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  • Fallstreak Holes

    Fallstreak Holes

    Occasionally clouds appear to have a hole in them; these are known as fallstreak holes or hole-punch clouds. To form, the water droplets in the cloud must be supercooled; in other words, they must be colder than their freezing point but still in liquid form. When disturbed — say, by the temperature drop caused by flowing over an airplane wing — the supercooled water droplets will suddenly freeze. This typically kicks off a chain reaction in which many droplets freeze and the heavy ice crystals fall out of the sky, leaving behind a void in the cloud. Because airplanes are particularly good at creating these fallstreak holes, they’re often seen near busy airports. (Image credit: J. Stevens/NASA; via NASA Earth Observatory)

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    “Radiolarians”

    “Radiolarians” is a short film by artist Roman De Giuli using ink, alcohol, and oil. Much of the fluid motion involves break-up into droplets. The effects appear to rely heavily on Marangoni bursting, the physics of which you can learn about in this previous post. (Image and video credit: R. De Giuli)

  • Chaotic Mixing in Porous Media

    Chaotic Mixing in Porous Media

    One of the peculiar characteristics of viscous, laminar flows is that they are reversible. Squirt dye into glycerin, stir it one way, then the opposite direction, and the dye returns to its initial position. But this neat trick only works in simple geometries; in a more complex environment, like the pores between packed gravel, flows cannot make their way back to their initial state.

    That’s the idea at the heart of this new study of mixing in porous media. Researchers took a bed of packed beads and pushed a slow, steady flow of dye into the bed. Then they steadily withdrew fluid to reverse the flow and observed how the dye they’d injected appeared at the surface of the bed (top image). If the flow were perfectly reversible, we’d expect the dye to return to its injection point. But instead the dye is spread chaotically across the surface, giving researchers a snapshot of the chaotic mixing taking place between beads. (Image and research credit: J. Heyman et al.; via APS Physics)

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    Wind Turbine Physics

    Over the years, wind turbines have gotten tall with long, thin blades. This MinutePhysics video delves into the reasons for those changes. They’re all aimed at generating more wind power and doing so with greater efficiency.

    I’ll add one caveat to the video, though, because you may wonder how modern wind turbines can be fast when they appear to rotate so slowly. That’s a trick of the reference frame. The power a turbine blade generates depends on the flow speed over it, and the relative air speed is greatest near the tip of the turbine blades.

    Think of the circle the blade tip traces. For a given rotation rate – say once revolution a minute – the blade tip has a much larger distance to travel than the blade’s base does. Divide that large distance by the rotation time and you get a large velocity. So even though the wind turbine appears to be rotating slowly, the flow the blade sees is quite fast. And the longer the wind turbine’s blades, the larger this effect. (Image and video credit: H. Reich/MinutePhysics)

  • The Sounds of Leidenfrost Stars

    The Sounds of Leidenfrost Stars

    On a hot surface, droplets can float on a layer of their own vapor and vibrate in star-like shapes. These so-called Leidenfrost stars also make noise, with distinct beats that match the oscillations of the vapor layer beneath them. Researchers found that the frequency of the sound shifts with droplet size, increasing as the drop size decreases. Physically, the droplets act much like a wind instrument! (Image and research credit: T. Singla and M. Rivera; via APS Physics)

  • Blue Jets

    Blue Jets

    Blue jets are a mysterious form of lightning that shoots upward from intense thunderstorms. The image above comes from one of the first color videos of blue jets, taken by an astronaut aboard the International Space Station. Scientist think blue jets form during an electric breakdown between the positively-charged upper region of a cloud and the negative charge at its boundary. Once the discharge starts, it can shoot to the stratopause in less than a second, forming a glowing, blue, nitrogen-based plasma. (Image credit: ESA/NASA/DTU Space; via NASA Earth Observatory)

  • Spiderwebs and Stratocumulus Clouds

    Spiderwebs and Stratocumulus Clouds

    Stratocumulus clouds cover about 20% of Earth’s surface at any given time, and they form distinctive patterns of lumpy cells separated by thin slits. Because of their interconnectedness, researchers nicknamed these narrow regions spiderwebs. New simulations show that evaporative cooling along the cloud tops drives the formation of these spiderwebs (Image 2). Without it (Image 3), the cloud pattern looks very different. (Image credits: featured image – L. Dauphin/MODIS, others – UConn ME 3250; research credit: G. Matheou et al.)

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    The Unsinkable Pygmy Gecko

    The Brazilian pygmy gecko is a tiny inhabitant of the Amazon rainforest, growing to no longer than 24 mm. But these tiny lizards have some incredible superpowers when it comes to surviving the rainforest’s deluges. The gecko’s surface is superhydrophobic — water repellent — thanks to millions of tiny hairs that create air pockets between water and the gecko’s skin. This superhydrophobic surface, combined with the gecko’s tiny stature, allow it to sit atop water, supported entirely by surface tension. (Image and video credit: BBC Earth)

  • Gathering Droplets

    Gathering Droplets

    In deserts around the world, plants have adapted to collect as much moisture as they can. Geometry aids them in this endeavor because droplets on the tip of a cone will move toward its thicker base. The motion takes place due to a imbalance in surface tension forces on either end of the droplet.

    As the droplet moves up a cone, it changes shape from a barrel-like drop that fully covers the conical surface to a clamshell-shaped droplet that hangs only from the bottom of the cone. (Image and research credit: J. Van Hulle et al.)

  • Jellyfish Make Their Own Walls

    Jellyfish Make Their Own Walls

    When we walk, the ground’s resistance helps propel us. Similarly, flying or swimming near a surface is easier due to ground effect. Most of the time swimmers don’t get that extra help, but a new study shows that jellyfish create their own walls to get that boost.

    Of course, these walls aren’t literal, but fluid dynamically speaking, they are equivalent. Over the course of its stroke, the jellyfish creates two vortices, each with opposite rotation. One of these, the stopping vortex, lingers beneath the jellyfish until the next stroke’s starting vortex collides with it. When two vortices of equal strength and opposite rotation meet, the flow between them stagnates — it comes to halt — just as if a wall were there.

    In fact, mathematically, this is how scientists represent a wall: as the stagnation line between a real vortex and a virtual one of equal strength and opposite rotation. It just turns out that jellyfish use the same trick to make virtual walls they can push off! (Image and research credit: B. Gemmell et al.; via NYTimes; submitted by Kam-Yung Soh)

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