FYFD

Videos

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    The Glory of a Roll Cloud

    Roll clouds stretch like a long horizontal tube, spinning as they process across the sky. This class of arcus cloud is relatively rare but occasionally forms in areas where cool air is sinking, along the downdraft of an oncoming storm or in coastal regions as a result of sea breezes. The cooler, sinking air displaces warmer, moist air to higher altitudes where the moisture condenses into a cloud. Winds then roll the cloud parallel to the horizon. Roll clouds are a form of soliton, a solitary wave with a single crest that moves without changing its shape or velocity; this is why the cloud appears so regular as it moves across the sky. These clouds are sometimes also called Morning Glory clouds and form regularly off the coast of Queensland, Australia around October. (Video credit: T. and B. Mask)

    A reminder, for those attending the APS DFD conference this weekend: my FYFD talk will be Sunday evening at 5:37pm in Rm 306/307. I will be discussing, among other things, the results of July’s reader survey and science communication.

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    The Challenges of Trapping Carbon Dioxide

    One way to reduce carbon dioxide in the atmosphere is to pump the CO2 into saline aquifers deep below the surface. Such aquifers are thin but stretch over large areas and are sometimes gently sloping. Since carbon dioxide is relatively buoyant, it may migrate up-slope after injection and potentially leak elsewhere. Dissolving the carbon dioxide into the groundwater helps prevent this undesirable migration. The video above shows a laboratory analog of the fluid instability at the heart of this trap. Imagine the video tilted by a few degrees so it slopes upward toward the right. The initially buoyant carbon dioxide, represented by the dark fluid, rises on the left and moves rightward, up-slope. As the CO2 dissolves into the ambient groundwater, the water becomes denser and fingers of the CO2-rich water drift downward, effectively halting the carbon dioxide’s escape. This is known as convective dissolution. (Video credit: C. MacMinn and R. Juanes)

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    Avoiding Splashback

    Here’s a likely Ig Nobel Prize candidate from the BYU SplashLab: a study of splashing caused by a stream of fluid entering a horizontal body of water or hitting a solid vertical surface. In other words, urinal dynamics. The researchers simulated this activity using a stream of water released from a given height and angle and observed the resulting splash with high-speed video. They found a stream falls only 15-20 centimeters before the Plateau-Rayleigh instability breaks it into a series of droplets, and that this is the worst-case scenario for splash-back. The video above shows how a stream of droplets hits the pool, creating a complex cavity driven deeper with each droplet impact. Not only does each impact create a splash, the cavity’s collapse does as well. Similarly, when it comes to solid surfaces, they found that a continuous stream splashes less. They’ve also put together a helpful primer on the best ways to avoid splash-back. (Video credit: R. Hurd and T. Truscott; submitted by Ian N., bewuethr, John C. and possibly others)

    For readers attending the APS DFD meeting, you can catch their talk, “Urinal Dynamics,” Sunday afternoon in Session E9 before you come to E18 for my FYFD talk.

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    Particle-Tracking in Granular Flows

    One of the challenges of experimental fluid dynamics is gathering sufficient data in environments that can be fast-changing, visually dense, and sometimes harsh. Ideally, researchers want to gather as much data–velocities, temperatures, pressures–at as many points as possible and do so without disturbing the flow with a probe. No technique can provide everything, and thus new diagnostics are always under development. This video shows a new particle tracking method developed for fluidized granular flows where the high concentration of particles makes other techniques unsuitable. Such flows are often seen in industrial applications in chemical processing, pharmaceuticals, and powder transport. Interestingly, the technique can also be used in particle-seeded fluid flows like those normally studied with particle image velocimetry (PIV). (Video credit: F. Shaffer and B. Gopalan; submitted by @ASoutolglesias)

