FYFD

Tag: fluid dynamics

  • Representing Rain’s Microphysics

    Representing Rain’s Microphysics

    Realistically modeling rainfall remains an extremely difficult problem. To be practical, results have to be on the scale of kilometers; no one is looking to find out whether rain will fall from one specific cloud over their head. But making that prediction depends on physics that happens at the microscale, where droplets tens of microns in size are condensing, colliding, and eventually growing large enough to fall as rain. A new study takes a look at three machine-learning models that could help describe those microscale physics with less computational overhead.

    The researchers used three different algorithms, all trained on high-quality simulations of microdroplet physics. The goal here was to represent the complex, nonlinear physics reflected in those results with an algorithm that’s less complicated and less computationally expensive than the methods used to create the training data. The team then tested the trained algorithms to see how they performed in conditions that were different than their training data.

    They found that the model with the best performance–in terms of giving more accurate predictions in the test cases–was actually the simplest of the three models. So it may be possible to get reasonable results for rain microphysics from simpler, easier-to-compute algorithms. The team does warn, though, that all of the models need more work before they’d be ready to add to commercial-grade weather prediction software. (Image credit: J. Fowler; research credit: E. de Jong et al.; via Eos)

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    How Strings Vibrate

    Many of us have a mental image of strings vibrating like a swinging jump rope, but that’s not quite what you actually see. Instead, plucked strings often look strangely triangular, as Henry explains in this Minute Physics video. This more complicated shape comes from the way the string’s tension responds to the applied force of plucking, striking, or bowing.

    That doesn’t mean that the jump rope picture is entirely wrong, though. The triangular waves seen in the video are the shape you get when all the sine waves of the jump rope model get added together! (Superposition is fun.) (Video and image credit: Minute Physics)

    The triangular wave of a string plucked near one end.
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  • Stripping Mars’ Atmosphere

    Stripping Mars’ Atmosphere

    Mars was once a warmer, wetter place, swathed in a thick and protective atmosphere. Unlike Earth, Mars lacks a strong global magnetic field, which allows the solar wind to strip its atmosphere, but the exact mechanisms of that process have been unclear. But a new study has caught the process in action.

    Illustration of the Kelvin-Helmholtz instability occurring as the solar wind and Mars' electric field interact.
    Illustration of the Kelvin-Helmholtz instability occurring as the solar wind and Mars’ electric field interact.

    By combining simultaneous measurements from two spacecraft–NASA’s MAVEN and China’s Tianwen-1–the team was able to monitor the upstream solar wind conditions and the atmospheric ions escaping. They found that previously-observed “plasma clouds” in the Martian atmosphere result from a Kelvin-Helmholtz instability between the solar wind and Mars’ electric field. Within these clouds, the ion flux is ten to a hundred times greater than at steady-state conditions.

    This mechanism directly couples ion escape from Mars’ atmosphere to the solar wind, and it’s likely that this process plays out for other unmagnetized planets as well. (Image credit: Mars – NASA, illustration – C. Zhang et al.; research credit: C. Zhang et al.; via Gizmodo)

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  • An Iceberg Flips

    An Iceberg Flips

    Footage of a massive iceberg flipping in Greenland has gone viral. The feat took place off the coast of Ilulissat and was caught on a nearby livestream camera. Icebergs shift like this because they melt unevenly, with submerged areas melting faster. As the underwater shape of the iceberg changes, the buoyant forces shift relative to the iceberg’s center of mass, and it flips, eventually settling into a (temporarily) stable orientation. This situation is simpler in the lab version, though; as the video here shows, real icebergs can also rapidly lose mass if they start breaking apart. That, too, can necessitate a flip. (Video and image credit: AfarTV; via Colossal)

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    Flex Pen and Ink

    It’s no secret that I’m a fan of fountain pens. They’re just one way I like to have little celebrations of fluid physics in my everyday life. That said, I don’t typically use anything with a monster flex nib like what you see here. This is extra. But it’s also an incredibly cool glimpse of fluid physics.

    Just check out that sheet of fluid stretching between the tines! To keep the liquid sheet intact to that kind of width you need the perfect balance of surface tension and flow. The ink needs to cling to the tines, cohere to itself (despite some impressive stretching), and make its way down from the reservoir and through the feed quickly enough (but not too quickly!) to replace the ink being laid down on the page as the nib moves. Gorgeous stuff. (Video and image credit: Flexperiments)

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  • Capillary Slinkies

    Capillary Slinkies

    Nature is full of helical fibers, including in plants and bird feathers. In this study, researchers explore how these soft springs react to droplets. When the pitch of the spring (roughly speaking, the spacing between coils) is small, droplets can flow down in a plug (not shown). But as the pitch increases, droplets can take on a caterpillar-like (or, eruciform) shape. These drops descend quickly, in part, the team found, because internal flows within the droplet help it along.

    A caterpillar-shaped droplet slides down a soft spring.
    A caterpillar-shaped droplet slides down a soft spring.

    Other drops maintain a spherical shape as they descend the widely-spaced coils of the spring. These drops tend to spin around the coil as they go, with their center of mass actually moving side-to-side as they descend. (Image and research credit: B. Bhatt and A. Carlson)

    A sphere-shaped droplet slides down a soft spring.
    A sphere-shaped droplet slides down a soft spring.
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    Levitating By Squeeze Film

    We’ve seen a few different kinds of levitation here over the years, including via acoustic waves, aerodynamics, and Leidenfrost effect. Here, Steve Mould describes something a bit different: a levitation squeeze film driven by ultrasonic transducers. Like a vibrating droplet, the thing keeping surfaces apart here is a thin, lubricating layer of air. The vibration of the ultrasonic transducer squeezes and replenishes the air layer quickly enough that it can keep transducers aloft. Steve even manages to create a miniature airless “air hockey” table with it! (Video and image credit: S. Mould)

  • Dragonfly Dogfights

    Dragonfly Dogfights

    Like fighter pilots of old, male dragonflies engage in aerial combat where each tries to outmaneuver the other to keep a sight on their rival’s tail. A recent study observed this combat in the field and uncovered some surprising similarities to dogfighting. Like pilots, dragonflies used spiraling turns and other high-g moves to gain an advantageous position behind the other. In human combat, that position favors the forward-facing weapons of the pilot in back; for dragonflies, it keeps their rival in the part of their vision that best detects movement.

    Interestingly, the team found that–even in the midst of combat–dragonflies spent at least a third of their time gliding. It’s unclear whether they glide to conserve energy or because it’s easier to track a rival when gliding.

    The authors dig into the control rules needed for dragonflies to execute these chases and found that even relatively simple control schemes–constrained by the dragonfly’s physical limits–result in complex flight contests. (Image and research credit: S. Fabian et al.; via Ars Technica)

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    Blowing Droplets Apart

    In raindrops, ink-jet printing, and spray painting, droplets get broken apart by the air flowing around them. Here, researchers investigate how a jet of fluid can push a droplet from its perch, or–with enough momentum–shatter the droplet. There are some lovely flow visualizations here; it’s especially cool to see the starting vortex ring of the jet push and deform the droplet. (Video and image credit: S. Jain et al.)

  • Salt and Dunes

    Salt and Dunes

    Photographer Barbara Brown captured these striking aerial views in Namibia. Coastal dunes and saltworks feature in the photos from Walvis Bay, where wind, waves, evaporation, and humankind have shaped the landscape. The colorful and dendritic dune images come from Sossusvlei, where the ephemeral Tsauchab River ends in the Namib Desert. Recent flooding left its mark on the dry landscape. (Image credit: B. Brown/IAPOTY; via Colossal)

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