FYFD

Tag: granular material

  • Forming Craters

    Forming Craters

    Asteroid impacts are a major force in shaping planetary bodies over the course of their geological history. As such, they receive a great deal of attention and study, often in the form of simulations like the one above. This simulation shows an impact in the Orientale basin of the moon, and if it looks somewhat fluid-like, there’s good reason for that. Impacts like these carry enormous energy, about 97% of which is dissipated as heat. That means temperatures in impact zones can reach 2000 degrees Celsius. The rest of the energy goes into deforming the impacted material. In simulations, those materials – be they rock or exotic ices – are usually modeled as Bingham fluids, a type of non-Newtonian fluid that only deforms after a certain amount of force is applied. An everyday example of such a fluid is toothpaste, which won’t extrude from its tube until you squeeze it.

    The fluid dynamical similarities run more than skin-deep, though. For decades, researchers looked for ways to connect asteroid impacts with smaller scale ones, like solid impacts on granular materials or liquid-on-liquid impacts. Recently, though, a group found that liquid-on-granular impacts scale exactly the way that asteroid impacts do. Even the morphology of the craters mirror one another. The reason this works has to do with that energy dissipation mentioned above. As with asteroid impacts, most of the energy from a liquid drop impacting a granular material goes into something other than deforming the crater region. Instead of heat, the mechanism for dissipation here is the drop’s deformation. The results, however, are strikingly alike.  

    For more on how asteroid impacts affect the moon and other bodies, check out Emily Lakdawalla’s write-up, which also includes lots of amazing sketches by James Tuttle Keane, who illustrates the talks he hears at conferences! (Image credits: J. Keane and B. Johnson; via the Planetary Society; additional research and video credit: R. Zhao et al., source; submitted by jpshoer)

     

  • Porous Fingers

    Porous Fingers

    If you inject a less viscous fluid, like air, into a narrow gap between two glass plates filled with a more viscous fluid, you’ll get a finger-like instability known as the Saffman-Taylor instability. If you invert the situation – injecting something viscous like water into air – the water will simply expand radially; you’ll get no fingers. But that situation doesn’t hold if there are wettable particles in the air-filled gap. Inject water into a particle-strewn air gap and you get a pattern like the one above. In this case, as the water expands, it collects particles on the meniscus between it and the air. Once the concentration of particles on the meniscus is too high for more particles to fit there, the flow starts to branch into fingers. This creates a greater surface area for interface so that more particles can get swept up as the water expands. (Image and research credit: I. Bihi et al., source)

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    Dam Failure

    In a recent video, Practical Engineering tackles an important and often-overlooked challenge in civil engineering: dam failure. At its simplest, a levee or dam is a wall built to hold back water, and the higher that water is, the greater the pressure at its base. That pressure can drive water to seep between the grains of soil beneath the dam. As you can see in the demo below, seeping water can take a curving path through the soil beneath a dam in order to get to the other side. When too much water makes it into the soil, it pushes grains apart and makes them slip easily; this is known as liquefaction. As the name suggests, the sediment begins behaving like a fluid, quickly leading to a complete failure of the dam as its foundation flows away. With older infrastructure and increased flooding from extreme weather events, this is a serious problem facing many communities. (Video and image credit: Practical Engineering)

  • Turning Sand Into a Fluid

    Turning Sand Into a Fluid

    Pumping air through a bed of sand can make the grains behave just like a liquid. This process is called fluidization. Air introduced at the bottom of the bed forces its way upward through the sand grains. With a high flow rate, the space between sand grains gets larger, eventually reaching a point where the aerodynamic forces on a grain of sand equal gravitational forces. At this point the sand grains are essentially suspended in the air flow and behave like a fluid themselves. Light, buoyant objects – like the red ball above – can float in the fluidized sand; heavier, denser objects will sink. Fluidization has many useful properties – like good mixing and large surface contact between solid and fluid phases – that make it popular in industrial applications. For a similar (but potentially less playful) process, check out soil liquefaction. (Image credits: R. Cheng, source; via Gizmodo; submitted by Justin)

  • Surge Flows

    Surge Flows

    Sandy beaches can be a great place to play with neat flows. In a recent video, Frank Howarth describes playing with beach rivers on the Oregon coast and observing a surge flow there. Under the right conditions, a current flowing over sand will build up sand ripples large enough that they form miniature dams in the flow. This traps additional water, which eventually collapses the sand ripples, releasing a surge of water. The surge tends to smooth out the sand and cause the ripple-making process to start over. It’s a fairly unusual phenomenon, but it’s one known to happen seasonally in a few specific places, like at Medano Creek in Colorado’s Great Sand Dunes National Park. There the snowmelt-fed creek surges during the late spring and early summer, releasing a fresh wave every 20 seconds or so. (Image credit: F. Howarth, source; h/t to Sebastian E.)

