FYFD

Category: Research

  • Stingray Eyes

    Stingray Eyes

    With their flexible, flattened shape, rays are some of the most efficient swimmers in the ocean. But, at first glance, it seems as if their protruding eyes and mouth would interfere with that streamlining. A new study uses computational fluid dynamics to tackle the effects of these protrusions on stingray hydrodynamics.

    With their digital stingrays, the team found that the animal’s eyes and mouth created vortices that accelerated flow over the front of the ray and increased the pressure difference across its top and bottom surfaces. The result was better thrust and the ability to cruise at higher speeds. Overall, the ray’s eyes and mouth increased its hydrodynamic efficiency by more than 20.5% and 10.6%, respectively. The lesson here: looks can be deceiving when it comes to hydrodynamics! (Image credit: D. Clode; research credit: Q. Mao et al.)

  • Twisting Free

    Twisting Free

    Anyone who’s dealt with hot glue guns is familiar with the long, thin tails of glue they leave behind. 3D printers suffer from a similar problem with the nozzle pulls away from viscoelastic materials like plastics and polymers. Little tails, like the ones seen above, are left behind on the part and must be cleaned away by hand. The source of the trouble is the elasticity of the fluid. Pulling on these liquids stretches them into long thin strands as the molecules inside the fluid resist. But researchers have found an alternate method to break the liquid cleanly: twisting.

    When a viscoelastic liquid bridge gets twisted, the liquid undergoes what’s known as edge fracture, an elastic effect that creates an indentation that forces its way inward and breaks the bridge’s connection cleanly. Since the technique only requires spinning the 3D printer’s nozzle when detaching, it should be relatively easy for printer manufacturers to implement! (Image credit: 3D-print – T. Claes, illustration – H. Hill/Physics Today, animation – S. Chan et al.; research credit: S. Chan et al.; via Physics Today)

  • Animals Lapping

    Animals Lapping

    Without full cheeks, cats, dogs, and many other animals cannot use suction to drink. Instead, these animals press their tongue against a fluid and lift it rapidly to draw up a column of liquid. They then close their mouth on the liquid before it breaks up and falls down. (Cats are a bit neater about it, but as the high-speed images above show, dogs use the same method.)

    A new study takes a look at the mathematics behind this feat, specifically how long it takes for the liquid column to break up. Normally, we describe that problem using the Plateau-Rayleigh instability, but in its usual form, the PR instability doesn’t account for the kind of acceleration drinking animals apply to the fluid. This new study modifies the equations to account for acceleration and finds that the predicted time it takes for breakup is consistent with the timing of animals closing their mouths on the water. In other words, cats and dogs are likely timing their lapping to maximize the amount of water they catch with each bite. (Image credits: top – C. van Oijen, others – S. Jung et al. 1, 2; research credit: S. Jung)

  • Why Creases Don’t Disappear

    Why Creases Don’t Disappear

    Flex your fingers and you’ll see your skin fold into well-defined creases. Many soft solids (including old apples) fold this way, and like your skin, the creases never fully disappear, even when the stress is removed. A recent study finds that surface tension and contact-line-pinning are critical to the irreversibility of these creases.

    The authors studied sticky polymer gel layers under a confocal microscope as the gel folded. In doing so, they found that surface tension dictates the microscopic geometry of a fold, causing the two sides of a surface to touch. They also found that completely unfolding a creased surface requires more energy than folding it in the first place did because the folded surfaces adhere to one another.

    When unfolded, the crease behaves somewhat like a droplet on a rough surface. Such droplets move in fits; their contact line stays pinned to the rough microscopic peaks of the surface until there’s enough energy to overcome that attachment and the contact line jumps to another position. Similarly, a creased surface cannot simply unfold smoothly. Adhesion ensures that part of the crease remains, serving as a starting point for the next fold-unfold cycle. (Image credit: C. Rainer; research credit: M. van Limbeek et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Spiral Shark Intestines

    Spiral Shark Intestines

    We’ve seen previously just how fluid dynamically impressive sharks are on the outside, but today’s study demonstrates that they’re just as incredible on the inside. Researchers used CT scans of more than 20 shark species to examine the structure of their intestines. Sharks have spiral intestines that come in four different varieties; two of those types look like a stacked series of funnels (either pointing upstream or downstream). These funnel-filled spirals, the researchers found, are incredibly good at creating uni-directional flow without any moving parts, much like a Tesla valve does. The spiral structure also seems to slow down digestion, which may factor into the shark’s ability to go long periods between meals. Incredibly, the fossil record indicates that spiral intestines — in some form — evolved in sharks about 450 million years ago — before mammals even existed! Clearly we engineers are way behind sharks when it comes to controlling flows!

    Animation of a 3D scan of a shark's intestine, showing the spiral internal structure.

