FYFD

Tag: Kelvin-Helmholtz instability

  • Lava Bomb

    Lava Bomb

    What you see above is a homemade lava bomb. To systematically study what happens when groundwater meets lava, scientists melted basalt and created their own meter-scale explosion-on-demand. Inside the container, they can inject water and observe the resulting dynamics.

    Beneath the lava, the water forms what scientists call a domain. Thanks to the Leidenfrost effect, it can be protected from direct contact with the lava by a thin vapor layer that boils off it. If the water domain is large enough, buoyancy will pull it upward through the lava. Whether the water maintains a spherical shape or begins to distort and break up into smaller domains depends on the speed of its rise.

    At some point, though, either naturally or through an external trigger (like the sledgehammer you see above), the water and lava can contact, resulting in explosive vaporization of the water and an explosion. What’s visible at the surface depends on the depth at which the explosion takes place. Scientists are eager to characterize these variations, which will help them better predict the explosive danger of eruptions like Kilauea and Eyjafjallajökull. (Image and research credit: I. Sonder et al.; video credit: NYTimes; submitted by Kam-Yung Soh)

  • Bringing the Stars Home

    Bringing the Stars Home

    One of my favorite aspects of fluid dynamics is the way that the same patterns and phenomena appear over and over again – sometimes in the most unexpected places. That’s the theme of my new article in American Scientist, which focuses on the connections between our daily lives and the stars:

    “Solar energy arises from nuclear fusion reactions in the core, but that energy is buried hundreds of thousands of kilometers beneath the surface, and most of the Sun’s overlying gas is nearly opaque; it hinders light from passing through, like a blanket thrown over a flashlight. Clearly the Sun does shine—but how? For the answer, you can simply go to your kitchen, fill a kettle, and flip on a burner.” #

    Click-through to read the full article. (Image credit: N. Sharp, Big Bear Solar Observatory, J. Blom, NASA/ESA, J. Straccia, NASA/JPL/B. Jonsson)

  • Breaking With a Wave

    Breaking With a Wave

    For rocket combustion and other applications, like watering your lawn with a hose, a stream of fluid may need to be broken up into droplets. While simply spraying a liquid jet will make it break up, waving that jet back and forth will break it up faster. A recent study simulated this problem numerically to determine the exact mechanisms driving that break-up. The researchers found two major culprits.

    The first is a Kelvin-Helmholtz, or shear-based, instability. When a jet leaves the nozzle, there’s friction between it and the comparatively still air surrounding it. This creates tiny ripples in the surface that eventually grow into the distortions we can see, and it’s found in all jets, regardless of their side-to-side motion.

    The second culprit, which is only found in the oscillating jet, is a Rayleigh-Taylor instability. By moving the jet side-to-side, you’re driving the dense liquid into less dense air, which creates a different set of disturbances that also help break up the jet. The final result: swinging the jet side-to-side breaks it into smaller droplets faster. (Image and research credit: S. Schmidt et al.)

  • Lincolnshire KH Clouds

    Lincolnshire KH Clouds

    These beautiful Kelvin-Helmholtz clouds were spotted over Lincolnshire on December 19th. They form between two layers of air, one of which is moving faster than the other. Although that situation is not very unusual, the conditions have to be just right for visible clouds to form at that interface between layers, and the clouds themselves are typically short-lived. This set is particularly lovely with its smooth curves and breaking wave form. If you, like me, love these clouds but never manage to see them yourself, you can always try wearing some instead! (Image credit: A. Towriss; via BBC News; submitted by Vince D.)

  • Sunglinting Seas

    Sunglinting Seas

    Sunlight reflecting off the Earth can reveal a remarkably rich picture of our planet’s activity. The silver-gray areas seen in this satellite image are sunglint, where lots of light is reflected back to space. Sunglint occurs in regions with very few waves; more waves – like in the bluer areas – mean more directions in which light can be scattered. The reason for these rough and smooth waters is atmospheric: the prevailing summer winds blow across the Aegean from the north. In open water, that wind drives up the waves, but rocky islands disrupt the flow, leaving “wind shadows” on their southern, leeward sides where the waves are smaller. (Image credit: J. Schmaltz; via NASA Earth Observatory)