  • The Surge in the Hourglass

    The Surge in the Hourglass

    When we watch sands running through an hourglass, we think their flow rate is constant. In other words, the same number of grains falls through the neck at the beginning and the end. In many practical granular flows, like those through industrial hoppers (left), this is not the case. Instead, emptying those containers involves a surge near the end where the discharge rate is higher.

    The surge is related to the interstitial fluid – the air, water, or other fluid in the space between the grains. On the right, you see an experiment in which brown grains submerged in green-dyed water are emptied. The dark layer is dyed water initially at the top of the grains. As the container drains, that dyed layer moves down more rapidly than the grains; this indicates that the interstitial fluid is actually being pumped by the draining of the grains. Researchers think this is an important factor affecting the final surge. (Image credits: hopper – T. Cizauskas; discharge graph – J. Koivisto and D. Durian, source; research credit: J. Koivisto and D. Durian; submitted by Marc A)

  • Self-Digging Seeds

    Self-Digging Seeds

    Some plants in the Pelargonium family produce seeds with long helical tails. These appendages, formally known as awns, are humidity-sensitive. On humid nights or after rainfall, the awn begins to straighten. With its end anchored on the ground, this unfurling spins the seed and helps it burrow into the soil. A study looking at the physics of this system found that rotating reduces the drag a burrowing seed experiences in a granular material. Normally much of the force that opposes motion into a granular material is the result of intergranular contacts creating what are known as force chains. (Many science museums have great displays that visualize force chains.) The rotating seed drags grains near its surface along with it, helping to break up the force chains and reduce resistance. (Image and research credit: W. Jung et al., source)

  • Boulder Sorting on Asteroid Itokawa

    Boulder Sorting on Asteroid Itokawa

    Itokawa is a small asteroid visited by the Japanese Hayabusa probe in 2005. Photographs of the asteroid revealed a surface covered in large boulders at high elevations and small pebbles in the valleys. The Brazil nut effect is often invoked to explain size separation in particle mixtures, but Itokawa is so small that any shaking sufficient to sort particles would likely exceed the asteroid’s meager escape velocity. Instead, researchers have suggested an alternative size sorting mechanism: ballistic sorting.

    The idea of ballistic sorting is that pebbles that strike boulders will impact and bounce a long way, whereas pebbles that strike other pebbles are likely to rebound only a short way. In both experiments and simulations, the researchers found that this was the case and that mixtures of large and small particles tended to separate just as on the asteroid. The effect is possible on Earth as well, but Itokawa’s small gravitational acceleration makes for more effective size sorting. (Image credit: JAXA; research credit: T. Shinbrot et al.)

  • Linear Dunes

    Linear Dunes

    The Namib desert of southern Africa is home to some of the most stunning dunes on Earth. They are primarily linear dunes, which form parallel to the winds that form them. On the left side of the image, the dunes are aligned north-to-south along the direction of the southerly winds that blow through this area. Toward the center of the image, however, the dunes are deflected by strong seasonal winds blowing from the east. On the far right, the dunes break from a linear pattern to one with rectangular criss-crossings. This is a mixture of old and new dunes, evidence that the dominant direction of the wind has shifted over time. (Image credit: NASA Earth Observatory)

  • A Particle-Filled Splash

    A Particle-Filled Splash

    A drop of water that impacts a flat post will form a liquid sheet that eventually breaks apart into droplets when surface tension can no longer hold the water together against the power of momentum flinging the water outward. But what happens if that initial drop of water is filled with particles? Initially, the particle-laden drop’s impact is similar to the water’s – it strikes the post and expands radially in a sheet that is uniformly filled with particles. But then the particles begin to cluster due to capillary attraction, which causes particles at a fluid interface to clump up. You’ve seen the same effect in a bowl of Cheerios, when the floating O’s start to group up in little rafts. The clumping creates holes in the sheet which rapidly expand until the liquid breaks apart into many particle-filled droplets. To see more great high-speed footage and comparisons, check out the full video.  (Image credit and submission: A. Sauret et al., source)