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  • Dunes Avoid Collisions

    Dunes Avoid Collisions

    The speed at which a dune migrates depends on its size; smaller dunes move faster than larger ones. That speed differential implies that small dunes should frequently collide into and merge with larger dunes, eventually forming one giant dune rather than a field of smaller separate ones. But that’s not what we observe in nature.

    To figure out why dunes aren’t colliding that often, researchers built a dune field of their own in the form of a rotating water tank. Inside the tank, their two artificial dunes can chase one another indefinitely while the researchers observe their interactions. What they found is that the dunes “communicate” with one another through the flow.

    As flow moves over the upstream dune, it generates turbulence in its wake, which the downstream dune then encounters. All that extra turbulence affects how sediment is picked up and transported for the downstream dune, ultimately changing its migration speed. For two dunes of initially equal size and close spacing, these interactions push the downstream dune further away until the separation between the dunes is large enough that they both migrate at the same speed. Even between dunes of unequal sizes, the researchers found that these repulsive interactions force the dunes away from collision and into migration at the same speed. (Image credit: dune field – G. Montani, others – K. Bacik et al.; research credit: K. Bacik et al.; via Cosmos; submitted by Kam-Yung Soh)

  • Featured Video Play Icon

    Sunlight Is Older Than You Think

    Joe Hanson over at “It’s Okay to Be Smart” has a great video on the random walk photons have to make to escape the core of the sun and other stars. Because the high-energy photons born in the star’s core have to bounce their way out rather than flying in a straight line, those photons can spend thousands of years escaping the sun. After that, the eight-and-a-half minute trip to Earth is nothing.

    But there’s a key element missing in this explanation: convection! That radiative random walk photons do doesn’t last all the way from the core of the sun to its surface. From a depth of about 200,000 km onward, the dominant mode of transport in the sun is convection, actual fluid motion that carries heat and light much faster than simple molecular diffusion, or Brownian motion, does. That’s why the surface of the sun shines with convection cells similar to the ones you’ll see in your skillet when heating a layer of oil.

    Fluid motion beyond molecular diffusion is also a big part of the other flows Joe describes in the video. If you had to wait on Brownian motion in order to smell your morning coffee, it would be cold long before you knew it was there! (Video and image credit: It’s Okay to Be Smart; sun surface image credit: Big Bear Solar Observatory/NJIT)

  • Listening to a Bubble’s Pop

    Listening to a Bubble’s Pop

    Sound is an important aspect of many flows, from the scream of a rocket engine to the hum of electrical wires vibrating in the wind. Critically, those sounds carry important information about the flow. A new study extends these acoustic diagnostics to the popping of soap bubbles.

    When a hole opens in a soap bubble, it throws the surface-tension-driven capillary forces of the bubble into disarray. The rim around the hole retracts, pushing fluid away from the expanding hole. At the same time, air is pushed out of the collapsing bubble. Using microphone arrays, the researchers found they could measure and distinguish sound from both sources — the escaping air and the expanding hole.

    From the sound, they developed a model that predicts the rupture location, bubble thickness profile, and other properties of the bubble. They confirmed the model’s results by comparing with high-speed photography. The authors hope their new acoustic technique will shed light on bubble bursting events that are hard to observe visually, like the bubbling of magma. (Image and research credit: A. Bussonnière et al.; via Science News; submitted by Kam-Yung Soh)

  • Nitro Bubble Cascades

    Nitro Bubble Cascades

    Animation of nitrogen bubbles cascading in Guinness

    Fans of nitro beers — particularly Guinness’ stout — have probably noticed the fascinating cascade of bubbles that form as the beer settles. It’s a non-intuitive behavior — bubbles rise since they’re lighter than the surrounding fluid. So why do the bubbles appear to sink in these beers?

    There are several effects at play here. Firstly, overall the bubbles in the beer are rising; even mixing nitrogen gas into a beer in place of carbon dioxide doesn’t change that. But pint glasses typically flare so that they’re wider at the top than at the bottom. Since the bubbles rise essentially straight up, this causes a bubble-less film to form near the upper walls. And as that heavier fluid sinks, it pulls some of the tiny nitrogen bubbles with it. (You don’t see this effect in typical beers because the bubbles there are larger and thus too buoyant to get pulled down by the falling fluid.)

    As for the cascading waves we see in the bubbles, this, too, comes from the shape of the glass. Hydrodynamically speaking, what’s happens as the fluid film slides down the pint glass is similar to what happens when rain runs downhill. Beyond a certain angle, the flow becomes unstable and will form rolls and waves of varying thickness instead of sinking in a thin, uniform layer. As the film goes, so go the bubbles being dragged along, giving everyone at the bar a brief but entertaining fluid dynamical show. (Image credits: pints – M. d’Itri; bubble cascade – T. Watamura et al.; research credit: T. Watamura et al.)

  • Submarine Canyons Focus Waves

    Submarine Canyons Focus Waves

    In winter months Toyama Bay in Japan can get hammered by waves nearly 10 meters in height. These waves, known as YoriMawari-nami, pose dangers to both infrastructure and citizens, and, thus far, are not captured by typical forecasting models.

    A new study indicates that these waves have their origin in the particular topography of Toyama Bay and the physics behind the double-slit experiment. The shape of Toyama Bay is such that only waves from the north-northeast can propagate all the way to shore. That restriction essentially creates a single, coherent source for waves in the bay.

