FYFD

Year: 2019

  • Dandelion Flight, Continued

    Dandelion Flight, Continued

    Not long ago, we learned for the first time that dandelion seeds fly thanks to a stable separated vortex ring that sits behind their bristly pappus. Building on that work, researchers have now published a mathematical analysis of flow around a simplified dandelion pappus. Despite their simplifications, the model captures the flow observed in the previous experiments (bottom image: experiments on left; model on right). 

    The model also allowed researchers to test various features – like the number of filaments in the pappus – and see how they affected the flow. Interestingly, they found that dandelion flight was most stable with about 100 filaments, which is right around the number of a typical pappus! (Image credits: dandelion – Pixabay, figure – P. Ledda et al.; research credit: P. Ledda et al.; via APS Physics; submitted by Kam-Yung Soh and Marc A.)

  • Prehistoric Filter Feeders

    Prehistoric Filter Feeders

    Earth’s earlier ages are filled with enduring mysteries about the plants and creatures that lived and died long before humanity. Many of these organisms, like the aquatic Ernietta shown above, are known only from scattered fossil remains. Yet fluid dynamics is helping us understand how Ernietta lived and fed some 545 million years ago.

    Ernietta were sack-like organisms consisting of stitched-together tubular elements. They had no way to move around and no obvious method for transporting nutrients into their bodies. Scientists hypothesized that they likely used one of two feeding methods: either Ernietta relied on its surface area to extract nutrients directly from the water or its shape enabled it to trap larger particles to feed on from the flow. To decide between these modes, scientists turned to computational fluid dynamics.

    By modelling both single Ernietta and small groups, they found that the shape of the organism generates a rotating current inside the bag that pulls flow down along one side and back up the other. Moreover, being near one another enhanced this effect, helping downstream Ernietta catch more particles than they otherwise would. All in all, the results suggest not only Ernietta’s likely feeding method but also that they lived in colonies and practiced one of the earliest known examples of communal feeding! (Image credit: D. Mazierski, source; research credit: B. Gibson et al.; via ArsTechnica; submitted by Kam-Yung Soh)

  • Titan’s Dragonfly

    Titan’s Dragonfly

    Last week, NASA announced its next New Frontiers mission: a nuclear-powered drone named Dragonfly heading to Titan. This astrobiology mission is set to search our solar system’s second largest moon for signs of life. It’s exciting aerodynamically, as well, since Titan’s thick atmosphere makes it uniquely suited for heavier-than-air flight. Therefore, rather than using wheeled rovers like we have on Mars, Dragonfly is a rotorcraft. It will be capable of traveling up to 8km per flight, which will quickly surpass the fewer than 21km the Curiosity Rover has managed on Mars! 

    Like Earth, Titan has rainfall and open liquid bodies on its surface. I, for one, can’t wait to see the alien vistas Dragonfly sends back as it cruises over methane lakes. (Image and video credit: NASA)

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    Fingers of Clay

    Take a mixture of a viscous liquid – like clay mud – and squeeze it between two glass plates and you’ll create a mostly-round layer of liquid. As you pry the two glass plates apart, air will push its way into that layer, forcing through the mud in a dendritic pattern. This is called the Saffman-Taylor instability or viscous fingering. It occurs because the interface between the air and mud is unstable.  (Image and video credit: amàco et al.)

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    “-N- Uprising”

    Although Thomas Blanchard’s latest short film, “-N- Uprising”, is less overtly fluid dynamical, fluids underlie almost every aspect of it. The blossoming of flowers is often driven by osmosis and water pressure. Spiders rely on hydraulic pressure to move their limbs, and many insects first unfurl their wings by pumping hemolymph through the network of veins that lace them. Even when hidden beneath the surface, fluid dynamics is everywhere. (Video credit: T. Blanchard; via Colossal)

  • Urban Centers During Hurricanes

    Urban Centers During Hurricanes

    As the climate warms, many urban centers are facing stronger and more frequent storms. Some, like New York City, are using numerical simulations to better understand the interactions of their complicated urban geometries with hurricane force winds. 

