FYFD

Category: Research

  • Never Break the Chain

    Never Break the Chain

    Pour water out of a bottle, and you’ll see a jet with a shape that resembles chain links. Sometimes known as a “liquid chain,” this phenomenon occurs when water pours through a non-circular hole. It’s quite a complex behavior, as shown in this recent study of the nonlinear effect. Even so, the authors found that the amplitude and wavelength of the chain’s sections are tied directly to the shape of the opening. Current models of the effect don’t account for the viscosity of the liquid, though, so future experiments will have to explore how fluids other than water behave. (Image and research credit: D. Jordan et al.; via APS Physics; submitted by Kam-Yung Soh)

    A comparison of oscillating jet shapes and metal chains.
    A comparison of an oscillating jet’s shape and metal chains. Each view is rotated 45 degrees from the one before.
  • Stabilizing Jupiter’s Polar Storms

    Stabilizing Jupiter’s Polar Storms

    Four years ago, Juno discovered an octagon of eight cyclones at Jupiter’s northern pole and a similar five cyclone structure at its southern pole. Since then, both polygons have remained intact. What keeps the storm systems so stable is still an open question, but a recent observational study using Juno measurements found that an anticyclonic ring sits between the central and outer cyclones. In line with a previous theoretical study, this ring structure helps shield and stabilize the storm system.

    The underlying convective mechanisms of the storm remain a mystery, though, as the current study is limited in resolution to a scale of about 200 kilometers. (Image credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM; research credit: A. Ingersoll et al.; via Gizmodo)

  • Searching for Stability

    Searching for Stability

    At present, there is no theory of relativistic fluid dynamics, which is problematic for those studying black holes, neutron star mergers, and heavy-ion collisions, where fluids may wind up moving at near-light speeds. Many current models for these systems allow energy to dissipate using equations that permit faster-than-light speeds. A new study shows that these assumptions lead to problematic results.

    The paper shows that, if the mathematical equations allow for faster-than-light speeds — thereby breaking causality — then the fluid system will behave stably to one observer and unstably to an observer in a different reference frame. In other words, there will always be a frame of reference where disturbances grow exponentially and destroy the system. That’s clearly not ideal.

    Fortunately, the paper also offers an important solution: if causality holds, the stability (or instability) of a system is the same regardless of reference frame. That’s incredibly powerful for researchers because it means that they only have to show the stability of the system in one reference frame to know that the result applies to all reference frames, so long as they’re not breaking causality. (Image credit: A. Pal; research credit: L. Gavassino; via APS Physics; submitted by Kam-Yung Soh)

  • Fluid Flow For Digestive Health

    Fluid Flow For Digestive Health

    During digestion, our intestines use two different patterns of muscle contraction to move food through our bodies. Scientists have long wondered why we have this added complexity. Using numerical simulations of the fluid flow created by these contractions, researchers have uncovered the answer.

    Our intestines use peristalsis, a forward-with-occasional-backward flow pattern, as the main driver. The strength of the muscle contractions determines how fast the average flow speed is. When the speed is slow, our bodies have more time to absorb nutrients, but that also allows more time for bacteria to flourish on those same nutrients. The other flow pattern, segmentation, creates a weaker flow overall but with much more mixing, which again enhances nutrient uptake.

    Switching between the two patterns, the researchers found, gives the body the best of both. Segmentation can enhance mixing and provide good nutrient uptake, then peristalsis can move the contents along quickly enough that bacteria don’t have time to grow before getting flushed out. (Image credit: Kindel Media; research credit: A. Codutti et al.; via APS Physics)

  • Mixing Effectively

    Mixing Effectively

    Mixing two fluids is a tougher task than you might think. One of my favorite asides from a fluids lecture concerned how to mix fruit into yogurt in an industrial setting. Mix too quickly, and you’ll obliterate the yogurt’s consistency, but mix too little and you may as well sell it as fruit-on-the-bottom. Apparently that particular problem got solved by sending the fruit and yogurt flowing through a series of specially-shaped ducts to slowly and carefully mix them together.

