FYFD

Tag: instability

  • Liquid Antispiral

    Liquid Antispiral

    Spiral formations are common in nature, from galaxies to chemical reactions. But most examples in nature rotate such that their arms trail the direction of rotation. Viewed side-on, this makes the arms appear to spiral outward from the the center. The opposite – an antispiral, where the arms appear to be drawn in toward the center – also exists, but there are far fewer examples. Which is why it’s notable that physicists have described a new one, seen above.

    You’re watching silicone oil draining through a plate with an array of holes in it. There’s a reservoir of oil on top supplying a constant flow rate. The patterns that form in this system vary widely – they can form between one and six arms – but the results are always antispirals. The driving mechanism seems to be the periodic nature of the discharge from individual holes, which is caused by a Rayleigh-Taylor instability. Hopefully systems like this can shed some light on why spirals are often preferred over antispirals. (Image and research credit: H. Yoshikawa et al.; via APS Physics)

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    Dripping Down the Rivulet

    If you’ve ever watched water running down the side of the street, you’ve probably noticed that it doesn’t flow smoothly. Instead, you’ll see waves, rivulets, and disturbances that form. That’s because the simple action of flowing down an incline is unstable. Water and other viscous liquids can’t flow downhill smoothly. Any disturbances – an uneven surface, the rumble of passing cars, a pebble in the way – will create a disruption that grows, often until the entire flow is affected. This video shows some of the complex and beautiful patterns you get then. (Video and image credit: G. Lerisson et al.)

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    “The Empire of C”

    Filmmaker Thomas Blanchard has once again released a beautiful, fluid-filled short to captivate us. Built from paint, oil, and liquid soap, “The Empire of C” feels like it gives viewers a birds-eye perspective over a fantastical land. I was particularly drawn to two fluid dynamical aspects of the film. The first were the dendritic sequences in the opening, which feel a bit like watching river deltas form in real time. Despite their resemblance to the Saffman-Taylor instability, I think these fingers are interfacially driven – meaning that they result from differences in surface tension between the different liquids Blanchard is using. 

    The second thing that caught my eye and made me rewind the video over and over were the glittery droplets. The glitter acts like tracer particles, allowing you to see the flow inside the droplets. Check out that counter-circulation compared to the paint flowing by outside! It’s a reminder that even inside a seemingly still droplet, there’s lots going on. (Video and image credit: T. Blanchard)

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    “If I Say”

    The new Mumford & Sons single “If I Say” features a fluid-dynamical music video. It’s full of dendritic fingers and flowing colors – likely from combinations of inks, paints, and other fluids. Although the fingers are reminiscent of the viscosity-dependent Saffman-Taylor instability, these appear to be driven by variations in surface tension between the different fluids. That’s a major feature throughout the video; although some of the flow is caused by the syringes depositing fluids, much of it seems to be a Marangoni effect, where flow moves away from areas of low surface tension to ones with higher surface tension. (Video credit: Mumford & Sons; filmed by P. Hofstede; via Katie M.)

  • Vortex Dome

    Vortex Dome

    Are you staring into the eye of a hurricane or watching the spin of a simple desk toy? Part of the beauty of fluid dynamics is recognizing how similar they both are. This is high-speed footage of a toy known as a “Vortex Dome,” which contains a fluid filled with tiny mica particles that react to local forces and allow users to “see” the flow. Before the video begins, the toy has been spinning for long enough that the fluid inside rotates as if it were a solid body. Then an unseen hand sets the disk spinning in the opposite direction and we observe what happens.

    Fluid at the outer edge of the toy has to immediately change direction due to friction with the wall. That change in momentum slowly passes from the wall inward as viscosity between one layer of fluid to the next passes that signal. This creates the rolls we see in the first animation. Initially, those rolls are smooth, but they quickly roughen as disturbances in them grow into full-blown turbulence. Meanwhile, viscosity continues to pass the change in rotation inward, ultimately swallowing the entire interior of the toy. Left spinning indefinitely, the disturbances will eventually quiet out and the entire fluid will spin as one. (Image and video credit: D. van Gils)

  • Using Instabilities for Manufacturing

    Using Instabilities for Manufacturing

    Manufacturing textured, flexible surfaces can be difficult, but researchers are exploring ways to use fluid dynamical instabilities to make the process easier. They begin with a pourable polymer mixture that cures and solidifies over time. By putting the mixture on a cylinder and rotating it, engineers trigger the Rayleigh-Taylor instability – the same instability that makes dense fluids sink into lighter ones. Here, the instability is driven not only by gravity but by the added acceleration caused by centrifugal force. It causes the fluid film to drain and form arrays of droplets, which then cure into dimples. The researchers can control the size, shape, and spacing of the droplets by changing parameters like the spin rate. And by repeating the process multiple times on the same piece, they can build up spikier shapes, like the ones shown on the poster below. (Image and research credit: J. Marthelot et al., poster)

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    Reminder for those at the APS DFD meeting! My talk is tonight at 5:10PM in Room B206. You’ll probably want to come early if you want a seat!

