FYFD

Category: Phenomena

  • Beijing 2022: Ski Jumping

    Beijing 2022: Ski Jumping

    In ski jumping, aerodynamics are paramount. Each jump consists of four segments: the in-run, take-off, flight, and landing. Of these, aerodynamics dominates in the in-run — where jumpers streamline themselves to minimize drag and maximize their take-off speed — and in flight. During flight, ski jumpers spread their skis in a V-shape and lift their arms to the sides to turn themselves into a glider. Their goal is to maximize their lift-to-drag ratio, so that the air keeps them aloft as long as possible. Because of the short flight time and high risk of taking jump after jump, many elite ski jumpers use wind tunnel time to practice and hone their flight positioning, as seen in the video below.

    Weather also plays a significant role in ski jumping; it’s one of the few sports where a headwind is an advantage to athletes. To try to adjust for wind effects, scoring for the sport uses a wind factor. (Image credit: T. Trapani; video credit: NBC News)

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    Acrylic Paint Fractals

    Here’s a simple fluids experiment you can try at home using acrylic paints, ink, isopropyl alcohol and a few other ingredients. When dropped onto diluted acrylic paint, a mixture of black ink and alcohol spreads in a fractal fingering pattern. The radial (outward) flow is driven by the alcohol’s evaporation, which increases the local surface tension and draws fluid outward. The shape and density of the fingers depends, at least in part, on the viscosity of the underlying paint layer; more viscous paint layers grow smaller and denser fractal patterns. (Image and video credit: S. Chan et al.)

  • Antarctic Meltwaters

    Antarctic Meltwaters

    Cerulean blue meltwater glints in this satellite image of the George VI Ice Shelf. Wedged between the Antarctic Peninsula on the right and Alexander Island on the left, the ice shelf itself floats on the ocean. When ice shelves collapse, they do not directly raise sea levels since their weight has already displaced water; but a collapsed ice shelf lets glaciers flow and break up faster, thereby raising water levels.

    In past ice shelf collapses, scientists have noted major buildup and sudden drainage of surface lakes like the ones seen here. Meltwater penetrating through snow and ice can destabilize the shelf and hasten collapse, but the exact mechanisms are hard to track. This Physics Today article summarizes our understanding of the process and some of the methods scientists use to study it. (Image credit: L. Dauphin/NASA Earth Observatory; see also Physics Today)

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    The Assassin’s Teapot

    The assassin’s teapot is a cleverly designed container that can pour from different reservoirs depending on how it’s held. Steve Mould digs into the physics in this video, and he builds a transparent cutaway version of the pot to show exactly how it works. This design uses two separate reservoirs, each with two holes — one in the spout and one concealed near the pot’s handle. By covering this breather hole, the server blocks air from flowing into the teapot, which also keeps the liquid inside from flowing out.

    What holds the liquid in? Air pressure, with an assist from surface tension. Atmospheric pressure is enough to hold the fluid inside the pot, provided air has no separate way in. To get in through the spout, air would have to push into the pot at the same time as water coming out. Surface tension prevents that, though, because the spout is too narrow. The same physics keeps water inside a larger bottle with a wire mesh over its mouth. The mesh’s tiny holes are smaller than the capillary length of water, which is the length scale at which surface tension and gravity balance one another. As long as the spout and holes are smaller than that length, surface tension will keep the liquid from deforming enough to get out. (Video and image credit: S. Mould)

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    The Yarning Droplet

    Marangoni bursting takes place in alcohol-water droplets; as the alcohol evaporates, surface tension changes across the liquid surface, generating a flow that tears the original drop into smaller droplets. Here researchers add a twist to the experiment using PMMA, an additive that dissolves well in alcohol but poorly in water. As the alcohol evaporates, the PMMA precipitates back out of the water-rich droplet, forming yarn-like strands. (Image and video credit: C. Seyfert and A. Marin)

  • The Return of the Ice Disk

    The Return of the Ice Disk

    Maine’s giant, spinning ice disk is taking shape again. In 2019, it reached about 91 meters across, rotating slowly in the Presumpscot River. How exactly these features form is still a matter of debate, but scientists have worked out a few relevant mechanisms. The spinning of the disk seems to depend on a vortex that forms beneath the ice as melting water sinks. (One of water’s peculiarities is that it’s densest around 4 degrees Celsius, so newly melted water is actually denser than ice. Otherwise ice wouldn’t float!) The plume of sinking water sets up a vortex that drags the ice disk with it as it spins in the river beneath. (Image credit: R. Bukaty/AP; via Gizmodo)

  • Volcanic Shocks

    Volcanic Shocks

    A violent underwater eruption at the Hunga Tonga-Hunga Ha’apai caldera on January 15th sent literal shock waves around the world. This animation, based on satellite images from Japan’s Himawari 8, shows the fast-moving shock waves and the growing ash plume coming from the uninhabited island. Although most recent eruptions from this volcano have been small, experts suspect that this latest eruption is part of a major event, similar to the volcano’s last big eruption about 1,000 years ago.

    The explosiveness of the eruption comes from the interaction of seawater and fresh magma. When the magma erupts quickly underwater, the hot liquid contacts seawater directly rather than forming a protective layer of vapor (as in the Leidenfrost effect). The resulting explosion tears the magma apart, exposing more hot surfaces to the cold water and further driving the chain reaction. (Image credit: S. Doran/Himawari 8; submitted by jpshoer; see also S. Cronin)

  • Cracking Droplets

    Cracking Droplets

    Droplets infused with particles — like coffee — can leave complex stains once they evaporate. Here researchers show the complex cracking pattern that develops as a droplet with nanoparticles evaporates. The central image in the poster actually shows the drop’s pattern changing in time. The initial drop is shown at 9 o’clock, and as you move clockwise around the drop, time passes and the crack structure becomes more complex. What a neat way to visualize the changes! (Image and research credit: P. Lilin and I. Bischofberger)

  • December’s Derecho

    December’s Derecho

    I confess I’d never heard the term derecho before moving to Colorado, but I’ve experienced a few of these wind storms now. They’re intense! Last December’s derecho formed when a high-pressure system in the western United States met a strong low-pressure system over the northern plains. In fluids, flow moves preferentially from areas of high pressure to those with low pressure, and that’s no different when it comes to weather. The strong pressure gradient drove high winds from the Rocky Mountains to Minnesota. The animation above shows the strongest winds in in yellow-white but even the “weaker” pink areas saw winds comparable to a fast-moving car in speed. The visualization is constructed from data reported by ships, buoys, aircraft, satellites, and other sources, all processed through a NASA weather algorithm. (Image credit: J. Stevens/NASA; via NASA Earth Observatory)

  • Swirls in the Wake

    Swirls in the Wake

    Rocky islands make excellent atmospheric swirls, as seen here around Guadalupe Island. Winds blowing in from the ocean get forced up and around the island’s topography, resulting in vortices that shed alternately from either side of the island. The pattern they form is known as a von Karman vortex street and is easily seen in satellite imagery, thanks to the swirls that can persist for tens of kilometers downstream. Personally, I never get tired of this one! (Image credit: NASA/GSFC/JPL; video credit: NOAA/CIRA; via Dakota Smith; submitted by @SellaTheChemist)