FYFD

Tag: buoyancy

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    How Particles Affect Melting Ice

    When ice melts in salt water, there’s an upward flow along the ice caused by the difference in density. But most ice in nature is not purely water. What happens when there are particles trapped in the ice? That’s the question this video asks. The answer turns out to be relatively complex, but the researchers do a nice job of stepping viewers through their logic.

    Large particles tend to fall off one-by-one, which doesn’t really affect the buoyant upward flow along the ice. In contrast, smaller particles fall downward in a plume that completely overwhelms the buoyant flow. That strong downward flow makes the ice ablate even faster. (Video and image credit: S. Bootsma et al.)

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    Bubbly Tornadoes Aspin

    Rotating flows are full of delightful surprises. Here, the folks at the UCLA SpinLab demonstrate the power a little buoyancy has to liven up a flow. Their backdrop is a spinning tank of water; it’s been spinning long enough that it’s in what’s known as solid body rotation, meaning that the water in the tank moves as if it’s one big spinning object. To demonstrate this, they drop some plastic tracers into the water. These just drop to the floor of the tank without fluttering, showing that there’s no swirling going on in the tank. Then they add Alka-Seltzer tablets.

    As the tablets dissolve, they release a stream of bubbles, which, thank to buoyancy, rise. As the bubbles rise, they drag the surrounding water with them. That motion, in turn, pulls water in from the surroundings to replace what’s moving upward. That incoming water has trace amounts of vorticity (largely due to the influence of friction near the tank’s bottom). As that vorticity moves inward, it speeds up to conserve angular momentum. This is, as the video notes, the same as a figure skater’s spin speeding up when she pulls in her arms. The result: a beautiful, spiraling bubble-filled vortex. (Video and image credit: UCLA SpinLab)

    Composite image showing far (left) and close (right) views of a bubbly vortex in a rotating water tank.
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  • “My Own Galaxy”

    “My Own Galaxy”

    Fungal spores sketch out minute air currents in this shortlisted photograph by Avilash Ghosh. The moth atop a mushroom appears to admire the celestial view. In the largely still air near the forest floor, mushrooms use evaporation and buoyancy to generate air flows capable of lifting their spores high enough to catch a stray breeze. (Image credit: A. Ghosh/CUPOTY; via Colossal)

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    How Cooling Towers Work

    Power plants (and other industrial settings) often need to cool water to control plant temperatures. This usually requires cooling towers like the iconic curved towers seen at nuclear power plants. Towers like these use little to no moving parts — instead relying cleverly on heat transfer, buoyancy, and thermodynamics — to move and cool massive amounts of water. Grady breaks them down in terms of operation, structural engineering, and fluid/thermal dynamics in this Practical Engineering video. Grady’s videos are always great, but I especially love how this one tackles a highly visible piece of infrastructure from multiple engineering perspectives. (Video and image credit: Practical Engineering)

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    Billowing Ouzo

    Pour the Greek liquor ouzo into water, and your glass will billow with a milky, white cloud, formed from tiny oil droplets. The drink’s unusual dynamics come from the interactions of three ingredients: water, oil, and ethanol. Ethanol is able to dissolve in both water and oil, but water and oil themselves do not mix.

    In this video, researchers explore the turbulent effects of pouring ouzo into water. In particular, pouring from the top creates a fountain-like effect, due to a tug-of-war between the ouzo’s momentum and its buoyancy. Momentum wants the ouzo to push down into the water, and buoyancy tries to lift it back up. For an extra neat effect, they also show what happens when the ouzo is confined to a 2D plane and what happens when momentum and buoyancy act together instead of oppositely. (Image and video credit: Y. Lee et al.)

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    Toying With Density and Miscibility

    Steve Mould opens this video with a classic physics toy that uses materials of different densities as a brainteaser. Two transparent, immiscible liquids fill the container, along with beads of a couple different densities. When you shake the toy, the liquids emulsify, creating a layer with an intermediate density. As the two liquids separate, the emulsified middle layer disappears, causing the beads (which have densities between that of the two original liquids) to come together.

    The rest of the video describes the challenges of expanding this set-up into three immiscible liquids and four sets of beads. Along the way, Steve had to contend with issues of miscibility, refractive index, and even chemical solvents. It’s amazing, sometimes, what it takes to make a seemingly simple idea into reality. (Video and image credit: S. Mould)

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    Convection in Action

    We’re surrounded daily by convection — a buoyancy-driven flow — but most of the time it’s invisible to us. In this video, Steve Mould shows off what convection really looks like with some of his excellent tabletop demos. The first half of the video gives profile views of turbulent convection, with chaotic and unsteady patterns. When he switches to oil instead of water, the higher viscosity (and lower Reynolds number) offer a more structured, laminar look. And finally, he shows a little non-temperature-dependent convection with a mixture of Tia Maria and cream, which convects due to evaporation changing the density. (Image and video credit: S. Mould; submitted by Eric W.)

  • Dancing Peanuts

    Dancing Peanuts

    Bartenders in Argentina sometimes entertain patrons by tossing a few peanuts into their beer. Initially, the peanuts sink, but after a few seconds they rise, wreathed in bubbles. Once on the surface, they roll, causing the bubbles to pop, and the peanut sinks once again. The cycle repeats, sometimes for as long as a couple hours.

    There are a couple physical processes governing this dance. The first is bubble nucleation. Most beers are carbonated; they contain dissolved carbon dioxide gas that remains in solution while the beer is under pressure. Once poured, that storage pressure is gone and bubbles start to form in the liquid. The shape of the peanut means that bubbles form more easily on it than on the glass walls or in the liquid. And once the peanut is covered in bubbles, buoyancy comes into play. The bubbles attached to the peanut reduce its density relative to the surrounding fluid, enabling the peanut to rise up and float.

    This same process is seen with other objects in carbonated fluids, too, such as blueberries in beer and lemon seeds in carbonated water. But it’s also reflected elsewhere in nature. For example, magnetite crystals are thought to float in magma due to a similar nucleation of dissolved gases on their surface. (Image and research credit: L. Pereira et al.; via APS Physics)

  • Bubble Cleaning

    Bubble Cleaning

    Removing dirt and bacteria from fruits and vegetables is a delicate job; too much force can bruise the produce and hasten spoiling. That’s why fluid mechanicians want to give the job to bubbles. Placing objects in a stream of air bubbles inside a bath is a surprisingly effective method for gently cleaning surfaces. A recent study finds that 22.5 degrees is the optimal angle for sliding bubbles to scrape a surface clean.

    As the bubbles slide past the surface, they exert a shear force that scrapes away debris, just as you might use a loofah in the shower. The angle the bubble makes with the surface determines how long it’s in contact and how much force the bubble exerts. Increasing the angle makes the bubble slide faster, increasing its shear force. But above 22.5 degrees, the bubble’s buoyancy means that it spends less time pressed against the surface, which decreases its cleaning ability.

    The team hopes to use their results to build a “fruit Jacuzzi” device that will direct bubble streams to gently and effectively clean fruits and vegetables in a matter of minutes. (Image and research credit: A. Hooshanginejad et al.; via APS Physics)

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    “Iridescent”

    Soft colors and sudden coalescence combine in this short film from Susi Sie’s team. The visuals rely on liquid lenses (likely oil) floating atop a water bath. You can see how the fluids get manipulated in their behind-the-scenes video, which also provides a peek at how the sound effects get made. (Video credit: S. Sie et al.)