FYFD

Tag: Kelvin-Helmholtz instability

  • Kelvin-Helmholtz and the Sun

    Kelvin-Helmholtz and the Sun

    Kelvin-Helmholtz instabilities (KHI) are a favorite among fluid dynamicists. They resemble the curls of a breaking ocean wave — not a coincidence, since KHI create those ocean waves to begin with — and show up in picturesque clouds, Martian lava coils, and Jovian cloud bands. The instability occurs when two layers of fluid move at different speeds and the friction between them causes wrinkles that grow into waves.

    Scientists have long suspected that KHI could occur in solar phenomena, too, like the coronal mass ejections that drive space weather. The Parker Solar Probe, a spacecraft designed to explore the sun, caught evidence of a series of turbulent eddies during a 2021 coronal mass ejection, and a recent study of those observations shows that the series of vortices are consistent with KHI. Put simply, the team found that the features are spaced and aligned as we’d expect for KHI and, during the probe’s measurements, the features grew at the rate Kelvin-Helmholtz eddies would. Although the instability itself may be common in the sun’s corona, it’s unlikely that we’ll see it often, simply because conditions need to be just right for them to be visible. (Image credit: NASA/Johns Hopkins APL/NRL/Guillermo Stenborg and Evangelos Paouris; research credit: E. Paouris et al.; via Gizmodo)

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  • Frozen Ripples

    Frozen Ripples

    Normally, freezing is a slow enough process that transient phenomena like ripples get smoothed out. But with the right conditions, even ripples can get frozen in time. This picture shows a backyard bird bath after a frigid winter storm passed overnight. For much of that time, the wind was active enough to keep the bath’s water from freezing. But when freezing did start, it happened so rapidly that the wavelets generated by the wind got frozen in place, too. Here’s a similar-looking effect (also in Colorado, ironically) that’s thought to have formed entirely differently. (Image credit: K. Farrell; via EPOD; submitted by Kam-Yung Soh)

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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)

  • Microscale Kelvin-Helmholtz

    Microscale Kelvin-Helmholtz

    When we think of cavitation in a flow, we often think of it occurring at a relatively large scale — on the propeller of a boat, for example. But cavitation takes place on microscales, too, including around fuel-injection nozzles. In this study, researchers investigated submillimeter-scale cavitation using a flow through a tiny Venturi tube. What they found was something we usually associate with larger scale flows: the Kelvin-Helmholtz instability.

    The Kelvin-Helmholtz instability takes place on this cavitation bubble.

    The wavy shape of a Kelvin-Helmholtz instability forms when two layers of fluid move past one another at different speeds and the interface where they meet becomes unstable. Here, that happens along a cavitation bubble, where the bubble and the flow meet. Interestingly, at these scales, the Kelvin-Helmholtz instability seems to be the primary method of break-up, instead of shock wave interactions.

    For those keeping track, we’ve now seen the Kelvin-Helmholtz instability from the quantum scale up to 160 thousand light-years. It’s hard to achieve a much wider range than that! (Image and research credit: D. Podbevšek et al.; submitted by M. Dular)

  • Rotating Waves of Grains

    Rotating Waves of Grains

    Rotating drums are a popular way to explore granular dynamics. Here, researchers fill a cylinder (seen below) with heavy grains and a low-viscosity fluid, then rotate the mixture about a horizontal axis. This sets up a contest between centrifugal forces and gravitational forces on the grains. At the right rotation rates, the grains form annular rings around the outside of the cylinder, where they rotate at a different speed than the fluid. This difference in speed between the two layers can trigger a Kelvin-Helmholtz instability and cause waves along the interface between the grains and the fluid, as seen in the examples above. (Image and research credit: V. Dyakova and D. Polezhaev; top image adapted by N. Sharp)

    Image of the experimental apparatus when not rotating.
  • Quantum Instability

    Quantum Instability

    In our everyday lives, two fluids moving past one another often form a wave-like pattern thanks to the Kelvin-Helmholtz instability. We see it in the curl of waves on the ocean, in clouds in the sky, and even in spirals of lava on Mars. Here researchers explore an analogous instability in the quantum world.

