FYFD

Videos

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    Un-Mixing a Flow

    This video demonstrates one of my favorite effects: the reversibility of laminar flow. Intuition tells us that un-mixing two fluids is impossible, and, under most circumstances, that is true. But for very low Reynolds numbers, viscosity dominates the flow, and fluid particles will move due to only two effects: molecular diffusion and momentum diffusion. Molecular diffusion is an entirely random process, but it is also very slow. Momentum diffusion is the motion caused by the spinning inner cylinder dragging fluid with it. That motion, unlike most fluid motion, is exactly reversible, meaning that spinning the cylinder in reverse returns the dye to its original location (plus or minus the fuzziness caused by molecular diffusion).

    Aside from being a neat demo, this illustrates one of the challenges faced by microscopic swimmers. In order to move through a viscous fluid, they must swim asymmetrically because exactly reversing their stroke will only move the fluid around them back to is original position. (Video credit: Univ. of New Mexico Physic and Astronomy)

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    Sandscapes

    Many of us have played with sand art–the rotating frames filled with water, sand, and air. In this video, Shanks FX demonstrates some of the realistic and surrealistic landscapes you can create using this toy. It also makes for a neat fluid dynamics demonstration. The buoyancy of the trapped air bubbles lets the sand sift slowly down instead of falling immediately. And the sand descends in a variety of ways–sometimes laminar columns and other times wilder turbulent plumes. (Video credit and submission: Shanks FX/PBS Digital Studios)

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    The Inverted Glass Harp

    You may be familiar with the glass harp, the instrument created by rubbing the rim of a partially-filled wine glass. But did you know that you can create the same effect by immersing an empty glass in water? In this video, Dan Quinn explains the physics behind both types of glass harps and why the pitch changes as you add or remove water. Vibration is the driving factor (as with most sound), and the key to the shifting pitches has to do with the change in mass of the material being vibrated. For more great physics, also be sure to check out Quinn’s previous video on tears of wine.  (Video credit: D. Quinn)

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    Printing in Glass

    A group at MIT have created a new 3D printer that builds with molten glass. This allows them to manufacture items that would difficult, if not impossible, to create with traditional glassblowing or other modern techniques. One of the coolest aspects of this technique is that it can use viscous fluid instabilities like the fluid dynamical sewing machine to create different effects with the glass. You can see this around 1:56 in the video. Varying the height of the head and the speed at which it moves will cause the molten glass to fall and form into different but consistent coiling patterns. All in all, it’s a very cool application for using some nonlinear dynamics! (Video credit: MIT; via James H. and Gizmodo)

  • The Angle of Repose

    [original media no longer available]

    Granular materials like sand tend to form heaps when poured. The steepness of the heap at rest is described by the angle of repose, which is determined by a balance between gravity, normal force, and friction on the grains. When a heap of grains is disturbed, it can trigger an avalanche. As can be seen in the video above, avalanches are a surface phenomenon, only moving the top few layers of grain while most of the heap remains stationary.  (Video credit: Peddie School Physics)

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    “The Chase”

    Sometimes it takes timelapse photography to truly appreciate the dynamic behavior of our atmosphere. In “The Chase” Mike Olbinski, whose work we’ve featured previously, has captured some of the most incredible and stunning weather timelapse footage I have ever seen. Despite watching it repeatedly, I continue to be awed to the point that I have no words. Seriously, just watch it. Be amazed by the drama of our sky. (Video credit: M. Olbinski)

  • Fire Tornadoes

    Fire tornadoes, despite their name, are more closely related to dust devils or waterspouts than to true tornadoes. Though rarely documented, they are relatively common, especially in wildfires. The heat of the fire creates an updraft of warm, rising air that leaves behind a low-pressure region. Air from outside is drawn toward this low-pressure area, gets heated, and rises. As the outside air gets pulled in, any vorticity or rotation it had gets intensified via conservation of angular momentum–the same way a spinning ice skater speeds up when she pulls her arms in. The result is the tightly-spinning vortex at the heart of a fire tornado. (Video credit: C. Fleur; via NatGeo)

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    Bubbles and Hurricanes

    You may think of soap bubbles as a childhood plaything, but there’s a lot to be learned from them. In her newest video, Dianna of Physics Girl explores some of the fascinating research scientists use soap bubbles for and how you can recreate some of their experiments at home. Scientists have used bubbles to explore how atmospheric vortices behave, how to tie knots in fluids, how grass waves in the wind, and even how explosive detonations occur. Just modeling bubbles and foams can be incredibly complex. It turns out the humble bubble has quite a lot to teach us. (Video credit: Physics Girl/PBS Digital Studios)

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    Turbulent Ink

    Turbulence is found throughout our lives, but rarely is it as startlingly beautiful as in this Slow Mo Guys video. Here they show high-speed videos of ink being injected into water. The resulting plumes are turbulent from the very start, with innumerable folds and eddies billowing outward as the plume expands. The large difference in length scales–from the millimeter-sized curls to the meter-sized length of the plume–is one of the classic characteristics of turbulence and part of what makes turbulent flows so difficult to model computationally. Energy in these flows is generated at the large scales, but it’s dissipated at the very smallest scales through viscosity. This means that to properly model a turbulent flow, you have to capture the largest scales, the smallest scales, and everything in between in order to represent this energy cascade from large to small. It’s a problem that engineers, mathematicians, meteorologists, and physicists have struggled with for more than a century. But, here, at least, we can all just sit back and enjoy the beauty. (Video credit: The Slow Mo Guys)

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    Leaping Mobulas

    Mobula rays engage in some pretty incredible aerial acrobatics. This species of ray, second only to manta rays in size, can jump up to 2 meters into the air. Large groups of mobula rays will engage in this behavior, including both males and females, but it remains unclear to scientists exactly what purpose the jumping serves. It may be a form of communication, which might explain the rays’ apparent preference for belly flopping. By striking the water surface with as much of their body as possible simultaneously, the rays generate both a large splash and a concussive clap that carries through the water. (Video credit: BBC; via J. Hertzberg)