FYFD

Month: March 2016

  • Mushrooms Make Their Own Breeze

    Mushrooms Make Their Own Breeze

    Plants and other non-motile organisms have developed some clever methods to disperse their seeds and spores for reproduction. Some plants use vortex rings for dispersal; others make their seeds aerodynamic. Low ground-dwellers like mushrooms must contend with a lack of wind to lift their spores and carry them away. Instead, they use evaporative cooling to generate their own air currents.

    Mushroom caps contain a lot of water and, as that water evaporates, it cools air near the mushroom, just as sweat evaporating off your skin cools you. That cooler, denser air tends to spread, carrying the spores outward. At the same time, the freshly evaporated water vapor is less dense than the surrounding air, so it rises. This combination of rising and spreading is capable of carrying spores tens of centimeters into the air, where the wind is stronger and able to carry spores further.  (Image credit: New Atlantis, source; research credit: E. Dressaire et al.)

  • Hovering Hummingbirds

    Hovering Hummingbirds

    Hummingbirds are incredible flyers, especially when it comes to hovering. To hover stationary and stable enough to feed, the hummingbird’s flapping pattern not only has to generate enough lift, or vertical force, to counteract their weight, but the bird must balance any forward or backward forces generated during flapping.

    As you can see in the animations above, when hovering the hummingbird’s wings move forward and back rather than up and down. When slowed down even further, the figure-8 motion of the wings becomes apparent. This careful motion is key to the hover; it allows the bird to generate about 70% of its lift on the downstroke when the wings move forward and creates the remainder of the lift needed on the upstroke. For much more high-speed footage of hummingbirds, check out the full BBC Earth Unplugged video, but be warned: you may experience a cuteness overdose! (Image credit: BBC Earth Unplugged, source)

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    Coffee-Making in Space

    In this video, Kjell Lindgren demonstrates his technique for making coffee aboard the Space Station. Astronauts usually drink coffee reconstituted from powder, or, on special occasions, enjoy a beverage from their special espresso machine. But Lindgren uses a pour-over method by attaching a pod of coffee grounds to the underside of a Capillary Beverage Experiment cup – a specially-designed 3D printed cup that uses capillary action and surface tension to guide fluids. Then, by forcing hot water from a syringe through the grounds and into the cup, he gets a result that’s not too different from the way many people enjoy their coffee here on Earth. I must say, though, that my favorite part of this video is how he just starts spinning to separate the air and water in the syringe! (Video credit: NASA; via IRPI LLC)

  • Eulerian vs. Lagrangian

    Eulerian vs. Lagrangian

    When I first studied fluid dynamics, one of the concepts I struggled with was that of Eulerian and Lagrangian reference frames. Essentially, these are just two different perspectives you can view the fluid from.  Physics is the same in both, but mathematically, you approach them differently. In the Eulerian perspective one sits at a location and watches the flow pass, like an observer watching a river go by. It’s demonstrated in the top animation, where turbulent flow sweeps past in a pipe. This is the usual perspective experimentally – you put an instrument at a certain point in the flow and you gather information as the fluid streams past in time.

    In the Lagrangian perspective, on the other hand, one follows a particular bit of fluid around and observes its changes over time. This means that one has to follow along at the mean speed of the flow in order to keep up with the fluid parcel one is observing. It would be like running alongside a river so that you can always be watching the same water as it flows downstream. The Lagrangian view of the same turbulent pipe flow is shown in the bottom animation. Notice how moving alongside the pipe makes it easier to see how the turbulence morphs as it goes along. Experimentally, this can be harder to achieve (at least in a flow with non-zero mean speed), but it’s a useful method of studying unsteadiness. (Image credit: J. Kühnen et al., source)

  • Martian Viscous Flow

    Martian Viscous Flow

    These images from the Mars Reconnaissance Orbiter show what are called viscous flow features. They are the Martian equivalent of glacial flow. Such features are typically found in Mars’ mid-latitudes.