    By spinning a gas of ultracold atoms, the team observed a spontaneous transition from a needle-like configuration to a crystal made up of spirals. It’s a quantum Kelvin-Helmholtz instability! The authors found that wave’s phase is random; it arises purely from quantum interactions between the atoms. (Image, research, and submission credit: B. Mukherjee et al.; see also MIT News)

    The spinning cloud of ultracold atoms breaks up into a series of spirals.
  • Superfluid Instabilities

    Superfluid Instabilities

    Superfluids — like Bose-Einstein condensates — are bizarre compared to fluids from our everyday experience because they have no viscosity. Without viscosity, it’s no surprise that they behave in unusual ways. Here, researchers simulated superfluids moving past one another. In each of these images, the blue fluid is moving to the left, and the red fluid is moving to the right. In a typical fluid, such motion causes ocean-wave-like curls due to the Kelvin-Helmholtz instability.

    Instead, with a low relative velocity and high repulsion between atoms of the two layers, the superfluids form a tilted, finger-like interface (Image 1) that the authors refer to as a flutter-finger pattern. (Repulsion essentially sets the miscibility between the superfluids. With a high repulsion, the superfluids resist mixing.)

    With a higher relative velocity (Image 2), the wavelength of the ripples becomes comparable to the thickness of the interface, and the superfluids take on a very different, zipper-like pattern. Note how the tips detach and reconnect to the neighboring finger!

    With lower repulsion, the interface between the two liquids is thicker and breaks down quickly (Image 3). The authors call this a sealskin pattern. (Image credits: water – M. Blažević, simulations – H. Kokubo et al.; research credit: H. Kokubo et al.; via APS Physics)

  • Brace For Impact

    Brace For Impact

    What happens in the moment before an object hits the water? That’s the question at the heart of a new study exploring how water deforms before an object’s impact. The researchers dropped circular disks onto a pool of water and, using a new reflection-based technique, measured micron-sized deflections in the water’s surface before impact, as seen below.

    Animation showing the deflection of the water's surface just before a circular disk impacts it.
    Movie of the water surface’s deflection as the circular disk approaches. Look for distortions in the grid pattern.

    The deflections are caused by the air getting squeezed out of the space between the oncoming object and the water surface. The team found that the deformation isn’t uniform. The air squeezing out along the edges moves fast enough to trigger a Kelvin-Helmholtz instability and actually pull up the water surface. So when the disk hits, it impacts along its edges first and traps an air bubble underneath. (Image credits: divers – E. Carter, experiment – U. Jain et al.; research credit and submission: U. Jain et al.)

  • Jovian Auroras

    Jovian Auroras

    Like Earth, Jupiter is home to polar auroras that light the sky as charged particles interact with the planet’s magnetosphere. A recent paper identifies interesting features in the aurora that appear similar to expanding vortex rings (see inset below). Although the researchers cannot yet identify the origin of the rings, they hypothesize that the process begins at the far edges of Jupiter’s magnetosphere where it interacts with the incoming solar wind. One theory posits that shear flows and Kelvin-Helmholtz instabilities where the magnetosphere and solar wind meet drive the phenomenon. (Image credit: Jupiter – NASA, ESA, and J. Nichols, aurora features – NASA/SWRI/JPL-Caltech/SwRI/V. Hue/G. R. Gladstone/B. Bonfond; research credit: V. Hue et al.; via Gizmodo)

    Diagram showing an inset of Jupiter's northern aurora, with further insets showing the expanding ring features.
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    Protecting From Storm Surge

    The most dangerous and destructive part of a tropical cyclone isn’t the wind or rain; it’s the storm surge of water moving inland. This landward shift of ocean takes place because of a cyclone’s strong winds, which drive the water via shear. The depth storm surges reach depends on the wind speed and direction, shape of the shoreline, and many other factors, making exact predictions difficult.

    Fortunately, engineers can — with enough foresight and investment — build structures and networks to help protect developed land from storm surge flooding. (Image and video credit: Practical Engineering)