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    The Show in the Sky

    There is a constant drama playing out overhead, though most of us do not take the time to watch. Fortunately, a few, like Blaž Šter, do and make timelapse videos that allow us to enjoy hours of atmospheric drama in only a few minutes. This timelapse shows a cloudy and rainy mid-July day in Slovenia, where an unstable atmosphere leads to turbulent and dramatic clouds. In an unstable atmosphere, it’s easier for vertical motion to take place between altitudes. For example, a parcel of warm air displaced upward will continue to rise because it will be lighter and more buoyant than the surrounding air. This is key to the strong convection that can generate thunderstorms. (Image and video credit: B. Šter, source)

  • The Challenges of Blowing Bubbles

    The Challenges of Blowing Bubbles

    Although every child has experience blowing soap bubbles with a wand, only in recent years have scientists dedicated study to this problem. It turns out to be a remarkably complex one, with subtleties that can depend on the size of the wand relative to the jet a bubble-blower makes as well as the speed at which the air impacts the film. A recent study found that, at low or
    moderate speeds, the film takes on a stable, curved shape (top image), but once you increase to a critical speed, the film will overinflate and burst. The key to forming a bubble, the authors suggest, is hitting that critical speed only briefly; if you slow down before the film ruptures, then the bubble has a chance to disconnect and form a sphere without breaking. 

    The work also suggests there are two reliable methods for bubble making in this way. One is to impulsively move the wand through the background fluid, as shown in the lower animation. The other is the one familiar to children: blow a jet just fast enough to overinflate the film, then let up so the bubble forms without breaking. (Image and research credit: L. Ganedi et al.; via Ars Technica; submitted by Kam-Yung Soh)

  • The Jumping Flea

    The Jumping Flea

    Nearly every lab has a magnetic stirrer for mixing fluids, but this ubiquitous tool still holds some surprises, like its ability to unexpectedly levitate. Magnetic stirrers consist of two main parts, a driving magnet that creates a rotating magnetic field, and a bar magnet – commonly referred to as the flea – that is submerged in the fluid to be stirred. When the driver’s rotating field is active, the flea will spin at the bottom of its container, keeping its magnetic field in sync with the driver.

    But if you place the flea in a viscous enough fluid, the drag forces on the flea can pull it out of sync with the driver’s field. Above a certain speed, the flea will jump so that its field repulses the driver’s. That makes the flea levitate as it spins. Depending on the interplay of viscous and magnetic forces, that spin can be unstable (left) or stable (right). The researchers suggest that this peculiar behavior could help artificial swimmers propel themselves or lead to new methods for measuring fluid viscosity. (Image and research credit: K. Baldwin et al.; via APS; submitted by Kam-Yung Soh)

  • Zones and Stars

    Zones and Stars

    Large-scale rotating flows, like planetary atmospheres, tend to organize themselves into zones. Within a zone, flow remains essentially in an east-west direction and serves as a barrier that keeps heat or other elements from mixing from one zone to another. This is, for example, how the tropical trade winds work here on Earth.

    Stars, on the other hand, don’t show this kind of zonal behavior. The reason, it turns out, is their magnetic fields. When there’s no magnetic influence, even weak shear in a rotating flow is enough to start organizing turbulent fluctuations and grow a zonal flow. This tendency toward growth is known as the zonostrophic instability. But when you add a magnetic field, instead of organizing the hydrodynamic disturbances, that weak shear strengthens the magnetic ones, which in turn suppress the flow fluctuations. As a result, the hydrodynamic disturbances cannot grow and no zonal flow forms.

    Researchers think this mechanism can explain both why stars have no zonal flows and just how deep zones can penetrate inside the atmospheres of gas giants like Jupiter and Saturn before their planet’s magnetic field suppresses them. (Image credit: NASA; research credit: N. Constantinou and J. Parker, arXiv; via LLNL News; submitted by Stephanie N.)