    Above you see a simulation showing predicted wind speeds in a Lower Eastside neighborhood. The incoming wind speed (from the left) is roughly 60 m/s (~134 mph), but the speeds around and between buildings are as much as 2 times higher than that. That means that, even if a storm is Category 3 or 4, there will be areas of a neighborhood that receive sustained winds well beyond the range of a Category 5 hurricane. Urban planners need this sort of data both for devising building requirements and for understanding what storm conditions warrant mandatory evacuations for residents. (Video and image credit: X. Jiang et al.)

  • Transporting Droplets

    Transporting Droplets

    Transporting droplets easily and reliably is important in many microfluidic applications. While this can be done using electric fields, those fields can impact biological characteristics researchers are trying to measure. As an alternative, a group of researchers have developed the concept of “mechanowetting,” a technique that uses surface tension forces to hold droplets on a traveling wave.

    Now visually, it’s a bit tough to see what’s going on here. In the animations, it looks like the droplets are just sticking to a moving surface, but that’s an illusion. The surface the droplet is sitting on is fixed and unmoving. It’s a thin silicone film that covers a ridged conveyor belt. The belt underneath can (and does) move. This creates a traveling wave. Instead of that wave simply passing beneath the droplet, it triggers an internal flow and restoring force that helps the drop follow the wave. The effect is strong enough that small droplets are even able to climb up vertical walls or stick upside-down. (Image, research, and submission credit: E. de Jong et al.)

  • Oil-on-Water Impact

    Oil-on-Water Impact

    Although many people have studied droplet impacts over the years, there’s been remarkably little work done with oil-on-water impacts. One of the things that makes this situation different is that the oil and water are completely immiscible, which means we can see aspects of the impact process that are invisible with, say, water-on-water impacts.

    The animation above shows an underwater view of the oil droplet’s impact. The energy of the initial impact creates an expanding crater and an unstable crown splash. That crown splash contains both water and oil. After it ejects some droplets, the rim stabilizes, but we can still see small perturbations along its edge as it starts to retract. In the water, high surface tension damps out these perturbations. Not so for the oil! As the crater retracts, the small disturbances along the rim get stretched into mushroom-shaped fingers that point inward toward the impact site. Because the index of refraction is different between oil and water, we can see the fingers clearly near the end of the animation. (Image and research credit: U. Jain et al.; submitted by Utkarsh J.)

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    Ink Explosion

    Sometimes beautiful flows come from simple combinations. Here the artists of Chemical Bouillon combine ink and hydrocarbons to create lovely explosions of color. Eschewing quick cuts between views, they allow us to linger and explore the flow ourselves as it changes. Differences in surface tension drive streaming flows along the surface, but there seem to be some chemical reactions contributing as well. Watch along the edges and you may even see convection pulling ink down and back. The whole video is only 2 minutes long and worth a full watch. (Image and video credit: Chemical Bouillon)

  • Artificial Microswimmers

    Artificial Microswimmers

    In a 1959 lecture entitled “There’s Plenty of Room at the Bottom”, Richard Feynman challenged scientists to create a tiny motor capable of propelling itself. Although artificial microswimmers took several more decades to develop, there are now a dozen or so successful designs being researched. The one shown above swims with no moving parts at all.

    These microswimmers are simple cylindrical rods, only a few microns long, made of platinum (Pt) on one side and gold (Au) on the other. They swim in a solution of hydrogen peroxide, which reacts with the two metals to generate a positively-charged liquid at the platinum end and a negatively-charged one at the gold end. This electric field, combined with the overall negative charge of the rod, causes the microswimmer to move in the direction of its platinum end. 

    Depending on the hydrogen peroxide concentration, the microswimmers can move as quickly as 100 body lengths per second, and they’re capable of hauling cargo particles with them. One planned application for artificial microswimmers is drug delivery, though this particular variety is not well-suited to that since the salty environment of a human body disrupts the mechanism behind its motion. (Image credits: swimmers – M. Ward, source; diagram – J. Moran and J. Posner; see also Physics Today)