FYFD

Category: Research

  • Stone Skipping Physics

    Stone Skipping Physics

    The current record for stone-skipping is about 88 skips. For most of us, that’s an unimaginably high number, but according to physicists, human throwers may top out around 300 or 350 skips. In the video above and the accompanying article, Wired reporter Robbie Gonzalez explores both the technique of a world-record-holding skip and the physics that enable it.

    The perfect skip requires many ingredients: a large, flat rock with good edges; a strong throw to spin the rock and hold it steady at the right angle of attack; and a good first contact with the right entry angle and force to set up the skips’ trajectory. The video is long, but it’s well worth a full watch. It gives you an inside look both at a master skipper and at the experts of skipping science. (Video and image credit: Wired; see also: Splash Lab, C. Clanet et al.; submitted by Kam-Yung Soh)

    ETA: Wired’s embed code is acting up, so if you can’t see the stone skipping video here, just go to the article directly.

    Heads up for those going to the APS DFD meeting! You can catch my talk Monday, Nov. 19th at 5:10PM in Room B206. I’ll be talking about how to use narrative devices to tell scientific stories. I’ll be around for the whole meeting, so feel free to come say hi!

  • Hovering

    Hovering

    Nectar-drinking species of hummingbirds and bats are both excellent at hovering – one of the toughest aerodynamic feats – but they each have their own ways of doing it. Hummingbirds (bottom) use a nearly horizontal stroke pattern that’s quite symmetric on both the up- and downstroke. To keep generating lift in the upstroke, they twist their wings strongly midway through the stroke. So although hummingbirds get most of their lift from the downstroke, they get quite a bit from the upstroke as well.

    Bats, on the other hand, use an asymmetric wingbeat pattern when hovering. Bats flap in a diagonal stroke pattern, using a high angle of attack in the downstroke and an even higher one during the upstroke. They also retract their wings partially during the upstroke. This flapping pattern gives them weak lift during the upstroke, which they compensate for with a stronger downstroke. Compared to non-hovering bat species, nectar-drinking bats do get more lift during the upstroke, but they’re nowhere near as good as the hummingbirds. The bats compensate by having much larger wings compared to their body size. Bigger wings mean more lift.

    In the end, the two types of hovering cost roughly the same amount of power per gram of body weight. That’s great news for engineers designing the next generation of flapping robots because it suggests two very different, but equally power-efficient methods for hovering. (Image credit: Lentink Lab/Science News, source; research credit: R. Ingersoll et al.; via Science News; submitted by Kam Yung-Soh) 

  • Carbonation in Microgravity

    Carbonation in Microgravity

    Bubbly beverages are popular among humans, but there’s surprising complexity underlying their seemingly simply carbonation, as explored in a new Physics Today article. Most drinks get their bubbles from carbon dioxide, which at higher than atmospheric pressures, can stay dissolved inside water and other liquids. When that pressure gets released, any carbon-dioxide-filled gas cavity in the liquid adopts the allowable saturation concentration for the ambient pressure, which sets up a concentration gradient of carbon dioxide  between the liquid and the bubble. That causes carbon dioxide gas to diffuse into the bubbles, making them grow. 

    Here on Earth, those growing bubbles are buoyant, and they form rising plumes of bubbles. They continue gathering carbon dioxide as they rise, making them grow ever larger (lower left). In microgravity, on the other hand, the bubbles congregate where they form and continue growing through diffusion (lower right). This is one reason carbonated beverages are unpopular in space – instead of rising to the surface and escaping, all the carbon dioxide in a drink gets consumed, leaving astronauts with no way to expel it aside from burping!

    For lots more fascinating facts about bubbly drinks – including how they relate to geology! – check out the full Physics Today article. (Image credits: beer – rawpixel; bubbles – P. Vega-Martínez et al.; see also: R. Zenit and J. Rodríguez-Rodríguez)

  • Featured Video Play Icon

    Inside Hurricane Maria

    In addition to looking outward, NASA constantly monitors our own planet using a suite of satellites. In this video, they visualize data taken by the Global Precipitation Measurement Core Observatory of Hurricane Maria two days before it hit Puerto Rico. Instruments on board the satellite measure both liquid and frozen precipitation, giving scientists – and now the public – a glimpse into the heart of a developing hurricane. Be sure to take a look around; it’s a 360-degree video, and I bet it’s even more spectacular in VR. Having a trove of data like this helps researchers better understand the processes that influence a strengthening hurricane, which ultimately allows them to make better predictions about hurricane behavior in order to save lives. (Video credit: NASA; via Francesco C.)

  • Boiling with Sound

    Boiling with Sound

    Ultrasonic vibrations can boil nanoscale liquid layers, according to a new simulation-based study. Above you see a layer of water initially about 2 nm thick. When the surface it’s on vibrates at frequencies in the 100 GHz range – about a billion times faster than a hummingbird flaps – it superheats the thin layer of water. In this case, the film undergoes nucleate boiling, forming the same kinds of bubbles you see when boiling a pot of water. When the water layer gets too thin to support nucleate boiling, it stops boiling but evaporation continues. The transition occurs when van der Waals forces become significant. The technique only works with ultrathin layers of a liquid, but the authors envision broad application possibilities in industry as well as in micro- and nano-scale fluid systems. (Image and research credit, and submission: R. Pillai et al.)

