FYFD

Category: Phenomena

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    Particles Separate When Flowing Downhill

    When particle-laden fluids like a mudslide flow downhill, even well-mixed particles can wind up separating. To explore how this works, researchers put glass spheres–of two different sizes but equal density–into silicone oil and let it flow down an incline. Their initially well-mixed oil soon turned red as the larger red particles overtook the smaller blue particles near the front. Looking at the flow from the side, the team observed a Brazil-nut-effect-like behavior where the larger particles move toward the top of the flow. That’s where the flow speed is fastest, and the particles are congregating there despite being denser than the oil carrying them! (Video and image credit: Y. Ba et al.)

  • Crowned Jets

    Crowned Jets

    If you fill a test tube with water and drop it, the impact causes a pressure wave that travels up from the bottom and creates a focused jet (left). If the impact is strong enough, cavitation bubbles form at the bottom and generate a sheet-like jet around the central one, like a crown (center and right). (Image credit: H. Watanabe et al.)

    Research poster with black and white images of jets with a crown-like liquid sheet around them.
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    Bioconvection

    Convection isn’t always driven by temperature. Here, researchers explore the convective patterns formed by Thiovulum bacteria. These bacteria are negatively buoyant, meaning they will sink if they aren’t swimming. They also have an asymmetric moment of inertia, so any flow moving past them tends to affect their swimming direction.

    When let loose in a Hele-Shaw cell with a oxygen levels that decrease with depth, the bacteria create complex convection-like patterns. They swim slowly upward in wide, slow plumes and sink in denser, narrow plumes. In other areas, they form large-scale rotating vortices. (Video and image credit: O. Kodio et al.)

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  • Colorful Tides

    Colorful Tides

    The colorful coastline of the Bazaruto Archipelago extends off East Africa. Regions of shallow waters, seagrass meadows, and coral reefs appear in shades of tan, green, and turquoise. Deeper waters appear blue. The coastlines, deltas, and tidal flats are shaped by moderate tides that rise and fall a few meters each day; strong currents run in the channels between islands, carving and reshaping the sediment. (Image credit: W. Liang; via NASA Earth Observatory)

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    Making Sound Visible

    Sound is not something we can typically see, though there are ways to visualize it, including cymatics and special acoustic cameras. This video pursues a different tactic: using schlieren photography and stroboscopic lighting to show how sound waves reflect and deflect. It’s no easy feat, but one worth enjoying–especially when others have already done the hard part for you! (Video and image credit: All Things Physics; submitted by David J.)

  • Mixing Bubble Caps

    Mixing Bubble Caps

    When bubbles form atop the ocean or in our cups, they typically live short lives. Although the bubble can exchange fluid with the pool below, this only happens at the foot of the bubble cap. There, thinner patches form and, due to their buoyancy, rise up along the bubble’s surface. Over time, these lighter, thinner patches reduce the amount of fluid in the cap–causing the bubble to thin and eventually burst.

    A research poster showing how external turbulence affects the plumes that thin a bubble cap.

    Here, researchers show that thinning–visible in the dark blue plumes rising up the bubble cap–when there’s no turbulence in the surrounding air. But as turbulence outside the bubble increases, the thinner patches stretch and deform across the cap. In the image series, turbulence increases moving from top to bottom. (Image credit: T. Aurégan and L. Deike)

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    Testing Structures Against Hurricane Storm Surge

    When hurricanes hit coasts, they bring with them incredible storm surge, which puts buildings right in the middle of ocean waves. To understand how to better protect against those conditions, engineers use facilities like the Directional Wave Basin to create smaller-scale versions of hurricanes. In this Practical Engineering video, Grady visited during a test that compared two identical one-third-scale houses subjected to the same storm conditions–except that one house had an additional foot (3ft at real-scale) of elevation. The results are pretty spectacular.

    This isn’t a short video, but it’s well-worth a watch. I think Grady does a great job of explaining why engineers need (admittedly) expensive facilities like this one to help guide both engineering and regulatory decisions. (Video and image credit: Practical Engineering)

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    Drag Reduction Via Bubbles

    To help reduce greenhouse emissions, businesses are exploring systems that reduce a container ship’s drag by releasing bubbles beneath them. But how do bubbles reduce drag? To find out, researchers simulated a bubbly flow that mimics the underside of a moving ship. By playing with the balance between inertial forces, buoyancy, and surface tension, they were able to sweep through conditions that the bubbles could experience.

    The best performance comes when bubbles stick together and coat the entire underside of the surface. In that case, they measured a nearly 40% reduction in the drag. But other conditions were not so fortuitous; in fact, with poorly chosen conditions, adding bubbles could actually increase the drag. (Video and image credit: S. Di Georgio et al.)

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    When the Meniscus Disappears

    When we first learn about states of matter, we’re taught about three: solid, liquid, and gas. In a solid, atoms are held close to one another–typically, but not always, in an orderly lattice structure. In liquids and gases, atoms are free to slip, slide, bounce, and move. So what really separates a liquid from a gas?

    Animation of liquid and gaseous carbon dioxide reaching the supercritical phase.

    That’s the question at the heart of this video by Steve Mould, in which he explores a weird fourth phase of matter: supercritical fluids. This phase has the diffusive properties of a gas and the solvent properties of a liquid–without really being either one.(Video and image credit: S. Mould)

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    Swirling Without Blades

    A ring of hydrogen bubbles rises, rotating clockwise, in this video of electrolysis. But there are no fan blades to cause this swirl, so why do the bubbles rotate? The answer is a Lorentz force induced by the electromagnetic set-up of the experiment. Watch to see how researchers manipulate the Lorentz force to affect the flow. (Video and image credit: Y. Cho et al.)