    In this study, researchers tackle a similar problem — mixing two fluids in a circular cross-section — through optimization. As you can see above, circular stirrers on their own don’t do a great job of mixing. So the researchers searched for the right combination of stirrer shape, mixing speed, and mixing trajectory to give the best mixing for a set mixing time and energy input. Their final stirrer shapes are anything but circular and often move in jerks and fits to help shed vortices that do the actual job of mixing. (Image and research credit: M. Eggl and P. Schmid; via APS Physics)

  • Sound Makes Stickier Bandages

    Sound Makes Stickier Bandages

    Keeping wounds safe and clean is hard when bandages are so prone to coming off. A team of researchers may have found a solution, though, using ultrasound to enhance adhesion. For their technique, they applied a layer of adhesive primer to the skin and covered it with a hydrogel bandage. Then they used an ultrasound transducer to generate cavitation bubbles in the primer. As the bubbles grew and collapsed, the primer and hydrogel pulled toward the tissue, creating adhesive bonds up to 100 times greater than without ultrasound. The extra adhesion had staying power, too, with between two and ten times more fatigue resistance than the bandage and adhesive alone. The researchers hope their technique will aid tissue repair, wound management, and attaching wearable electronics. (Image and research credit: Z. Ma et al.; via Physics World)

  • Escaping the Sun

    Escaping the Sun

    One enduring mystery of the solar wind — a stream of high-energy particles expelled from the sun — is how the particles get accelerated in the first place. The sun frequently belches out spurts of plasma, but without further momentum, that material simply falls back to the sun’s surface under the star’s gravity. Mechanisms like shock waves can further accelerate particles that are already moving quickly, but they cannot explain how the particles get going in the first place.

    A recent study used supercomputers to tackle this challenging problem in turbulent plasma physics. Each simulation tracked nearly 200 billion particles, requiring tens of thousands of processors. The results showed that turbulence itself provides the necessary initial acceleration and serves as the first step to getting particles moving fast enough to escape the sun. (Image credit: NASA SDO; research credit: L. Comisso and L. Sironi; via Physics World)

  • Testing Full-Size Engines

    Testing Full-Size Engines

    Engineers can often use small-scale models to test the physics of their creations, but sometimes there’s no substitute for going large. In this photo, we see a full-size commercial engine used on an airplane, mounted at the Instituto Nacional de Tecnica Aeroespacial (INTA) in Madrid.

    Behind the engine, in red, is an optical rig used for a brand-new measurement technique that allows engineers to directly measure the carbon dioxide emissions of the engine as it runs. The optical frame is 7 meters in diameter and uses 126 beams of near-infrared laser light to probe the engine’s exhaust without interrupting the flow. It’s the first chemically specific imaging of a full-scale gas turbine like those found on commercial aircraft. Given the high carbon emissions associated with air travel, the technique will be important for engineers building greener aircraft engines. (Image and research credit: A. Upadhyay et al.; via The Engineer; submitted by Simon H.)

  • Droplet Bounce

    Droplet Bounce

    A droplet falling on a liquid bath may, if slow enough, rebound off the surface. Its impact sends out a string of ripples — capillary waves — on the bath’s surface and sends the droplet itself into jiggling paroxysms. A new pre-print study delves into this process through a combination of experiment, simulation, and modeling. Impressively, they find that the most of the droplet’s initial energy is not dissipated during impact. Instead it’s fed into the capillary waves and droplet deformation that follow. (Image and research credit: L. Alventosa et al.; via Dan H.)

    A droplet falls on a bath, partially coalesces and rebounds. The process repeats until the droplet is small enough to coalesce completely.
    A droplet falls on a bath, partially coalesces and rebounds. The process repeats until the droplet is small enough to coalesce completely.
  • Turbulence in Accretion Disks

    Turbulence in Accretion Disks

    Accretion disks form everywhere, from around young, planet-building stars to massive black holes. As matter circles in the disk, it slowly loses angular momentum and falls inward toward the central gravitational body. But the details of this process have long vexed astronomers. The low-viscosity environment of gas and dust in accretion disks simply is not sufficient to account for the level of angular momentum lost. Turbulence is expected to provide a boost to the effect, but neither astronomical observations or Taylor-Couette experiments have shown how.

    A new study uses a liquid metal, confined in a disk using radial and vertical electrical fields. Unlike prior experiments, this set-up creates a more gravity-like force to rotate the liquid. With it, researchers can tune both the rotation speed and the level of turbulence. They found that turbulence is, indeed, responsible for the loss of angular momentum that transports mass inward — even at the limit of zero intrinsic viscosity.

    Unfortunately, the apparatus isn’t a perfect analog for astrophysical disks; in the experiment, the turbulence originates from secondary flows that aren’t present in real systems. So while the team demonstrated that turbulence can drive the accretion disk’s behavior, it can’t pinpoint where that turbulence originates in real accretion disks. (Image credit: NASA; research credit: M. Vernet et al.; via Physics World)