FYFD

Tag: fluid dynamics

  • Swimming Intermittently

    Swimming Intermittently

    Many fish do not swim continuously; instead, they use an intermittent motion, swimming in a sudden burst and then coasting. This intermittent swimming is tough to simulate, due to its unsteady nature, but a new study does so using some clever computational techniques.

    Animation showing a fish swimming in a burst-and-coast pattern.
    Animation showing a fish swimming in a burst-and-coast pattern.

    Researchers suspected that the energy intensity of a fish’s burst could be balanced by the low-drag, low-effort phase of coasting. And, indeed, that’s consistent with the team’s results. But they found that the swimming method does require careful optimization; with the wrong cadence, the burst-and-coast technique can be incredibly energy intensive. In nature, of course, fish have had millions of years to optimize their technique, but the results serve as a warning to those building fish-based robots. Those biorobots will need careful optimization to benefit from this swimming style. (Image credit: tetra – Adobe Stock Images, simulation – G. Li et al.; research credit: G. Li et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Beneath the Cavity

    Beneath the Cavity

    When a drop falls into a pool of liquid, it creates a distinctive cavity, followed by a jet. From above the surface, this process is well-studied. But this poster offers us a glimpse of what goes on beneath the surface, using particle image velocimetry. This technique follows the paths of tiny particles in the fluid to reveal how the fluid moves.

    As the cavity grows, fluid is pushed away. But the cavity’s reversal comes with a change in flow direction. The arrows now point toward the shrinking cavity — and they’re much larger, indicating a strong inward flow. It’s this convergence that creates the Worthington jet that rebounds from the surface. And, as the jet falls back, its momentum gets transferred into a vortex ring that drifts downward from the point of impact. (Image credit: R. Sharma et al.)

  • “Elements”

    “Elements”

    Photographer Mikko Lagerstedt specializes in Nordic landscapes, like the windswept snow seen here. I love the way he’s captured the snow that gets picked up and blown by the wind. Notice the hazy layer of snow hovering over the foreground. This snow is saltating, just as sand does in the desert. When flakes get picked up by the wind, they follow a ballistic trajectory, much like a cannonball in a high-school physics class. As the snow crashes back down, its impact knocks up more flakes, and the process continues. Repeat enough times, and you’ll see this hazy layer of blowing snow blanketing a snowscape. (Image credit: M. Lagerstedt; via Colossal)

  • Mixing With E. Coli

    Mixing With E. Coli

    What happens when a flow meets swimming micro-organisms? Does the flow affect the swimmers? And how do the swimmers affect the flow in turn? Those are the questions behind the experiment seen here. The apparatus contains a thin layer of saline water with swimming E. coli. Electromagnetism is used to mix the fluid in an array-like pattern that triggers chaotic mixing. To visualize what’s going on, dye is introduced into the right half of the image, while the left half remains undyed.

    On the right side of the image, bright blue and white mark areas of high dye concentration, where strong mixing occurs. On the undyed left side of the image, pale blue streaks mark areas where E. coli are clustered. By comparing the two, we see that the micro-swimmers are clustered in the very same regions of flow marked by strong mixing. This result suggests strong interactions and the potential for feedback between the mixing flow and the swimmers. (Image and research credit: R. Ran et al.; see also 1 and 2)

  • A Starry Nursery

    A Starry Nursery

    This mountain of interstellar gas and dust lies in the picturesque Eagle Nebula. Though it appears solid in this near-infrared image from JWST, the density of the structure is actually quite low. Jets and solar winds from the glowing, young stars inside the region sculpt the pillar’s shape. Over the next 100,000 years, the stars’ energetic jets, solar winds, and destructive supernovas will destroy the dusty nursery. (Image credit: NASA/ESA/CSA/STScI/M. Özsaraç)

  • Surface Fat Gives Chocolate’s Mouthfeel

    Surface Fat Gives Chocolate’s Mouthfeel

    Understanding the interactions of food and our mouths is incredibly difficult. There are lots of changes going on: shape changes from chewing, viscosity changes as saliva lubricates the food, and, sometimes, phase changes from the heat of our bodies. Add to that the sensitivity of our papillae-covered tongues, and it’s a lot to manage all at once. Recently, researchers have turned to 3D-printing to create a more realistic lab version of our mouths.