    The bay is also home to submarine canyons that stretch like underwater valleys from the continental shelf down toward the deeper ocean. To the incoming waves, these canyons act much like the slits in the double-slit experiment, creating two sets of waves whose fronts can interfere. In some positions, a wave crest will combine with a wave trough, cancelling one another out. But in other spots, two wave crests will meet and combine, creating the much larger YoriMawari-nami wave.

    Diagram illustrating the similarity of the YM-wave phenomenon to Young's double-slit experiment. By H. Tamura et al.

    Toyama Bay is not the only spot in the world where this phenomenon happens. The same physics is behind some of the most popular surf spots in the world, including Half-Moon Bay in California and Nazaré, Portugal. In all of these cases, properly predicting wave heights requires tracking an extra variable — wave phase — that most models leave out. That’s why forecasters have struggled with Toyama Bay’s waves. (Image credit: wave – M. Kawai, diagram – H. Tamura et al.; research credit: H. Tamura et al.; via AGU Eos; submitted by Kam-Yung Soh)

  • Pearls On a Puddle

    Pearls On a Puddle

    Leave a drop of coffee sitting on a surface and it will leave behind a ring of particulates once the water evaporates. But what happens to a droplet made up of multiple liquids that evaporate differently? That’s the subject of this new study. Researchers mixed a volatile drop (isopropyl alcohol) with a smaller amount of a non-volatile liquid and observed how this changed the droplet’s splash rim and evaporation pattern.

    When the surface tension difference between the two liquids was large, the researchers found that the splash formed fingers along its rim (Image 1). The fingers consist almost entirely of the non-volatile component, driven to the outskirts of the drop by Marangoni forces. The dark and light bands you see in the image are interference fringes, which the researchers used to track the film’s thickness.

    When the researchers used liquids with similar surface tensions, the droplet rim instead formed pearl-like satellite droplets. Once the volatile liquid evaporated away, the remaining liquid merged into a thick film. (Image and research credit: A. Mouat et al.; via APS Physics; submitted by Kam-Yung Soh)

  • The Sand Sea’s End

    The Sand Sea’s End

    The northern extent of Africa’s Namib Sand Sea ends where the reddish dunes meet the Kuiseb River and the hard, rocky land on its other side. Within the sand sea, dunes stretch as high as 300 meters while the prevailing winds create and march them across the desert. Although dunes rarely occur in isolation, the mechanisms that regulate dune-dune interactions are still poorly understood, though new experiments are beginning to shed light on the processes. (Image credit: USGS/NASA Earth Observatory)

  • Hydrodynamics of Sheep

    Hydrodynamics of Sheep

    As we’ve discussed previously, not all fluid-like behavior occurs within a literal fluid. Many groups of organisms — humans included — behave like a fluid en masse. Herds of sheep are a fantastic example of this, and now researchers have actually analyzed footage of sheep as a fluid!

    The authors find strong evidence for emergent collective behavior among the sheep, as well as a tendency for the flock to minimize its perimeter. In other words, even though the sheep do not physically exert an attractive force on one another, they behave as though the flock has surface tension! For a herd animal, this behavior makes sense since it minimizes the exposure of individuals to predators. (Image credit: top image – S. Carter, drone footage – M. Bircham; research credit: M. de Marcken and R. Sarfati; submitted by Kam-Yung Soh)

    ETA: Thanks to commenter gib for finding the original author of the drone footage!

  • Ice Rings Caused By Underlying Eddies

    Ice Rings Caused By Underlying Eddies

    Observations of strange ice rings on Lake Baikal, the world’s deepest lake, have puzzled scientists for decades. Surveys of satellite imagery have revealed rings on Baikal and two other lakes dating back to the 1960s and some of our earliest satellite images. The rings are roughly 5-7 km in diameter, with a dark layer of thin ice about 1 km wide around a brighter layer of thick ice.

    A new study, buoyed in part by on-the-ground observations during Siberian winter, argues that the ice rings observed on the surface are related to eddies of warmer water circulating below. The researchers were able to capture several eddies in their measurements, including one migratory one. The size, shape, and location of these sub-surface eddies are consistent with ice ring appearance. The kilometers’ wide eddies are several degrees warmer at shallow depths and rotate approximately once every 3 days.

    The researchers suspect the eddies form long before the ice does. Infrared observations in late autumn suggest the eddies form from a combination of wind and influx of river water into the lakes. Then, as ice does form, it’s affected by the underlying circulation. (Image credits: NASA, 1, 2; research credit: A. Kouraev et al.; via Gizmodo)

  • To Beat Surface Tension, Tadpoles Make Bubbles

    To Beat Surface Tension, Tadpoles Make Bubbles

    For tiny creatures, surface tension is a formidable barrier. Newborn tadpoles are much too small and weak to breach the air-water surface in order to breathe. Researchers found that, instead, the 3 millimeter creatures place their mouths against the surface, expand their mouth to generate suction, and swallow a bubble consisting largely of fresh air.

    When they’re especially small, some of these species are essentially transparent (Image 1), allowing researchers to see the bubble directly. But even as the tadpoles aged (Images 2 and 3) and grew strong enough to breach the surface, they observed many instances in which the tadpoles continued this bubble-sucking method to breathe. (Image and research credit: K. Schwenk and J. Phillips; via Cosmos; submitted by Kam-Yung Soh)

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