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  • Escaping the Limits of Viscosity

    Escaping the Limits of Viscosity

    For large creatures, it’s not hard to feel the evidence of someone else swimming nearby. But to tiny swimmers water is incredibly viscous and hard to move. These creatures have to swim very differently than their larger cousins, and evidence of their motion dies out quickly. But at least one microorganism,  Spirostomum ambiguum, has discovered a method for overcoming the limits of size and viscosity.

    The single-celled swimmer, when threatened, contracts its body in milliseconds, generating accelerations greater than those seen by fighter pilots. That acceleration is strong enough that it generates a burst of turbulence powerful enough to overcome the natural damping of its viscous surroundings. Within their colonies, S. ambiguum seem to use contraction to send out hydrodynamic signals to neighbors, who pass on the call to arms. To see the colonies in action, check out this previous article. (Image and research credit: A. Mathijssen et al.; via Physics Today; submitted by Kam-Yung Soh)

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    Why Do Backwards Wings Exist?

    Over the years, there have been many odd airplane designs, but one you probably haven’t seen much is the forward-swept wing. While most early aircraft featured straight wings, rear-swept wings are fairly common today, especially among commercial airliners. A rear-swept wing has its forward-most point at the root of the ring, where it attaches to the fuselage. The sweep breaks up the incoming flow into a chordwise component that flows from the leading edge to the trailing edge of the wing and a spanwise component that flows along the wing. Compared to straight wings, a swept wing provides better stability and control when flying at transonic speeds where shock waves can form on the wing (even though the plane itself is not supersonic).

    The trouble with rear-swept wings is that when they stall, they do so from the wingtips inward. Since the ailerons that control the plane’s orientation are out near the wingtips, that’s a problem. Forward-swept wings were supposed to solve this issue because they would stall from the root outward. But they came with a whole new set of problems, which included the need for robust onboard computers controlling them constantly to keep them in stable flight. In the end, the disadvantages outweighed any gains and so, for the most part, the forward-swept wing design has seen little flight time. (Image and video credit: Real Engineering)

  • Arctic Swirls

    Arctic Swirls

    These colorful swirls show sediment and organic matter carried into the Arctic Ocean. Like dyes or tracer particles in a lab experiment, this run-off reveals the complicated patterns of mixing where freshwater and salt water mix. Delicate as they appear, these eddies are tens of kilometers across. Zoom in on the full resolution image to really appreciate the details, like the feathery edges between layers. (Image credit: N. Kuring; via NASA Earth Observatory)

  • If You Teach a Goose to Fly

    If You Teach a Goose to Fly

    Scientists do all manner of odd things in the name of science. To teach bar-headed geese – birds capable of flying at the altitude of Everest – to fly in a wind tunnel, one group of researchers fostered a group of geese from the moment they hatched. They taught them to fly, first by chasing their bicycling parent and then following her on a motor scooter. Only then could they train the geese to fly in a wind tunnel designed to test how these birds manage to keep flying with only 30% of the oxygen found at sea level*.

    The birds’ secret, it turns out, is metabolic. As the oxygen dropped, so did the temperature of the geese’s blood. Hemoglobin, which binds oxygen in blood cells, is more efficient at lower temperatures, allowing the birds to get more oxygen. At the same time, though, their overall metabolism slowed down, meaning that they required less oxygen overall to function. Taken together, these adaptations make the geese excellent fliers in conditions most animals cannot tolerate. (Image and research credit: J. Meir et al.; via WashPo; submitted by Marc A.)

    * Occasionally I get comments pointing out that drag decreases with altitude, thereby making it easier to cut through the air. While this is true, I can say from my own experience of living and exercising at altitude that, for most of us, the effects of low oxygen levels far outweigh the savings in drag. It’s hard to appreciate a tiny drop in drag when your heart rate is sky high!

  • What Controls an Avalanche?

    What Controls an Avalanche?

    In an avalanche, grains spontaneously flow when a slope reaches a critical angle, and they continue flowing until they settle at a new, lower angle. Scientists have long debated why this angle mismatch occurs, and, in recent years, the general opinion was that the avalanche’s inertia kept it flowing long enough to settle at a lower angle. But a new experiment, using a slowly-rotating drum similar to the one above*, shows that friction, not inertia, is the key player. 

    The researchers used silica beads suspended in water, which allowed them to cleverly control the interparticle friction. In water, silica beads build up negative electrostatic charges, which push the grains apart and eliminate friction. In that frictionless state, the researchers found that the beads tumbled smoothly; their starting and ending angles were always the same. 

    By adding salt to the water, the researchers were able to eliminate some of the electrostatic charge and thereby tune the friction. When they did that, the difference between starting and stopping angles came back and grew more substantial as the friction increased. All in all, the results indicate that friction between particles is what makes an avalanche avalanche. (Image credit: J. Gray and V. Chugunov, source; research credit: H. Perrin et al.; via APS Physics; submitted by Kam-Yung Soh)

    * If you’re curious about the patterns in the image, I explain them in this previous post.

