FYFD

Tag: flow visualization

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    Draining a Bottle

    Turn a bottle upside-down to empty it, and you’ll hear a loud glug-glug-glug as the liquid in the bottle empties and air rushes in. In this video, researchers aim a high-speed camera at the very first bubble that forms during the process. Once the bubble reaches the wider area of the bottle, it tends to pinch off in the bottle’s neck. That creates a narrow jet that pierces the bubble and flies all the way to the other side, leaving a column of liquid inside the rising bubble. Increasing the fluid’s viscosity has remarkably little effect, at least until the liquid is extremely viscous. (Image and video credit: H. Mayer et al.)

  • Waves in Liquid Crystals

    Waves in Liquid Crystals

    Liquid crystals are now ubiquitous in displays, but scientists are still discovering new properties for this state of matter. Here, a team explores nematic liquid crystals, whose rod-like shape rotates in three dimensions as they apply a voltage. The layer of liquid crystals is held between polarizing filters, creating regions of light and dark that depend on the liquid crystals’ orientations.

    Traveling waves and other wave patterns form in this liquid crystal as the voltage applied to it increases.
    Traveling waves and other wave patterns form in this liquid crystal as the voltage applied to it increases.

    As the researchers increase the voltage, traveling waves form. With higher voltages, the waves appear to slow a stop. The slowing waves result from the molecules tilting far from a vertical orientation, which makes it harder for individual molecules to rotate since they experience greater resistance from their neighbors. (Image, video, and research credit: V. Panov et al.; via APS Physics)

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    Classifying Waves

    In a lab, researchers create their waves in a long, clear-sided tank, where they can observe how the waves form, travel, and interact. To generate the wave, they use a plate, attached to a piston. Push the water at one end, and a wave forms. The type of wave that forms depends on both the velocity and the stroke length of the piston, as shown in this video. By mapping out these two variables, researchers can observe all different sorts of waves, from peaceful solitary waves to wild, plunging breakers. (Image and video credit: W. Sarlin et al.)

  • Airflow in the Opera

    Airflow in the Opera

    Like so many other performers, the singers and musicians of New York’s Metropolitan Opera House were left without a way to safely perform when the SARS-CoV-2 pandemic began in early 2020. In search of safe ways to perform and rehearse, the Met turned to researchers at nearby Princeton University, who worked directly with the performers to explore aerosol production and airflow in the context of professional opera.

    Through visualization and other experiments, the team found that the highly-controlled breathing of opera singers actually posed a lower risk for spreading pathogens than typical speaking and breathing. Most of a singer’s voiced sounds are sustained vowels, which produce a slow, buoyant jet that remains close to a singer. The exception are consonants, which created rapid, forward-projected jets.

    In the orchestra, the researchers found that placing a mask over the bell of wind instruments like the trombone reduced the speed and spread of air. One of the highest risk instruments they found was the oboe. Playing the oboe requires a long, slow release of air, but between musical phrases, oboists rapidly exhale any remaining air from their lungs and take a fresh breath. That rapid exhale creates a fast, forceful jet of air that necessitates placing the oboist further from others. (Image credit: top – P. Chiabrando, others – P. Bourrianne et al.; research credit: P. Bourrianne et al.; via APS Physics; submitted by Kam-Yung Soh)

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    Walking in the Wake

    Flow visualization is an important tool in fluid dynamics, and scientists have many ways to capture and visualize flow information. But our methods are not the only — or even the best — ways to express a flow. Here, engineers teamed up with architects and artists to explore the flow behind an oscillating cylinder. When free to move forward-and-backward the cylinder’s wake takes on three distinctively forms. The team explored many ways to display the wakes — drawings, 3D-printed sculptures, and more — before ultimately building an art installation that lets visitors walk through the wake to experience it. I love the creativity of these interdisciplinary efforts. To see a similar, yet very different, take on the wake of a cylinder, check out this interpretative dance. (Image and video credit: P. Boersma et al.)

  • Why Moths Are Slow Fliers

    Why Moths Are Slow Fliers

    Hawkmoths and other insects are slow fliers compared to birds, even ones that can hover. To understand why these insects top out at 5 m/s, researchers simulated their flight from hovering to forward flight at 4 m/s. They analyzed real hawkmoths flying in wind tunnels to build their simulated insects, then studied their digital moths with computational fluid dynamics.

    During hovering flight, they found that hawkmoths generate equal amounts of lift with their upstroke and downstroke. As the moth transitions into forward flight, though, its wing orientation shifts to reduce drag, and the upstroke stops being so helpful. Instead, the upstroke generates a downward lift that the downstroke has to counter in addition to the insect’s weight. At higher forward speeds, this trend gets even worse.

    The final verdict? Hawkmoths don’t have the flexibility to twist their wings on the upstroke the way birds do to avoid that large downward lift. Since they can’t mitigate that negative lift, the insects have a slower top speed overall. (Image and research credit: S. Lionetti et al.; via APS Physics; submitted by Kam-Yung Soh)

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    Pumping With Faraday Waves

    Vibrate a liquid pool vertically, and it will form a pattern of standing waves known as Faraday waves. Here, researchers confine those waves to a narrow ring similar in size to the wave. The confinement causes a type of secondary flow — a streaming flow — beneath the water surface. As a result, the wave pattern rotates around the ring. The applications of this rotation are pretty neat. As the team demonstrates, it can drive complex fluid networks and even create a pump! (Image and video credit: J. Guan et al.)

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    “Reconfiguring It Out”

    Leaves flutter and bend in the breeze, changing their shape in response to the flow. Here, researchers investigate this behavior using flexible disks pulled through water. The more flexible the disk and the faster the flow, the more cup-like the disk’s final shape. Adding tracer particles to the water allows them to visualize the flow behind the disk. Every disk leaves a donut-shaped vortex ring spinning in its wake, but the more reconfigured the disk, the narrower the vortex. This, ultimately, reduces drag on the disk. That’s why trees in heavy winds streamline their branches and leaves; that flexibility lowers the drag the tree’s roots have to anchor against. (Image and video credit: M. Baskaran et al.)

  • Mixing Effectively

    Mixing Effectively

    Mixing two fluids is a tougher task than you might think. One of my favorite asides from a fluids lecture concerned how to mix fruit into yogurt in an industrial setting. Mix too quickly, and you’ll obliterate the yogurt’s consistency, but mix too little and you may as well sell it as fruit-on-the-bottom. Apparently that particular problem got solved by sending the fruit and yogurt flowing through a series of specially-shaped ducts to slowly and carefully mix them together.

    In this study, researchers tackle a similar problem — mixing two fluids in a circular cross-section — through optimization. As you can see above, circular stirrers on their own don’t do a great job of mixing. So the researchers searched for the right combination of stirrer shape, mixing speed, and mixing trajectory to give the best mixing for a set mixing time and energy input. Their final stirrer shapes are anything but circular and often move in jerks and fits to help shed vortices that do the actual job of mixing. (Image and research credit: M. Eggl and P. Schmid; via APS Physics)

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    Cleaning the Skies

    Those of us who live in urban environments have experienced the clear, pollution-free air that comes after a rainstorm. But how exactly does rain clean the air? Air pollution typically has both gaseous and particulate components to it. As a raindrop falls, it experiences collision after collision with those particles. Depending on the particle’s surface characteristics — is it hydrophilic or hydrophobic? — and its momentum during impact, it can get trapped in the raindrop, skip off, or even pass through entirely. The physics, it turns out, are identical to those of a rock falling into or skipping off a lake — even though the raindrop and particle might be 1000 times smaller! (Image and video credit: N. Speirs et al.)