FYFD

Tag: fluid dynamics

  • When Rivers Jump

    When Rivers Jump

    Avulsions — sudden changes in the course of a river — are a river’s equivalent of an earthquake, and they can be similarly devastating for those in the river’s path. In a recent study, authors combed through 50 years’ worth of satellite data to catalog over 100 avulsions and categorize them into three regimes. About a quarter of the observed avulsions took place in the river delta’s fan, where the river spreads out once it exits a canyon or valley. These avulsions, they found, occur when rivers lose confinement and sediment can build up.

    This animation of satellite images shows the sudden avulsion -- a dramatic change in the river's course -- that took place on the Kosi River in 2008.
    This animation of satellite images shows the sudden avulsion — a dramatic change in the river’s course — that took place on the Kosi River in 2008.

    Among the other observations, the team linked avulsion location to the river’s flow properties. Most of these remaining avulsions took place in the river’s backwater region, where the river begins to slow down before its outlet. The last category of avulsion took place far upstream of the backwater region on rivers with high sediment flows. During flood conditions, erosion can travel far upstream on these rivers, causing avulsions in unexpected places. Changes in sediment load due to human activities, like deforestation, could even cause rivers to change from the backwater regime to the high-sediment load one. (Image credit: top – R. Simmon/USGS, bottom – S. Brooke et al.; research credit: S. Brooke et al.; via AGU Eos; submitted by Kam-Yung Soh)

  • Fish-Scale Tides

    Fish-Scale Tides

    On 31 July 2022, an unusual tidal phenomenon, a fish-scale tide, took place on the Qiantang River’s estuary in Zhejiang Province, China. Here are a couple videos. I’ve not found any explanations for it thus far, so I’m assembling my own. The Qiantang River and its estuary, Hangzhou Bay, are home to the world’s largest tidal bores, reaching 9 meters in places. That means the area regularly sees trains of large waves moving upstream against the normal current.

    The area is also known to have rotating currents, meaning that the tide does not simply move inland and then smoothly reverse direction. Instead, a rotating current can change its direction of flow over the course of a tidal cycle without changing its speed. Taken together, this makes the Qiantang River region perfect for winding up with groups of waves colliding at oblique angles, similar to a cross sea. I believe that’s what’s going on here with the fish-scale tide. Two sets of tidal-bore-induced waves are colliding at an angle, creating some gnarly conditions and a very cool pattern. (Image credit: VCG; submitted by Antony B.)

  • Liquid-in-Liquid Printing

    Liquid-in-Liquid Printing

    With 3D printing and other recent technologies, manufacturing options are always in flux. Here, researchers explore a method for printing a liquid inside of a liquid. Their materials are specially chosen such that the injected liquid forms an emulsion at its interface with the surrounding fluid. Once injection ends, the interface forms a wrinkly, viscoelastic skin that acts like a tube. As shown below, the tube is robust enough that it can be pumped full of yellow-dyed water without any loss of structure. (Image and research credit: P. Bazazi et al.)

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    Groundwater-Structure Interactions

    Groundwater can sometimes wind up in unexpected places, given the way it interacts with subsurface structures. In this Practical Engineering video, Grady discusses the paths that groundwater takes around structures and how civil engineers account for groundwater-related forces on dams and other buildings. As always, he illustrates with excellent model demos, allowing viewers to see groundwater interactions for themselves. (Image and video credit: Practical Engineering)

  • Saffman-Taylor Instability

    Saffman-Taylor Instability

    Air and blue-dyed glycerin squeezed between two glass plates form curvy, finger-like protrusions. This is a close-up of the Saffman-Taylor instability, a pattern created when a less viscous fluid — here, air — is injected into a more viscous one. If you reverse the situation and inject glycerin into air, you’ll get no viscous fingers, just a stable, expanding circle. Although you sometimes come across this instability in daily life — like in a cracked smartphone screen — the major motivation for studying this phenomenon historically has been oil and gas extraction. (Image credit: T. Pohlman et al.)

  • Inhibiting Marine Lightning

    Inhibiting Marine Lightning

    Thunderstorms over the ocean have substantially less lightning than a similar storm over land. Scientists wondered whether this difference could be due to lower cloud bases over the ocean or differences in the cloud droplets’ nuclei. But a new study instead implicates coarse sea spray as the deciding factor. By tracking the full lifetime of storm systems through remote sensing, the team found that fine aerosols can increase lightning activity over both land and ocean. But adding coarse sea salt from sea spray reduced lightning by 90% regardless of fine aerosols. With sea salt in the mix, clouds seem to develop fewer but larger condensation droplets, providing less opportunity for the electrification necessary to generate lightning. (Image credit: Z. Tasi; research credit: Z. Pan et al.)

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    Seeing the Flow

    Experimentalists often need a sense for the overall flow before they can decide where to measure in greater detail. For such situations, flow visualization techniques are a powerful tool since they provide quick ways to see and compare flows.

    Here, researchers paint a viscous oil atop their flying wing model and observe how the oil moves once the air flow starts up. This oil flow visualization shows the large-scale shifts in how air flows over the craft as the angle of attack increases. The disadvantage is that these techniques often give only a qualitative sense of the flow. But they can allow experimentalists to test many different conditions to decide which specific cases they should examine quantitatively. (Image and video credit: V. Kumar et al.)

  • Rain-Driven Prey Capture

    Rain-Driven Prey Capture

    Pitcher plants often entice their insect victims with sweet nectar before trapping them in inescapable viscoelastic goo. But some species go even further. Nepenthes gracilis, a species native to Southeast Asia uses its leafy springboard to lure its prey. Once an ant crawls to the underside of the leaf, a falling rain drop will spell its doom. When drops hit the leaf, it deflects down and jerks up, thanks to its shape and stiffness. The motion catapults insects into the pitcher, where digestive fluids await. While we’ve seen some fast-moving plants before, this is a rare example of a plant with an externally-driven speed mechanism. With it, the pitcher plant doesn’t have to wait or expend any metabolic effort to reset for the next insect. (Image credit: GFC Collection/Alamy; research credit: A. Lenz and U. Bauer; via New Scientist)

  • Reefs Along New Caledonia

    Reefs Along New Caledonia

    Brown reefs edge a turquoise lagoon in this astronaut snapshot of the New Caledonian coastline. Reefs like these form a natural barrier that protects coastlines from storms by breaking up waves (seen here as those white edges) before they reach the shore. The lagoon is streaked with lines of tan where sediment flows from the uplands into the water. Similarly, the color variations from green to blue in water hint at changes in depth, organic content, and more. (Image credit: NASA; via NASA Earth Observatory)

  • Stunning Waves

    Stunning Waves

    Photographer Lloyd Meudell captures breathtaking images of ocean waves off his home shores of New South Wales. The waveforms and lighting combine to create infinite variety in shape and texture. Some waves look like towering mountain landscapes; some look like glass sculptures. Every one of them draws you into the ocean’s power. (Image credit: L. Meudell; via Bored Panda)