    (Image credit: top – D. Torobekov, scan – S. Leigh; research credit: S. Leigh et al.; via NYTimes; submitted by Kam-Yung Soh)

  • Megaripples Beneath Louisiana

    Megaripples Beneath Louisiana

    Approximately 66 million years ago, a 10-km asteroid struck our planet near Chicxulub on the Yucatán Peninsula. The impact was globally catastrophic, causing tsunamis, wildfires, earthquakes, and so much atmosphere-clogging sediment that about 75% of all species on the planet — including the non-avian dinosaurs — died out. A new study points to another remnant of the impact: giant ripples buried in the sediment of Louisiana.

    Seismic data shows giant ripples left behind by the tsunami following the Chicxulub impact.

    Using seismic data collected by petroleum companies, the researchers describe the ripples as approximately 16 meters tall with a spacing around 600 meters, making them the largest known ripples on the planet. Currently, they are buried about 1500 meters underground, just below a layer of fine debris associated with the impact. The ripples show no evidence of erosion from storms or wind, leading the authors to conclude that they were deposited by an impact-associated tsunami and remained unaffected by smaller natural disasters before their burial. It’s very likely, according to the authors, that many other such megaripples exist, hidden away in proprietary petroleum data sets. (Image credits: top – D. Davis/SWRI, ripples – G. Kinsland et al.; research credit: G. Kinsland et al.; via Gizmodo)

  • Hydrodynamic Spin Lattices

    Hydrodynamic Spin Lattices

    Droplets bouncing on a fluid bath display some strikingly quantum-like behaviors thanks to the interactions between a drop and its guiding surface wave. Here, researchers use submerged wells beneath the drop to confine each droplet into a space where it bounces in a clockwise or anticlockwise trajectory.

    (a) An illustration of the experimental set-up and (b) top-down image of the spin lattice.

    With an array of these wells, the droplets form a lattice. Each drop remains in its well, but its wave travels beyond and interacts with nearby wells. Through this interaction, the researchers found that lattices tended to synchronize, similar to the way groups of fireflies will synchronize their flashing. This sort of behavior is also observed in quantum systems, and the researchers hope that further studying their bouncing droplets will give insight into quantum spin systems and their behaviors. (Image and research credit: P. Saenz et al.; via Nature; submitted by Kam-Yung Soh)

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    Keeping Cool in the Cretaceous

    I love that fluid dynamics can bring new insights to other subjects, like this study on how heavily-armored ankylosaurs avoided heat stroke. Scans of ankylosaur skulls show a complicated, twisty nasal cavity that researchers likened to a child’s crazy straw. Using numerical simulations, they showed that the airflow through these passages acts like a heat exchanger. As air gets drawn into its body, it warms up from exposure to blood vessels lining the nasal cavity; that means that, simultaneously, the hot blood is getting cooled. Those blood vessels lead up to the animal’s brain, indicating that these twisted cavities essentially serve as air-conditioning for the sauropod’s brain! (Image and video credit: Scientific American; research credit: J. Bourke et al.; via J. Ouellette)

  • Flying Out of the Water

    Flying Out of the Water

    Flying fish and diving birds often navigate the interface between water and air in their flight, but few studies have actually looked at the effects of this transition on lift. In this work, researchers measured forces on a small, fixed wing as it egresses from water into air at a constant velocity.

    The tests showed that exit velocity had a large effect on lift generation. At low speeds, an exiting wing experienced a strong, positive lift spike as soon as the leading edge broke the surface. But that lift changed to strongly negative as the wing continued out of the water. At higher speeds, the wings had no lift reversal but also reached lower peak lift coefficients. The team studied the effects of angle of attack and starting depth as well, concluding that any vehicles intended to navigate the water-air transition will need robust control systems prepared to deal with fast-changing forces. (Image credit: fish – J. Cobb, wing – W. Weisler et al.; research credit: W. Weisler et al.)

  • Microjets and Needle-Free Injection

    Microjets and Needle-Free Injection

    Some people don’t mind needles, and others absolutely detest them. But to replace needles with needle-free injections, we have to understand how high-speed microjets pass through skin. Given skin’s opacity, that’s tough, so researchers are instead using droplets as a model. If we can understand the dynamics of a microjet passing through different kinds of droplets, getting jets of medicine into arms becomes easier.

    Researchers found that jets passed completely through a droplet if they impacted above a critical velocity. For Newtonian droplets, the jet creates a cavity and shoots straight through because the inertia of the impact outweighs the countering force of surface tension. But with viscoelastic drops, the jet goes through, slows down, and gets sucked back into the droplet. In this case, the combination of surface tension and viscoelasticity can, eventually, overpower the jet’s inertia. (Image, research, and submission credit: M. Quetzeri-Santiago et al.)