FYFD

Tag: instability

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    Convection in Action

    We’re surrounded daily by convection — a buoyancy-driven flow — but most of the time it’s invisible to us. In this video, Steve Mould shows off what convection really looks like with some of his excellent tabletop demos. The first half of the video gives profile views of turbulent convection, with chaotic and unsteady patterns. When he switches to oil instead of water, the higher viscosity (and lower Reynolds number) offer a more structured, laminar look. And finally, he shows a little non-temperature-dependent convection with a mixture of Tia Maria and cream, which convects due to evaporation changing the density. (Image and video credit: S. Mould; submitted by Eric W.)

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    “Lucid”

    Artist Roman Hill made this official music video to go with Thomas Vanz’s “Lucid.” The imagery, formed from ink and other fluids, warps our sense of scale. Though the camera focuses on an extremely small area, to our eyes the results shift from nebulas to oceans and back again. There are likely a whole host of phenomena going on here, but without knowing more about Hill’s ingredients, I can only speculate that there are Marangoni flows driven by variations in surface tension and maybe some density instabilities going on between fluid layers. I’m also fairly confident that Hill has played with time reversal in the video editing. Regardless of the secrets in its making, the film is captivating and gorgeous. (Image and video credit: R. Hill)

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    “Perfect Sky”

    It’s all blue skies in Roman De Giuli’s short film, “Perfect Sky.” Created with paint, ink, and glitter on paper, it’s a relaxing piece of fluid art. Put on your headphones, take a deep breath, and plunge in. You’ll see lots of gorgeous Marangoni effects, some low Reynolds number mixing, and various instabilities. (Video and image credit: R. De Giuli)

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    Tracking Break-Up

    In fluid dynamics, researchers are often challenged with complicated, messy flows. With so much going on at once, it’s hard to work out a way to keep track of it all. Here, researchers are looking at the break-up of two colliding liquid jets. This setup is often used to break rocket fuel into droplets prior to combustion. This video shows off a new data analysis tool that lets researchers break the flow into different parts, track them in time, and extract data about the changes that happen along the way. (Video and image credit: E. Pruitt et al.)

  • Mocha Diffusion

    Mocha Diffusion

    These firework-like patterns spread when dyes are added atop a viscous but miscible lower fluid layer. Here, researchers use lower layers like corn syrup and xanthan gum; then they spread dye mixtures including ammonia and vinegar atop those layers. Because the upper and lower layers of fluid are miscible and can diffuse into one another, they together form elaborate patterns. The mixing of the two layers creates gradients in surface tension that can drive the flow and create these mocha diffusion patterns. (Image credit: T. Watson and J. Burton)

  • The Sound of Bubbles

    The Sound of Bubbles

    Every day I stand in front of my refrigerator and listen to the water dispenser pouring water into my glass. The skinny, fast-moving jet of water plunges into the pool, creating a flurry of bubbles. Those bubbles come from air the water jet pulls in with it, and the sound the water makes (minus the fridge’s noises) comes from those bubbles. A short, laminar jet will make fewer bubbles and, therefore, be quieter than a a jet that falls farther before hitting the water.

    The reason? That tall jet falls for long enough that its walls start to wobble or even break up completely into separate droplets. Compared to a smooth jet, these wobbly or broken-up jets pull in more air and create more bubbles. That makes them louder. Researchers even suggest that listening to these bubbles can give a noninvasive method for finding how much fresh oxygen is in the water. (Image credit: R. Piedra; research credit: M. Boudina et al.; via APS Physics)

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    Liquid Lace

    3D printers are a neat apparatus for exploring flow instabilities. If too much material is extruded compared to the speed of the printer head, coiling takes place. But under-extrusion creates patterns, too. Here, researchers show how under-extrusion can create a stable lace-like pattern. Once dried, the material can stretch, but only in certain directions, a bit like many textiles. (Video and image credit: L. Dreier et al.)

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    “High Flow”

    Roman De Giuli’s “High Flow” is vibrant and energetic. Colorful paints and inks flow across the page, creating complex patterns. I love the blossoming flows, feathery fronds, and spreading Marangoni effects. De Giuli’s films never disappoint! (Video and image credit: R. De Giuli)

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    “Alive”

    In “Alive,” filmmaker Christopher Dormoy explores acrylic paints and the variety of ways in which the medium can be used. From a fluids perspective, there’s dripping, viscous flow, turbulent eddies, billowing plumes, and “accidental painting” due to density-driven instabilities. It’s a fun tour of fluid phenomena in art. What examples do you spot? (Video and image credit: C. Dormoy)

  • Controlling Finger Formation

    Controlling Finger Formation

    When gas is injected into thin, liquid-filled gaps, the liquid-gas interface can destabilize, forming distinctive finger-like shapes. In laboratories, this mechanism is typically investigated in the gap between two transparent plates, a setup known as a Hele-Shaw cell. In the past, researchers looking to control the instability have explored how surface tension, viscosity, and the elasticity of the gap itself affect the flows. But a new set of studies look at the compressibility of the gas being injected.

    The team found that viscous fingers formed later the higher the gas’s compressibility. That provides a potential control knob for people trying to exploit the mechanism, especially geologists. For geologists trying to extract oil, viscous fingering is detrimental, but, on the flip side, viscous fingers are desirable when injecting carbon dioxide for sequestration. With these results, users can tweak their injection characteristics to match their goals. (Image credit: C. Cuttle et al.; research credit: C. Cuttle et al. and L. Morrow et al.; via APS Physics)