FYFD

Tag: instability

  • Aqueous Chandeliers

    Aqueous Chandeliers

    Colorful dyes falling through water form chandelier-like, branching shapes. These formations are the result of a slight density difference between the heavier dyes and the surrounding water. As the dye falls, Rayleigh-Taylor instabilities cause the mushroom-like blobs and their branches. With creativity and photographic skill, Mark Mawson turns these ephemeral shapes into bold liquid sculptures, frozen in time. See more of his work in these previous posts, on his website, and on Instagram. (Image credit: M. Mawson)

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    “Life and Chaos”

    In “Life and Chaos,” artists Roman Hill and Paul Mignot shot fluid flows live in a 1 cm x 1 cm square, then projected those images across 3,300 square meters. There’s something incredible about art on this immersive scale. It is literally impossible for any one visitor — or even the artists themselves — to experience the full piece; each person, by definition, can only take in a small part of the whole. That makes it all the more incredible to derive such a piece from a tiny, tiny canvas. As venues for this sort of immersive art spread, I can only imagine the amazing art we’ll see! (Image and video credit: R. Hill and P. Mignot)

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    “I See You”

    In “I See You,” filmmaker Rus Khasanov captures fluid flows that give the screen an eye with which to gaze back at us. The textures visible in the flows are incredible at mimicking the details of a human iris. These are some seriously neat Marangoni flows. For a similar effect, check out this film of his. (Image and video credit: R. Khasanov)

  • Asperitas Formation

    Asperitas Formation

    In 2017, the World Meteorological Organization named a new cloud type: the wave-like asperitas cloud. How these rare and distinctive clouds form is still a matter of debate, but this new study suggests that they need conditions similar to those that produce mammatus clouds, plus some added shear.

    Using direct numerical simulations, the authors studied a moisture-filled cloud layer sitting above drier ambient air. Without shear, large droplets in this cloud layer slowly settle downward. As the droplets evaporate, they cool the area just below the cloud, changing the density and creating a Rayleigh-Taylor-like instability. This is one proposed mechanism for mammatus clouds, which have bulbous shapes that sink down from the cloud.

    When they added shear to the simulation, the authors found that instead of mammatus clouds, they observed asperitas ones. But the amount of shear had to be just right. Too little shear produced mammatus clouds; too much and the shear smeared out the sinking lobes before they could form asperitas waves. (Image credit: A. Beatson; research credit: S. Ravichandran and R. Govindarajan)

  • When Seeing a Flow Changes It

    When Seeing a Flow Changes It

    Adding dye to a flow is a common technique for visualization. After all, many flows in fluids like air and water are invisible to our bare eyes. But for some classes of flows — especially those driven by variations in surface tension — adding dye can have unforeseen effects. A recent study shows how true this is for bursting Marangoni droplets, where evaporation and alcohol concentration can pull a water-alcohol droplet apart.

    Composite series of photos showing the effect of increased dye concentration on Marangoni bursting.
    As more dye is added to the experiment, the daughter droplets grow larger and more ligaments form. In the first three images, a dashed black line has been added to show the location of the droplet rim.

    Without dye, it’s nearly impossible to see the phenomenon since the refractive indices of the two component liquids are so close. But the researchers found that, as they added more methyl blue dye, it did more than increase the contrast in the flow. It changed the flow, making the droplets larger and creating ligaments between them. They believe that the dye’s own surface tension creates local gradients that alter the flow. It’s a reminder that experimentalists have to be careful to consider how our efforts to measure and observe a flow can change it. (Image credit: top – The Lutetium Project, bottom – C. Seyfert and A. Marin with modification; research credit: C. Seyfert and A. Marin)

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    “Velocity”

    In this short film by Vadim Sherbakov, macro shots of glittery ink and pigments look like astronomical vistas. The title of the film, “Velocity,” is spot on; every shot is full of flow and motion driven by the mixture of ink, alcohol, soap, and other fluids. That means lots of surface-tension-driven flow, and the glitter particles act as excellent tracers, giving a real sense of depth and direction for our gaze to follow. Watching films like this, I always want to pull out some odds and ends and try it for myself, but I’m certain my results would pale in comparison! (Video and image credit: V. Sherbakov; via Colossal)

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    Whistle Physics

    Ever wondered how whistles work? Depending on the type of whistle, there are a few different phenomena in play, but the most fundamental one is the oscillation of a fast-moving air stream. Any small deviation in the air stream can set up a situation where the flow shifts side-to-side, and most whistles use this oscillation to drive the sound they produce.

    Many whistles direct the air flow onto a wedge-shape to strengthen the oscillation; then they have a cavity that amplifies the sound using resonance. Water whistles — which warble in a bird-like way — do the same thing, but the water inside them creates a shape-changing cavity, thereby changing the pitch to create an unsteady, warbling sound. You can see all these whistles and more deconstructed in Steve’s video. (Video and image credit: S. Mould)

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    Inside Viscous Fingers

    Sandwich a viscous fluid between two transparent plates and then inject a second, less viscous fluid. This is the classic set-up for the Saffman-Taylor instability, a well-studied flow in which the interface between the two fluids forms a wavy edge that develops into fingers. Despite its long history, though, there is still more to learn, as shown in this video. Here, researchers alternately injected a dyed and undyed version of the less viscous fluid. The result (Image 3) is a set of concentric dye rings that show how the fluid moves far from the fingers along the edge. Notice that the waviness of the fingers appears in the flowing fluid well before it approaches the interface. (Image and video credit: S. Gowan et al.)

  • Blowing Up Euler

    Blowing Up Euler

    The mathematics of fluid dynamics still have many unknowns, which makes them an attractive playground for mathematicians of all stripes. One perennial area of interest is the Euler equations, which describe an ideal (i.e., zero viscosity), incompressible fluid. Mathematicians suspect that these equations may produce impossible answers — vortices with infinite velocities, for example — under just the right circumstances, but so far no one has been able to prove the existence of such singularities.

    A recent Quanta article delves into this issue and the race between researchers using traditional methods and those using new deep learning techniques. Will the singularities be found and who will get there first? It’s well worth a read, whether theoretical mathematics is your thing or not. (Image credit: S. Wilkinson; see also Quanta; submitted by Jo V.)

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    Fast Fractal Fingers

    With the right balance of viscosity and surface tension, many fluid combinations can form fractal or dendritic patterns. Here, researchers use a drop of food coloring atop a mixture of water and xanthan gum. Depending on the concentration of gum (and the age of the viscous fluid) different fractal patterns spread quickly across the surface. (Image and video credit: R. Camassa et al.)