FYFD

Tag: 2022gofm

  • Flow Over an AT-AT

    Flow Over an AT-AT

    Having previously examined the re-entry characteristics of an X-Wing, a group of engineers are back to look at Imperial vehicle physics. In this poster, they look at what happens to the AT-AT walker when strong crosswinds, like those seen in the Battle of Hoth, blow across the vehicle’s path. Given its boxy body and gangly legs, it will come as no surprise that the AT-AT is not at all streamlined and instead causes lots of separated flow. Those flow separations come with strong side forces that can tip the walkers.

    Be sure to take a closer look at the text on the poster. It’s written from the perspective of Imperial engineers, complete with recommendations for the next generation of AT-AT. (I don’t think those got built, at least not by the Empire!) May the 4th be with you! (Image credit: Y. Yuan et al.)

  • Giant Droplet Splashes

    Giant Droplet Splashes

    When droplets get larger than 0.27 cm, they no longer stay spherical as they fall. Here, researchers look at very large droplets (equivalent to 3.06 cm in diameter) falling into water. On their way to the pool, the droplets oscillate — some lengthening, some flattening, and some bulging into a bag. The droplet’s shape at impact (and its speed) determine what shape of splash and cavity form. Wider drops make wider and shallower cavities. (Image credit: S. Dighe et al.)

  • A 2D Splash

    A 2D Splash

    We see plenty of droplets splash when they fall into a pool, but what happens when the drop and pool are two-dimensional? Here researchers captured the familiar process of a splash in an unfamiliar way by looking at a falling drop contained within a soap film. As the drop reached the thicker lower boundary of the soap film (which acts like a pool), its impact sent up ejecta that stretch and curl, much like the three-dimensional splashes we’re accustomed to. (Image credit: A. Alhareth et al.)

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    Magnetic Soap Films

    Soap films naturally thin over time as fluid evaporates and differences in film thickness cause surface-tension-driven flows. In this video, researchers experiment with adding magnetic nanoparticles to the soap film. In the second image, the white structures near the center of the film contain nanoparticles, and they’re drawn toward the magnet that sits (out-of-frame) to the left of the film. With more nanoparticles and a stronger magnetic field (Image 3), the entire soap film takes on a distinctive profile that thins from left to right. The effect is so strong that the film quickly thins to the point of rupture. (Image and video credit: N. Lalli et al.)

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    Surfactants and Waves

    In the ocean, waves often curl over and trap air, becoming plunging breakers. How do surfactants like soap or oil affect this process? That’s the question behind this video, where researchers visualize breaking waves with differing amounts of added surfactant. In the case of pure water, the wave forms a smooth jet that curls over and traps air when the wave breaks. As more and more surfactant gets added, the shape of that jet and cavity change. In one case, they become irregular. In another, they disappear entirely, and with the most surfactant added, the wave suddenly looks just like the water-only case.

    The key to these behaviors, it turns out, is not how much surfactant there is, but how much the concentration of surfactant varies along the length of the wave. When there are significant changes in the surfactant concentration along the wave, local Marangoni flows try to even out the surface tension, causing the wave to break up in an irregular fashion. (Image and video credit: M. Erinin et al.)

  • Beneath the Cavity

    Beneath the Cavity

    When a drop falls into a pool of liquid, it creates a distinctive cavity, followed by a jet. From above the surface, this process is well-studied. But this poster offers us a glimpse of what goes on beneath the surface, using particle image velocimetry. This technique follows the paths of tiny particles in the fluid to reveal how the fluid moves.

    As the cavity grows, fluid is pushed away. But the cavity’s reversal comes with a change in flow direction. The arrows now point toward the shrinking cavity — and they’re much larger, indicating a strong inward flow. It’s this convergence that creates the Worthington jet that rebounds from the surface. And, as the jet falls back, its momentum gets transferred into a vortex ring that drifts downward from the point of impact. (Image credit: R. Sharma et al.)

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    Collapsing Cavitation Bubbles

    Cavitation bubbles live short, violent lives. Triggered here with a laser, these bubbles rapidly expand and then collapse, sending out shock waves. In this video, researchers explore how bubbles collapse when they’re near a plate with holes in it. For bubbles sitting between holes, collapse becomes asymmetric, eventually splitting the bubble into two as it falls in on itself. Bubbles centered over a hole perform a disappearing act, sucking themselves down into the hole during collapse. (Image and video credit: E. Andrews et al.)

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    Hollow Drops

    When a partially-air-filled drop hits a surface, it splashes and rebounds in a complex fashion. This video breaks down the physics of the process. Upon impact, a lamella spreads, eventually becoming wavy and unstable along its rim. At the same time, a counterjet forms, growing until it pierces the remaining bubble of the drop. The jet continues to stretch upward due to its momentum, pinching off and forming wobbly satellite drops that finally fall back to the surface. (Image and video credit: D. Naidu and S. Dash)

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    Kelvin-Helmholtz Flows Downhill

    Gravity currents carry denser fluids into lighter ones, like cold air drifting under your door in winter or dense fogs flowing downhill in San Francisco. Here, researchers visualize the situation using denser salt water flowing into fresh water. Once the gate separating the two fluids rises, the salt water slides down an artificial slope into the fresh water.

    Very quickly the flow forms a Kelvin-Helmholtz instability due to the different flow speeds between the two fluids. Kelvin-Helmholtz waves form distinctive swirls and billows that are reminiscent of a cat’s eye. As the swirls rotate, they can flow over one another, and break up into turbulence. (Image and video credit: C. Troy and J. Koseff)

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    Exascale Simulations

    Capturing what goes on inside a combustion engine is incredibly difficult. It’s a problem that depends on turbulent flow, chemistry, heat transfer, and more. To represent all of those aspects in a numerical simulation requires enormous computational resources. It’s not simply the realm of a supercomputer; it requires some of the fastest supercomputers in existence.

    Exascale computing, like that used for the simulations in this video, is defined as at least 10^18 (floating-point) operations per second. For comparison, my PC has a recent, high-end graphics card, and it’s about a million times slower than that. These are absolutely gigantic simulations. (Image and video credit: N. Wimer et al.)