  • Jupiter’s Atmosphere

    Jupiter’s Atmosphere

    Jupiter’s atmosphere is fascinatingly complex and stunningly beautiful. This close-up from the Juno spacecraft shows a region called STB Spectre, located in Jupiter’s South Temperate Belt. The bluish area to the right is a long-lived storm that’s bordering on very different atmospheric conditions to the left. Shear from these storms moving past one another creates many of the curling waves we see in the image. These are examples of the Kelvin-Helmholtz instability, which generates ocean waves here on Earth, creates spectacular clouds in our atmosphere, and is even responsible for waves in galaxy clusters. Check out some of the other amazing images Juno has sent back of our solar system’s largest planet. (Image credit: NASA/JPL-Caltech/SwRI/MSSS/R. Tkachenko; via Gizmodo)

  • Jupiter On Display

    Jupiter On Display

    The rich detail of Jupiter’s atmosphere is on full display in this enhanced-color image from the Juno spacecraft. (Full resolution version here – trust me, you want to click that link.) To the north, on the left side of the image, Jupiter’s Great Red Spot swirls. To the center and right, the cloud bands of Jupiter’s southern region are coming into view. The color enhancements really highlight eddies on the edge of these bands. These are examples of Kelvin-Helmholtz instabilities caused by shear between cloud bands moving at different speeds. Within the bands, smaller vortices spin. Some of these are anti-cyclones, high-pressure storm systems found all over the planet. Jupiter’s atmosphere still holds many mysteries for scientists, but I love how every gorgeous image Juno sends back shows fluid physics written larger than life across our solar system’s biggest planet. (Image credit: NASA/JPL-Caltech/SwRI/MSSS/G. Eichstädt /S. Doran; via Gizmodo)

  • Featured Video Play Icon

    Kelvin-Helmholtz Instability

    Sixty Symbols has a great new video explaining the laboratory set-up for demoing a Kelvin-Helmholtz instability. You can see a close-up from the demo above. Here the pink liquid is fresh water and the blue is slightly denser salt water. When the tank holding them is tipped, the lighter fresh water flows upward while the salt water flows down. This creates a big velocity gradient and lots of shear at the interface between them. The situation is unstable, meaning that any slight waviness that forms between the two layers will grow (exponentially, in this case). Note that for several long seconds, it seems like nothing is happening. That’s when any perturbations in the system are too small for us to see. But because the instability causes those perturbations to grow at an exponential rate, we see the interface go from a slight waviness to a complete mess in only a couple of seconds. The Kelvin-Helmholtz instability is incredibly common in nature, appearing in clouds, ocean waves, other planets’ atmospheres, and even in galaxy clusters! (Image and video credit: Sixty Symbols)

  • Featured Video Play Icon

    Perijove

    The Juno spacecraft continues to send back incredible photos of Jupiter’s atmosphere. This video animates images from the sixth close pass of Jupiter to give you a sense of what Juno sees as it swoops by our system’s largest planet. The trajectory passes from the north pole to the south, showing Jupiter’s whitish zones, dark belts, and massive storms. Up close Jupiter looks like an Impressionist painting, all vortices and shear instabilities. The large white spots you see are enormous counterclockwise rotating vortices known as anticyclones – many of them larger than our entire planet. (Video credit: NASA / SwRI / MSSS / G. Eichstädt / S. Doran)

  • Breaking Waves in the Sky

    Breaking Waves in the Sky

    Under the right atmospheric conditions, clouds can form in a distinctive but short-lived breaking wave pattern known as a Kelvin-Helmholtz cloud. The animation above shows the formation and breakdown of such a cloud over the course of 9 minutes early one morning in Colorado’s Front Range region. Kelvin-Helmholtz instabilities occur when fluid layers with different velocities and/or densities move past one another. Friction between the two layers moving past creates shear and causes the curling rolls seen above.

    In the background, you can also see a foehn wall cloud low to the horizon. This type of cloud forms downwind of the Rocky Mountains after warm, moist Chinook winds are forced up over the mountains, cool, and then condense and sink in the mountains’ wake. (Image credit and submission: J. Straccia, more info)