FYFD

Tag: conservation of angular momentum

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    Playful Martian Dust Devils

    The Martian atmosphere lacks the density to support tornado storm systems, but vortices are nevertheless a frequent occurrence. As sun-warmed gases rise, neighboring air rushes in, bringing with it any twisted shred of vorticity it carries. Just as an ice skater pulling her arms in spins faster, the gases spin up, forming a dust devil.

    Black and white video illustrating a small Martian dust devil catching up to and getting swallowed up by a larger dust devil.

    In this recent footage from the Perseverance Rover, four dust devils move across the landscape. In the foreground, a tiny one meets up with a big 64-meter dust devil, getting swallowed up in the process. It’s hard to see the details of their crossing, but you can see other vortices meeting and reconnecting here. (Video and image credit: NASA/JPL-Caltech/LANL/CNES/CNRS/INTA-CSIC/Space Science Institute/ISAE-Supaero/University of Arizona; via Gizmodo)

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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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  • The Variable Venusian Day

    The Variable Venusian Day

    Venus is a thoroughly unpleasant place thanks to its hellish temperatures and acidic clouds, but a new study adds another wrinkle to our strange sister planet: Venus’s day varies by up to 21 minutes in length. This peculiar factoid is the result of 15 years spent monitoring Venus’s rotation via radar. Previous attempts to pin down the exact length of Venus’s day produced differing answers; those disagreements make more sense in light of the new study, where individuals measurements of Venus’s rotation rate could differ by 3 minutes just from one (Earth) day to the next!

    So why does Venus’s rotation rate change so dramatically? Venus’s atmosphere is massive — 100 times more massive than Earth’s — and it spins incredibly fast. The upper layers of Venus’s atmosphere can complete a rotation in 4 Earth days, while the solid ground requires 243 Earth days. As the atmosphere spins and sloshes, some of its angular momentum gets transferred to the ground, changing the planet’s rotation rate. (Image credit: NASA/JPL-Caltech; research credit: J. Margot et al.; via AGU Eos; submitted by Kam-Yung Soh)

  • Waterspouts

    Waterspouts

    Despite their ominous appearance, these waterspouts – like most of their kind – are fair-weather phenomena unrelated to tornadoes. They can form when cold, dry air moves over warm waters. As warm, moist air rises from the water’s surface, air is drawn in from the surroundings to replace it. Any vorticity in that air comes with it, growing stronger as it gets pulls in, thanks to conservation of angular momentum. That action creates the waterspout, which becomes visible when the warm, humid air cools enough to condense and form a cloud wall. (Image credit: R. Giudici; via EPOD)

  • Putting a Spin on Splashes

    Putting a Spin on Splashes

    Researchers put a spin on splashing droplets with selective wetting. When a drop impacts on a water-repellent, superhydrophobic surface, it will spread circularly, then pull back together and rebound off the surface. That’s because the surface coating resists actually touching – or being wetted by – the water. But just as there are surface coatings that resist water, there are those that attract it.

    Above, researchers have coated a surface so that it’s mostly superhydrophobic, but it also has narrow pinwheel-like arms that are hydrophilic. As the drop impacts, it spreads across the surface and then retracts. But where the hydrophilic arms are, the drop lingers. This creates the four lobes we see on the droplet, and the asymmetric retraction gives the drop angular momentum. As it leaves the surface, the spin continues. In some configurations, the researchers could make the drop spin at more than 7300 rpm. (Image and research credit: H. Li et al; via Science; submitted by Kam-Yung Soh)

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    Fire Tornado in a Bubble

    File this one under awesome tricks you shouldn’t try at home. Here bubble artist Dustin Skye demonstrates his handheld inverted fire tornado. First, he blows a large encapsulating bubble, then blows butane and smoke into a smaller secondary bubble. When he breaks the wall between the two, the mixture swirls into the larger bubble. Then, by breaking a narrow hole into the remaining bubble, Skye forms a swirling tornado. He’s using conservation of angular momentum here to concentrate the vorticity he created by blowing into the original butane bubble. As the big bubble shrinks, the vorticity inside gets pulled inward and speeds up – like when a spinning ice skater pulls his arms in. That’s how you get the tornado. And from there, it’s just a matter of lighting the exiting butane and air mixture. (Video credit: D. Skye; via Gizmodo)

  • Water Bottle Flipping Physics

    Water Bottle Flipping Physics

    In 2016, a senior talent show launched a new viral craze: water bottle flipping. As improbable as it seems at first glance, physics is actually on your side when it comes to pulling this trick off. As explained in this classroom-oriented paper and the video abstract below, the sloshing of the water in the bottle as it flips slows its rate of rotation, which creates the stable landing. You don’t even need water to make the trick possible. Using two tennis balls will also give a stable flip – provided they have room to spread out. When they fly apart, they change the bottle’s moment of inertia and that slows down the rotation rate. All in all, it’s a great lesson in conservation of angular momentum.

    And, in case you’re wondering whether the water helps with sticking that landing, we’ve got you covered there, too. (Image credit: A. Johnson, source; video and research credit: P. Dekker et al.)

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    From Firenado to Water Spout

    Just a few years ago, fire tornadoes were almost fabled because they were so rarely captured on video. Now, with worsening wildfire seasons and cell phone cameras everywhere, there are new videos all the time. This video captures a fire tornado that sets off a water spout as it reaches the river (~1:15 in).

    Neither the fire tornado or the water spout is truly tornadic; instead they are more like dust devils. They are driven by the rising heat of the fire. As cooler, ambient air flows inward to replace the rising air, it brings with it any vorticity it had. And, like an ice skater, the incoming air spins faster as it moves inward. This sets up both the fire tornado and the water spout’s vortices.

    Although this is the first example I’ve seen video of, fire tornadoes have been known to create water spouts before. Lava flowing into the ocean can create whole trains of them. (Video credit: C. & A. Mackie; via Jean H.)

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    Rainbow Paint on a Speaker

    Every year brings faster high-speed cameras and better quality imaging, so the Slow Mo Guys like to occasionally revisit topics they’ve done before, like paint vibrated on a speaker. The physics involved here are fantastic, so I’ll revisit the topic, too! In this version, Gav and Dan are using a pretty beefy speaker at a relatively high volume, so the paint gets a strong acceleration. As they note, the paint colors mix to brown almost immediately. In the high-speed footage, we can see why. 

    Watch how the individual strands of paint behave. As they fly upward, they stretch out and get thinner. That stretching has a side effect: it makes the paint spin. This is angular momentum of the paint being conserved. Just like a spinning ice skater who pulls his arms in, the paint spins faster as it gets thinner. This provides a lot of the mixing. Just look at how the different colors twist together! (Image and video credit: The Slow Mo Guys)

  • Caught in a Whirl

    Vortex rings may look relatively calm, but they are concentrated regions of intensely spinning flow, as this poor jellyfish demonstrates. The rings form when a high-speed fluid gets pushed suddenly (and briefly) into a slower fluid. In the case of this bubble ring, a burst of air is pushed by a diver into relatively still water. The vorticity caused by the two areas of fluid trying to move past one another forms the ring. Like a spinning ice skater who pulls his arms inward, the narrow core of the vortex spins fast due to the conservation of angular momentum. Meanwhile, the bubble ring moves upward due to its buoyancy, pulling nearby water in as it goes. This catches the hapless jellyfish (who relies on vortex rings itself) and gives it quite a spin. But. don’t worry, the photographer confirmed that the jelly was okay after its ride. (Video credit: V. de Valles; via Ashlyn N.)