  • Waves in the Sky

    Waves in the Sky

    Even when the sky is mostly blue, there’s a lot going on at different altitudes. The winds do not move in a consistent direction or at the same speed, something which becomes apparent when watching clouds move relative to one another. When different layers of air move past one another, there is shear between them, not unlike the friction you feel when running your hand along a table. Under the right circumstances, this shear creates Kelvin-Helmholtz waves like the ones in this image over Helena Valley, Montana. Fast-moving winds (blowing right to left in the image) above a layer of clouds created these breaking wave-like curls. The same phenomenon creates many of the ocean’s waves from the shear caused by wind blowing across water. (Image credit: H. Martin, via EPOD)

  • The Impressive Take-Off of Pigeons

    The Impressive Take-Off of Pigeons

    One reason that peregrine falcons are such amazing fliers is that their prey, pigeons, are no slouches in flight, either. Able to take off vertically and accelerate to 100 kph in two seconds, pigeons are pint-sized powerhouses. With this high-speed video, BBC Earth highlights the mechanics of this vertical take-off. Pigeons begin by bending their legs and jumping high enough that their first downstroke can extend fully and still clear the ground. That gives them a headstart on generating the force they need to propel themselves upward. 

    Note the way the pigeon’s wings move, sweeping from directly behind the bird’s back to a full extension in front of it. With the bird moving vertically, this motion tells us that it’s thrust – not aerodynamic lift – from the wingstroke that’s powering this take-off. In that sense, the pigeon is something like a Harrier jet, using the thrust of air downward to take off vertically. (Image and video credit: BBC Earth)

  • Inside the Canopy

    Inside the Canopy

    If you’ve ever gone into the woods on a windy day, you know that conditions there are drastically different than in the open. To blowing wind, trees of different sizes act like enormous roughness that disturbs the flow. Inside the canopy, flows can become incredibly complicated and many of the common techniques used by researchers no longer hold. 

    You can get a sense for this complexity with the second image above, which visualizes data from a wind tunnel experiment. The gray blocks represent roughness elements – the trees of this wind-tunnel-scale forest – and the large, blue arrow shows the direction of the flow. The thin colored lines show the paths taken by particles in the flow. The lines’ colors indicate what height the trajectory began at. 

    Notice how the blue and purple lines are relatively straight and oriented in the direction of the flow. This indicates that the flow here is relatively steady and uncomplicated. At the lower heights, though, especially in the green and yellow regions, the pathlines are far more twisted and complex. The flow here is turbulent, and the particles’ trajectories don’t necessarily correlate at all to the winds higher up. (Image credit: T. Japyassu and R. Shnapp et al.; research credit: R. Shnapp et al.; submitted  by Ron S.)

  • Reader Question: White Caps

    Reader Question: White Caps

    Reader eclecticca asks:

    I really like the last two posts about waves, and they left me with another question…  My dad had a little boat he used to take us ocean fishing on quite a bit.  I always noticed that some days we just had big waves (swells) when out from the coastline and in fairly deep water (a hundred feet to hundreds of feet according to the depth sounder) and other days those swells would “break” and curl and foam and crash in on themselves, being what we called “breakers” or “white caps”.  There is no shore to create the breakers in this case, so what is happening?  Is it due to wind? current  a combination of factors?    Always been kind of curious about this really…

    You’re exactly right: those open ocean white caps are due to wind. Strictly speaking, the wind is what’s causing all* of the waves out in open, deep waters. But once the wind is strong enough, it starts breaking up the crests of waves, creating those foamy white tops. 

    According to one study, the break-up happens when the wind transfers more energy to the wave than surface tension can withstand. When the wave crest breaks up into a mixture of air, spray, and foam, it effectively gives the wind more surface area to push against and continue transferring energy. (Image credit: M. Moers)

    * With a few notable exceptions, like in the case of a tsunami.

  • Ferrofluid in a Cell

    Ferrofluid in a Cell

    Ferrofluids are a colloid consisting of magnetically sensitive nanoparticles suspended in a carrier liquid, like oil. They’re often associated with a distinctive spiky appearance when exposed to a magnet, but this isn’t their only magnetic response. Above we see a ferrofluid confined to a Hele-Shaw cell – essentially two glass plates with a small gap between them. In the upper image, the ferrofluid is exposed first to an axial magnetic field, which stretches it to form spidery arms. Then the magnetic field switches to a rotating configuration, which curls the arms around and causes the ferrofluid to slowly rotate.

    In the lower image, you see the reverse. First, the ferrofluid feels a rotating magnetic field. When this is changed to an axial field, the ferrofluid bursts into a cell-like center with straight arms. As the magnitude of the axial field increases further, the arms begin to curl. For more fantastical ferrofluid formations, check out these previous posts featuring artists Linden Gledhill and Fabian Oefner. (Image credit: M. Zahn and C. Lorenz, source; via Ashlyn N.)

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