FYFD

Category: Phenomena

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    Taylor Columns

    When rotating, fluids often act very differently than we expect. For example, an obstacle in a rotating flow will deflect flow around it at all heights. This is known as a Taylor column.

    In this video, we see the phenomenon recreated in a simple rotating tank (that’s easy to build yourself). Once all the water in the tank is rotating at the same rate, there is very little variation in flow with height. Food coloring dropped into the tank forms tight vertical columns. Even with a short obstacle in place and induced flow in the tank from a change in rotation rate, the dye continues to move uniformly in height. Because the dye cannot travel through the obstacle, it goes around and does so at every height, leaving the space above the obstacle dye-free.

    The same phenomenon occurs in planetary atmospheres; this rotating tank is basically a mini-version of our own atmosphere. Where there are obstacles — like mountains — on our planet, air has an easier time flowing around the mountain instead of over it! (Image and video credit: DIYnamics)

  • 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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    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)

  • 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)

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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.)

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    Coastal Erosion

    The same dynamic forces that make coastlines fascinating create perennial headaches for engineers trying to maintain coastlines against erosion. This Practical Engineering video discusses some of the challenges of coastal erosion and how engineers counter them.

    In a completely undeveloped coastline, waves and storms erode the shoreline while rivers and currents replenish sand through sedimentation. Manmade structures tend to strengthen erosion processes while disrupting the sedimentation that would normally counter it. Beach nourishment — where sand gets dredged up and deposited on a beach — is an engineered attempt to replace natural sedimentation.

    Dunes, mangrove forests, and wetlands are all nature’s way of protecting and maintaining coastlines. We engineers are still learning how to both utilize and protect shorelines. (Image and video credit: Practical Engineering)

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    Ultrasonic Vibrations

    Ultrafast vibrations can break up droplets, mix fluids, and even tear voids in a liquid. Here, the Slow Mo Guys demonstrate each of these using an ultrasonic homogenizer, a piece of lab equipment capable of vibrating 30,000 times a second. At that speed generating cavitation bubbles is trivial, and the flow induced by that cavitation is well-suited to emulsifying otherwise immiscible liquids like oil and water. They also show how a lone droplet gets torn into many microdroplets, a process formally known as atomization. (Image and video credit: The Slow Mo Guys)

  • Snowflake Velocimetry

    Snowflake Velocimetry

    In our era of remote learning, students don’t always have a chance to do hands-on lab experiments in the usual fashion. But that doesn’t mean they can’t explore important flow diagnostic techniques. Here a simple smartphone video of snow falling gets turned into a lesson on particle image velocimetry, or PIV, a major technique for measuring flow velocities.

    A nearby house acts as a fixed backdrop, and by comparing snowflake positions from one frame to the next, students can measure the instantaneous flow patterns in the snowfall. Of course, that’s a tedious task to do by hand, but luckily there are computer programs that do it automatically. Simply run the smartphone video through the software, and analyze the patterns it reveals!

    As a bonus, students don’t have to get distracted by the complexities of laser sheets and flow seeding that are normally a part of PIV. Instead, the flow and the lighting are already right outside their window, and they can concentrate instead on learning the principles of the technique and how to use the software. (Image and submission credit: J. Stafford)

  • Strings of Swirls

    Strings of Swirls

    Von Karman vortex streets are the rows of alternating vortices shed off isolated objects interrupting a flow. Here, the volcanic peaks of Cabo Verde disrupt an atmospheric flow accustomed to an empty ocean. In a steady wind, air wraps around the volcanoes and detaches first on one side, creating a vortex, then from the other side, making a vortex of the opposite rotation. Although these structures are always present, we only see them when they stir up the cloud layer, leaving these strings of swirls for hundreds of kilometers behind the islands. (Image credit: L. Dauphin/NASA; via NASA Earth Observatory)