    The team 3D-printed a papillae pattern matching the size and distribution of an actual human tongue, then molded that pattern onto a silicone elastomer. The result? A replica tongue that matches a human one in terms of softness, wettability, and surface roughness. They then attached their tongue to a rheometer to measure the friction between the tongue and dark chocolate.

    Their experiments simulated licking, eating, and swallowing the confection. During licking and eating, they found that the chocolate was lubricated by a layer of fat directly between the tongue and the food. Their results suggest that one way to make healthier chocolate options is to concentrate fat into the surface layer of the chocolate while lowering the fat content inside the bar. (Image credit: D. Ramoskaite; research credit: S. Soltanahmadi et al.; via APS Physics)

  • Founts of Enceladus

    Founts of Enceladus

    In its exploration of Saturn, Cassini discovered that the moon Enceladus is home to icy eruptions. Beneath its shell of ice, Enceladus has a global ocean of salty liquid water. The average thickness of the ice is 20 kilometers, putting the ocean seemingly out of reach — except at the moon’s southern pole, where icy plumes of ocean water jet out.

    Here, where the ice is thinnest, the tidal forces Enceladus experiences from Saturn and its fellow moon Dione break through the ice. As the cracks open and close, liquid from the ocean sprays out, freezing into plumes that Cassini measured. Plans are underway for new missions that prioritize further sampling of Enceladus’ ocean. For now, we can only imagine what hides in its interior ocean. (Image credit: NASA/JPL-Caltech/SSI; for more, see M. Manga and M. Rudolph)

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    Paint Ejection

    Shaking paint on a speaker cone and filming it in high speed is an oldie but a goodie. Here, artist Linden Gledhill films paint ejection at 10,000 frames per second, giving us a glorious view of the process. As the paint flies upward, accelerated by the speaker, it stretches into long ligaments. As the ligaments thin, surface tension concentrates the paint into droplets, connected together by thinning strands. When those strands break, they snap back toward the remaining paint, imprinting swirling threads of different colors, thanks to their momentum. Eventually, surface tension wins the tug-of-war and transforms all the paint into droplets. (Video and image credit: L. Gledhill)

  • Stabilizing Paper Airplanes

    Stabilizing Paper Airplanes

    Making a good paper airplane is tough. Drop a simple sheet of paper and it will tumble and flip its way to the floor instead of gliding. The folds of a proper paper airplane add weight in just the right spots to stabilize its flight and let it glide smoothly through the air. To better understand what makes paper fly, researchers looked at how sheets of paper flew when weighted (with metallic tape) in different spots.

    Trajectories of pieces of paper with different weighting.
    Trajectories of pieces of paper with different weighting.

    An unweighted sheet of paper tumbled end-over-end. Shifting the center-of-mass too far forward or backwards also resulted in tumbles and nosedives. But when the weighting placed the center of mass between these two extremes, there was a sweet spot where the paper glided smoothly. In this situation, the aerodynamic forces on the paper could correct for changes in flight angle; if the paper tilted too far upward, the forces pushed it back down — and vice versa. This ability of the thin wing to self-stabilize is different than most large-scale aircraft, which need tails and other structures to provide stability to the main wing. (Image credit: paper airplane – K. Eliason, paper trajectories – H. Li et al.; research credit: H. Li et al.; via Ars Technica; submitted by Kam-Yung Soh)

  • Featured Video Play Icon

    How Spillway Gates Work

    Dams and reservoirs need a way to control their water level, and for many, that’s managed using spillway gates. In this video, Grady from Practical Engineering introduces several types of spillway gates, including their advantages and disadvantages. As always, he’s got neat tabletop demonstrations of each type, and he digs into the practical issues engineers face when building, maintaining, and repairing these critical pieces of infrastructure. (Image and video credit: Practical Engineering)