FYFD

Tag: science

  • Rip Currents

    Rip Currents

    Rip currents — also known as rips — are a threat to beachgoers around the world, and, unfortunately, they’re often underestimated or misunderstood. As waves crash on the shore, water must find a path back out to sea, often through deeper channels that provide a break between the waves. These flow paths are rip currents, and they can form, shift, and intensify with little warning.

    Over the years, researchers have found that efforts to educate beachgoers through signs, flags, and other methods once at the beach have done little to help visitors understand, avoid, or escape rips. Instead, it’s better to educate people long before the water is in sight. Since no one method is guaranteed success for escaping a rip, it’s better to learn to recognize and avoid these dangerous areas. Check out the video below for advice on spotting rips, and here’s a video showing rips from a surfer’s perspective, as well as one using dye flow visualization to mark a rip. Be safe and smart out there! (Image credit: P. Auitpol; video credit: Surf Life Saving Australia; via Hakai Magazine; submitted by Kam-Yung Soh)

  • Inside a Champagne Pop

    Inside a Champagne Pop

    When the cork pops on a bottle of champagne, the physics is akin to that of a missile launch in more ways than one. In this study, researchers used computational fluid dynamics to closely examine the gases that escape behind the cork. They identified three phases to the flow. In the first, the exhaust gases form a crown-shaped expansion region, complete with shock diamonds. Once the cork has moved far enough downstream, the axial flow accelerates to supersonic speeds and a bow shock forms behind the cork. Finally, the pressure in the bottle drops low enough that supersonic conditions cannot be maintained and the flow becomes subsonic. (Image credit: top – Kindel Media, simulation – A. Benidar et al.; research credit: A. Benidar et al.; via Ars Technica; submitted by Kam-Yung Soh)

    A numerical simulation showing the ejection of a champagne cork from a bottle. The colors indicate the speed of gases escaping from the bottle.
    A numerical simulation showing the ejection of a champagne cork from a bottle. The colors indicate the speed of gases escaping from the bottle.
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    Pop-Pop Boats

    I confess I’ve never heard of the pop-pop boat toys Steve Mould uses in this video. They feature a tank filled with water and a small source of heat in the form of a tea light candle. Together, these features generate propulsion and a distinctive popping sound from the toy. As he is wont to, Mould explains the physics behind the toy using a transparent version to show the water/steam oscillations that drive the boat. Having watched, I have to say that this set-up seems ready made for an undergrad fluids class and a control volume analysis! (Image and video credit: S. Mould)

  • A Forest of Ferrofluids

    A Forest of Ferrofluids

    Ferrofluids are made up of ferrous nanoparticles suspended in a carrier fluid like an oil. Under magnetic fields, they take on an array of shapes — from pointed spikes to elaborate labyrinths — depending on the field strength and what fluids they’re surrounded by. This photographic series by Linden Gledhill captures some of that fantastic variety, with ferrofluids that look like cells and nebulas in addition to mazes and tridents. See more of Gledhill’s work at his website and in previous posts. (Image credit: L. Gledhill)

  • Spinning Off-Axis

    Spinning Off-Axis

    To make a vortex in the laboratory, researchers typically set a tank on a rotating platform and allow the water to drain out a hole in the center of the tank. In that case, a vortex forms over the drain (like in your bathtub!) and remains centered over the hole. In nature, though, vortices rarely follow such a simple path.

    In this experiment, researchers moved the drainage hole so that it is not aligned with the tank’s axis of rotation. Although the vortex forms over the drain (marked by a yellow dot in the lower image), it quickly moves away, following a roughly circular path around the axis until it comes to a stop. Green dye marks fluid from the tank’s bottom boundary layer, which eventually gets entrained up into the vortex. (Image and research credit: R. Munro and M. Foster; via Physics Today)

    Timelapse animation showing the development of the vortex. The yellow dot marks the location of the drain.
    Timelapse animation showing the development of the vortex. The yellow dot marks the location of the drain.

  • Particle-Filled Coatings

    Particle-Filled Coatings

    Pulling a solid object from a liquid bath can coat it in a thin layer of liquid. The thickness of the coating layer depends on the speed at which the object is removed. Introducing particles into the liquid bath adds a new dimension to the coating problem, namely the size of the particles. In this poster, researchers demonstrate some of the coatings possible in a mixture with particles of more than one size. It’s even possible, they note, to filter out particles of a certain size by carefully selecting the removal speed. (Image credit: D. Jeong et al.)

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    Stably Jammed

    Granular materials like sand, gravel, and medications can become a rigid mass when squeezed or sheared. Even with a relatively loose packing, these materials can jam together to act like a solid if the contacts between grains no longer allow particles to shift or rotate. In this video, researchers explore how stable these jammed states are by repeatedly shearing the mixture and observing how it changes.

    Most of the videos are set up as a triptych, where all three panels show the same material. On the left, you see a simple view showing the position of each particle. In the middle, the disks are viewed through polarized filters, so that the material looks brightest where it is stressed. This view lets us see the force chains that run through the material. On the right, UV-sensitive ink on each marker glows to show any rotation particles experience.

    In the first sample, repeated shearing slowly unjams the mixture and allows it to shift and flow once more. We see this from the decreasing brightness in the middle panel. The slow fade to black means that the force chain network has disappeared entirely. In contrast, the second sample ultimately reaches an “ultra-stable” jammed state, in which further shear cycles cause no change to the network. Once again, this is easiest to observe in the middle image, where the bright force network stops changing after 2,000 cycles or so. (Image and video credit: Y. Zhao et al., research pre-print)

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    How Wells and Aquifers Work

    When rain falls, some of that water turns into run-off in storm systems but much of it seeps into the ground. What happens to that water? In most places, it joins the local aquifer, infusing the spaces between soil particles underground. In this video, Grady takes us through some of the interactions between surface water, aquifers, and the wells we use to access water underground. He’s even built some great demonstrations to show how aquifers and surface water like rivers pass water back and forth. (Image and video credit: Practical Engineering)

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    “I See You”

    In “I See You,” filmmaker Rus Khasanov captures fluid flows that give the screen an eye with which to gaze back at us. The textures visible in the flows are incredible at mimicking the details of a human iris. These are some seriously neat Marangoni flows. For a similar effect, check out this film of his. (Image and video credit: R. Khasanov)

  • Asperitas Formation

    Asperitas Formation

    In 2017, the World Meteorological Organization named a new cloud type: the wave-like asperitas cloud. How these rare and distinctive clouds form is still a matter of debate, but this new study suggests that they need conditions similar to those that produce mammatus clouds, plus some added shear.

    Using direct numerical simulations, the authors studied a moisture-filled cloud layer sitting above drier ambient air. Without shear, large droplets in this cloud layer slowly settle downward. As the droplets evaporate, they cool the area just below the cloud, changing the density and creating a Rayleigh-Taylor-like instability. This is one proposed mechanism for mammatus clouds, which have bulbous shapes that sink down from the cloud.

    When they added shear to the simulation, the authors found that instead of mammatus clouds, they observed asperitas ones. But the amount of shear had to be just right. Too little shear produced mammatus clouds; too much and the shear smeared out the sinking lobes before they could form asperitas waves. (Image credit: A. Beatson; research credit: S. Ravichandran and R. Govindarajan)