  • Ants Avoid Traffic Jams by Giving Up

    Ants Avoid Traffic Jams by Giving Up

    Both ants and traffic are well-connected to fluid dynamics, even if they are not, strictly speaking, fluids. As it happens, ant traffic has interesting implications not only for human transit but for avoiding clogs in crowds or when pouring granular materials. 

    Ants tend to dig narrow tunnels. This helps individual ants recover from potential slips, but it also makes clogging more likely. Researchers studying the behavior of individual ants during tunnel digging found that ants entering the tunnel often turn around without collecting a grain and carrying it away. When they encounter heavy traffic, they simply reverse direction and give up. So 70% of the work of digging was done by only 30% of the ants. This seemingly unfair division of labor actually optimizes the overall traffic flow and work output for the ants as a whole. Without this instinct to turn around and ease the jam, incoming ants would cascade the traffic and worsen the jamming. (Image and research credit: J. Aguilar et al.; see also Physics Today)

  • Settling in Straws

    Settling in Straws

    At some point in your life, you’ve probably stuck your finger over the end of a straw and used it to pick up the liquid you’re drinking. If you lift the straw so that the end is still in your drink and remove your finger from the top, the liquid level in the straw will drop, then bounce up and down a couple times before it settles. This is what we see happen in the series of snapshots in the top image. Eventually, the liquid level settles at its equilibrium position, marked by the red arrow at the far right.

    The liquid has to bounce before settling because capillary forces and the liquid’s inertia are battling it out moment by moment. Just how long the rebound takes depends on the initial height of the fluid and the depth the straw is immersed at, but it doesn’t depend on the fluid’s viscosity. Lower viscosity fluids do sometimes have a neat jet (bottom image) that forms at the immersed end of the straw, though. (Image and research credit: J. Marston et al.)

  • Featured Video Play Icon

    Flying on Flexible Wings

    Bats are incredible and rather unique among today’s fliers. Like birds, they flap to produce their lift and thrust, but where birds have relatively stiff wings, a bat’s wings are flexible. The thin webbing of skin stretched between the bat’s finger joints has muscles inside it that fire as the mammal flaps. This means that the bat may actively control just how stiff its wing is as it flies.

    Compared to other natural and manmade fliers, the bat is incredibly agile and stable, able to recover from wind gusts in less than a full wingbeat cycle. They also have some incredible acrobatic capabilities. When preparing to perch, a bat loses almost all of its aerodynamic lift but still manages to maneuver itself so it flips over and grabs hold. Check out the full video above to learn more about these fascinating animals. (Video and image credit: Science Friday; research credit: S. Swartz and K. Breuer)

    Editor’s Note: I’ll be travelling through the end of the month with limited email access. The blog should continue posting uninterrupted, but if you contact me, just know it may be awhile before I can get back to you. Thanks! – Nicole

  • Inside Avalanches

    Inside Avalanches

    Avalanches have traditionally been difficult to model and predict because of their complex nature. In the case of a slab avalanche, the sort often triggered by a lone skier or hiker, there is a layer of dense, cohesive snow atop a layer of weaker, porous snow. The presence of the skier can destabilize that inner layer, causing a fracture known as an anticrack to propagate through the slab. Eventually, it collapses under the weight of the overlying snow and an avalanche occurs.

    What makes this so complicated is that the snow behaves as both a solid – during the initial fracturing – and as a fluid – during the flow of the avalanche. Researchers are making progress, though, using new models capable of simulating the full event (shown above) by leveraging techniques developed and used in computer animation for films. That’s right – the physics-based animation used in films like Frozen is helping researchers understand and predict actual avalanche physics! (Image and research credit: J. Gaume et al.; via Penn Engineering; submitted by Kam-Yung Soh)

  • Merging Black Holes

    Merging Black Holes

    At the heart of many galaxies, including our own, lies a supermassive black hole millions of times the mass of our sun. Scientists have yet to observe the merger of two such black holes, but using simulations, they are trying to learn what such collisions might look like. Simulations like the one shown here require combining relativity, electromagnetism, and, yes, fluid dynamics to capture what happens during the in-spiral.

    Supermassive black holes like these are surrounded by gas disks that flow around them. Magnetic and gravitational forces heat the gas, causing it to emit UV light and, at times, high energy X-rays, both of which may be observable.

    Gravitational wave detectors, similar to LIGO, may also measure evidence of supermassive black hole mergers, but physicists expect that will require a next-generation observatory, like the space-based LISA to be launched in the 2030s.   (Image and video credit: NASA Goddard; research credit: S. d’Ascoli et al.; submitted by @lh7)