    Ground-penetrating radar studies of Mars have shown that some of these features contain water ice covered in a protective layer of rock and dust, making them true glaciers. Another study of similar Martian surface features found that their slope was consistent with what could be produced by a ~10 m thick layer of ice and dust flowing superplastically over a timescale equal to the estimated age of the surface features. Superplastic flow occurs when solid matter is deformed well beyond its usual breaking point and is one of the common regimes for glacial ice flow on Earth. (Image credit: NASA/JPL/U. of Arizona; via beautifulmars)

  • Rotating Jet

    Rotating Jet

    This photo, one of the winners of the Engineering and Physical Sciences Research Council’s (EPSRC) annual photography contest, shows a rotating viscoelastic jet. Rotating liquid jets are common to many manufacturing processes, and their sometimes-wild appearance comes from a balance of gravitational forces and centrifugal force against surface tension. But because this fluid contains a small amount of polymer additive, surface tension has the additional aid of some elasticity to help hold the jet together and keep the globules and ligaments you see from flying off. As centrifugal forces fling the fluid outward, it stretches the polymer chains within the fluid, and they pull back against that tension like a stretched rubber band. To see some of the other contest winners–including other fluids entries!–check out the Guardian’s run-down. (Image credit and submission: O. Matar et al., ICL press release)

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    Filter-Feeding Mantas

    Large filter-feeders like the manta ray face the interesting challenge of obtaining enough small particulates like plankton to sustain an animal the size of a car. They do this through what is known as ram filter-feeding, essentially swimming open-mouthed through food-laden waters, filtering out the food, and releasing the water through their gills. Their internal filtration doesn’t simply catch particles like a colander does, though – it would be too easy for the ray’s filters to clog. Instead, the animals use several alternative methods to catch and redirect particles toward their esophagus. One, known as crossflow filtration, causes water to turn sharply through the filters. Heavier particles cannot accelerate that quickly, so they are carried onward. Another method, vortex filtration, works like a tiny centrifuge, spinning the water and ejecting the heavier particles back toward the esophagus. (Video credit: Science Friday; research credit: E. Paig-Tran, thesis)

  • Wrinkling Fluids

    Wrinkling Fluids

    What you see here is a viscous drop falling into a less viscous fluid. Shear forces between the drop and the surrounding fluid cause the drop to quickly deform into a shape like an upside-down mushroom as it descends. The cap forms a vortex ring that curls the viscous fluid back on itself. As it does, that motion compresses the viscous sheet, causing it to wrinkle, as seen in the close-up in the bottom animation. Check out the full video here. (Image credit: E. Q. Li et al., source)

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    Singing Sand Dunes

    Reports of singing sand dunes date at least as far back as 800 C.E. Strange as it sounds, about forty sites around the world have been associated with this phenomenon, in which avalanches of sand grains on the outer surface of the dune cause a deep, booming hum for up to several minutes. As you can hear in the video above, the sound of the dune is somewhat like a propeller-driven airplane. A leading explanation for this behavior is that it results not from the size or shape of the sand grains but from the structure of the underlying dune.

    Measurements show that the booming sand dunes contain a hard packed layer of sand 1-2 meters below the surface. When sand at the surface is disturbed by the wind or sliding researchers, it creates vibrations. Those disturbances have trouble crossing into the air or into the harder layers below. Instead they resonate in the upper surface of the sand, which acts as a waveguide, reflecting and enhancing the sound, just as the body of a violin resonates to enhance the vibration of its strings. For more, check out this video from Caltech or the research paper linked below. (Video credit: N. Vriend; research credit: M. Hunt and N. Vriend, pdf)

  • Review: “Life in Moving Fluids”

    Review: “Life in Moving Fluids”

    If you liked the prairie dog post earlier this week and you’re interested in more examples of biological fluid dynamics, you may enjoy Steven Vogel’s “Life in Moving Fluids”. I’m often asked for suggestions of readable textbooks for those who want an introduction to fluid dynamics, and this book is a great option. It addresses a wide variety of basic fluids concepts without getting as bogged down mathematically as many of the engineering texts do. It is written as an introduction to fluid dynamics for working biologists, though, so it contains plenty of technical detail – including relevant equations, discussions of basic flow measurement techniques, and overviews of the early academic literature.

    It is also chock full of interesting biological applications of fluid dynamics with examples ranging from the growth patterns of barnacles to the shape-shifting drag capabilities of trees. Vogel keeps a light-hearted tone and dry humor throughout and doesn’t shy away from puns.

    I read a first edition of the book (copyright 1981). The second edition, from the mid ‘90s, has updated coverage of the research literature, but I dare say the the topic has exploded within the last 20 years, so your mileage may vary with regard to the literature review. However, age in no way impacts the quality of Vogel’s coverage of the basics of fluid dynamics, and I feel confident in recommending this as an introductory text for those who’d like to pursue fluids in more depth.  (Images: S. Vogel/Princeton U. Press; h/t to Chris R.)