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    Wavy Swimmers

    Animals often move in ways engineers find counter-intuitive. Take, for example, the glass knifefish, an undulatory swimmer that controls its motion through wavelike oscillations of its fin. One might expect the knifefish to move its fin so that a single continuous wave moves from one end to the other. Instead two opposing waves move down the knifefish’s fins, one travelling from head to tail and the other travelling from the tail forward. The intersection of these waves is the nodal point, and, by shifting the nodal point fore or aft, the knifefish can hover in place, move forward or swim backward. At first glance, this seems like a wasteful system since a significant portion of each wave cancels the other, but, through mathematical modeling and experiments with a biomimetic robot, the researchers found that the dual-wave locomotion increases both the stability and maneuverability of the fish. (Video credit: N. Cowan et al.; via phys.org)

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    Engineering Sediment Transport

    Sediment transport via fluid motion is a major factor in engineering, geology, and ecology. This video shows two common forms of sediment transport: particle suspension and saltation. Suspension, in which the fluid carries small solid particles, is visible high in the blue water layer. Saltation occurs closer to the surface when loose particles are picked up by the flow before being redeposited downstream. Watch some of the individual particles near the surface to see the process. Kuchta has several more demo videos of flow in this desktop flume, sold by Little River Research & Design. (Video credit: M. Kuchta; submitted by gravelbar)

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    Holey Splashes

    A liquid’s surface tension can have a big effect on its splashes. In this video, a 5-mm droplet hits a surface covered in a thin layer of a liquid with lower viscosity and surface tension. The result is a dramatic effect on the spreading splash. As the initial curtain grows and expands, the lower surface tension of the impacted fluid thins the splash curtain. Fluid flows away from these areas due to the Marangoni effect, causing holes to grow. The sheet breaks up into a network of liquid filaments and ejected droplets before gravity can even bring it all to rest. For more, see this previous post and review paper. (Video credit: S. Thoroddsen et al.)

  • Liquid Sculptures

    [original media no longer available]

    Water sculptures–a marriage of liquids, photography, and timing–are spectacular form of fluid dynamics as art. Artist Markus Reugels is a master of the form. This video captures the life and death of such water sculptures at 2,000 fps, beginning with the fall of the initial blue droplet. The droplet’s impact causes a rebounding Worthington jet, which reaches its pinnacle just as a second droplet strikes. The impact spreads into an umbrella-like skirt consisting of a thin, expanding liquid sheet with a thicker rim. The rim itself is unstable, breaking into regularly spaced filaments and tiny satellite droplets that shoot outward before the entire structure collapses into the pool. One especially cool aspect of watching this in video is seeing how the blue dye from each droplet spreads as the water splashes and rebounds. You can see the set-up Reugels uses for his photography here. (Video credit: M. Reugels and L. Lehner)

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    Cornstarch Physics

    Oobleck, a non-Newtonian fluid made up of water and cornstarch, is a perennial Internet favorite for its ability to dance and the fact that one can run across a pool of it. It’s typically described as a shear-thickening fluid and only exhibits solid-like behavior under impact. Strictly speaking, oobleck is a suspension of solid grains of cornstarch in water. When struck, the initially compressible grains jam together, creating a region more like a solid than a liquid. From this point of impact, a solidification front expands through the suspension, jamming more grains together and enabling the fluid to absorb large amounts of momentum. The process is known as dynamic solidification. (Video credit: University of Chicago; research credit: S. Waitukaitis & H. Jaeger)

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    Floating Water Bridges

    Water bridges that seem to float on air are an electrohydrodynamic phenomenon. By filling two beakers with extremely pure deionized water and applying a large voltage across them, flow is induced from one beaker to the other, as seen in the first few seconds of the video above. This flow is stable enough that the beakers can then be separated by a few centimeters without disturbing the bridge. Gravity tends to make the water bridge sag and capillary action tries to thin the bridge, but both effects are countered by the polarization forces induced in the water by the electric field. You can learn much more about the effect and see both photos and videos of it in action at Elmer Fuchs’ webpage. The flow visualization videos are especially neat! (Video credit: E. C. Fuchs)