs but choosing too large a neck will make setting the timing difficult. Interestingly, it doesn’t matter whether the hourglass is filled with air or with water–the same principle holds.

    Where this knowledge becomes especially useful, though, is when dealing with crowds. We’ve all experienced the frustration of being in a large crowd trying to fit through a small exit. Paradoxically, the fastest way to get a large number of particles (or sheep or people) through a narrow opening is to slow each individual down. This can either be done by instructing everyone to slow down or by forcing that same result by placing an obstacle immediately before the exit. The reduction in speed reduces clogging, which means everyone gets through faster! (Video credit: A. Marin et al.)

  • Frost Spreading

    Frost Spreading

    Frost typically forms when supercooled droplets of water scattered across a surface freeze together. The freezing spreads via tiny ice bridges that link droplets together into a frozen network. The animation above shows this process in action. Freezing starts in a droplet off-screen on the right and quickly spreads. Watch carefully, and you can see the ice bridges growing toward the unfrozen droplets. This is because the ice bridges are fed by water vapor evaporating from the droplets. If one can spread the droplets far enough from one another, it’s possible for a droplet to evaporate completely before the ice bridge reaches it, thereby disrupting the spread of frost.  (Video credit: J. Boreyko et al.; research paper)

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    Researching Wind Turbines

    Two of the most awesome things (in my admittedly biased opinion) about fluid dynamics are the amazing facilities we build for experiments and the tests they allow us to do. In this video, you get a behind-the-scenes look at one such facility, used for wind turbine research at Princeton.

    One challenge of wind turbine research is accurately capturing the aerodynamic effects of full-scale wind turbines in the controlled-environment of a laboratory. At Princeton, they match conditions between their model turbines and the real ones by drastically raising the density in their wind tunnel. This means that running the tunnel requires a series of compressors and storage tanks full of compressed air, and it also means that the wind tunnel itself has to be quite hefty to handle the pressure difference inside and out. Definitely check out the full video for more on their wind tunnel and what it can help them learn about wind turbines. (Video credit: M. Miller and J. Keifer; submitted by M. Miller)

  • Spore Squirting

    Spore Squirting

    The fungus Pilobolus spreads its spores with a squirt cannon. Each spore sits on the end of a round fluid-filled pod. Like many plants, the fungus uses a process called osmosis to pump water into the pod. Through osmosis, the fungus increases the concentration of certain molecules inside the pod, which draws water into the pod and increases its pressure. Eventually, the pod ruptures, sending the spore aloft on a jet of fluid that accelerates it at 20,000+g! (Image credit: BBC Earth Unplugged, source; research credit: L. Yafetto et al.)

  • Hummingbird Drinking

    Hummingbird Drinking

    Hummingbirds are master acrobats, able to hover and drink simultaneously before flitting off to the next flower. At first glance, you might expect that their tongues are simply tiny straws that use surface tension and capillary action to draw up nectar. But it turns out that process is just too slow for the fast-paced birds.

    Instead, hummingbirds use a forked tongue with a long groove on either half. When the hummingbird extends its tongue, its beak compresses the grooves and squeezes them together. Once the tongue reaches nectar, the grooves expand, which draws nectar up along the full length of the tongue grooves. This allows the bird to fill its tongue much faster than it could otherwise, enabling the hummingbird to lick up nectar more than 10 times a second.

    There’s a neat excerpt from a documentary including this research over here (Tumblr won’t allow the embedded version); the full documentary premieres today on PBS. (Image credits: A. Rico-Guevara et al., sources 1,2; submitted by mypronounsareherrchancellor)

  • Floating on a Granular Raft

    Floating on a Granular Raft

    A thin layer of hydrophobic particles dispersed at an oil-water interface is strong enough to prevent a water droplet from coalescing. The researchers refer to this set-up as their granular raft. As the red-dyed water droplet gets larger (top row), it deforms the raft more and more, but the grains continue to keep the drop separate from the fluid beneath (middle row). When water is removed from the droplet, wrinkles form on the raft as the drop’s volume shrinks. This is because the contact line – where the droplet, grains, and air meet – is pinned. The grains already touching the drop are held there by adhesion. But since the drop is shrinking, the area on the raft has to shrink, too – thus wrinkles! (Photo credits: E. Jambon-Puillet and S. Protiere, original)

  • A Molecular View of Boiling

    A Molecular View of Boiling

    All matter is made up of molecules. But most of the time we treat fluids as materials with given properties – like density, viscosity, and surface tension – without worrying about the individual molecules responsible for those material characteristics. Now that we have much more powerful computers, though, we can begin to simulate fluid behavior in terms of molecules.

    The animations above show some examples of this. In the top animation, we see a gas condensing into a liquid. As the temperature decreases, molecules start clumping together, and eventually settle into a droplet on the solid surface. The lower animation shows the opposite situation – boiling – in which bubbles of vapor nucleate next to the solid surface and grow as more liquid changes phase. To see more examples, including droplets pinching off, check out the full video.   (Image credit: E. Smith et al., source; submitted by O. Matar)

  • Hairy Surfaces Keep Skin Dry

    Hairy Surfaces Keep Skin Dry

    Big animals like whales and sea lions stay warm in cold waters by having thick layers of insulating blubber. But smaller mammals, like beavers and sea otters, have a different mechanism for staying warm – their thick fur traps air near their skin, keeping the cold water at bay. Researchers used flexible, 3D-printed “hairy” surfaces to see how hair density, diving speed, and fluid viscosity affected the amount of air trapped between hairs. This enabled them to build a mathematical model describing the physics, which can now be used to predict, for example, the characteristics needed for a hairy wetsuit that could keep surfers warm in and out of the water. For more on this research check out MIT News’ video, and for a closer look at sea otter fur – not to mention a healthy overdose of pure adorable – check out the video below.  (Photo credit: F. Frankel; video credit: Deep Look; research credit: A. Nasto et al.)