FYFD

Tag: avalanche

  • Predicting Landslide Speeds

    Predicting Landslide Speeds

    Knowing what speed a landslide will reach helps us predict how much damage they can cause. That speed depends on many factors: the steepness of the terrain, the sliding distance, the thickness of the flowing layer, and the type of grains making up the flow. Researchers found that predictions from previous studies often underestimated the speeds reached by thicker flows. Through laboratory experiments with grains of different shapes, a team found that those models mistakenly assumed a fully-developed flow — in other words, one where the grains have reached a constant final speed. While spherical grains reach that state over a short sliding distance, that’s not the case for other grains.

    Instead, the team used their results to build a new predictive model for landslide speeds. This one still depends on incline angle and flow thickness, but it also uses a dynamical friction coefficient to describe the granular material and capture how the flow’s speed varies with distance down the incline. (Image credit: W. Hasselmann; research credit: Y. Wu et al.; via APS News)

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  • Forests Slow Avalanches

    Forests Slow Avalanches

    In snowy mountainous regions, avalanches are a dangerous and destructive problem. Researchers studying the mechanisms of these flows have a suggestion: plant more trees. A group of researchers found that a “forest” of regularly spaced pillars slowed avalanches by as much as two-thirds. On an empty slope, the avalanche picked up speed as its thickness grew. But with regularly-spaced pillars the slower flow rate became almost completely independent of avalanche thickness.

    The researchers with their avalanche set-up, which releases glass beads through a forest of pillars.
    The researchers with their avalanche set-up, which releases glass beads through a forest of pillars.

    For now, the researchers suggest placing trees every 3 meters on steep, avalanche-prone slopes — a technique that, admittedly, only works for slopes below the treeline. In their next round of experiments, the researchers plan to see how a randomly arranged forest affects an avalanche. (Image credit: top – N. Cool, apparatus – Université Paris-Saclay/FAST; research credit: B. Texier et al.; via Physics World)

  • Sliding on Sand

    Sliding on Sand

    Getting around on sandy slopes is no easy feat. On steep inclines, even small disturbances will cause an avalanche. The predatory antlion takes advantage of this fact by building a conical pit that makes ants that walk in slide down into its waiting jaws. But a new study shows that it’s more than just pressure that determines when an object slides down the slope.

    To simulate hapless ants sliding into an antlion’s pit, researchers used plexiglass disks with four smaller disks that act as legs on the granular slope. By varying the distance between these points of contact, researchers found that stance also affects when a slide starts. The closer together the contacts are, the more likely the disk would slide. In contrast, spreading the points of contact increased stability, meaning that adopting a wider stance could keep an animal, human, or robot from sliding as easily. (Image credit: NEOM; research credit: M. Piñeirua et al.; via APS Physics)

  • Slab Avalanche Physics

    Slab Avalanche Physics

    Slab avalanches like the one shown here begin after weak, porous layers of snow get buried by fresher, more cohesive snow layers. On a steep slope, the weight of the new snow can be too great for friction to hold the slab in place, causing the upper layer to crack and slide at speeds up to 150 meters per second. Scientists had two competing theories for how slab avalanches began. One theory presumed that the weak layer of snow failed under shear; the other argued that the collapse of the lower, porous layer was at fault.

    In a new study combining large-scale numerical simulation with real-life observations, scientists came to a new conclusion: cracks began to form in the porous layer as the weight of heavier snow crushed down, but once the cracks formed, the shear mechanism took over. Cracks formed by shear could propagate along the existing cracks in the porous layer, allowing faster crack propagation than through undamaged snow. In the end, it’s the combination of the two mechanisms that triggers the avalanche. (Image credit: R. Flück; research credit: B. Trottet et al.; via Physics World)

  • 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.

  • Digging Sandpits

    Digging Sandpits

    Antlion larvae dig sandpits to catch their prey, and, according to a new study, they rely on the physics of granular materials to do so. The antlion digs in a spiral pattern (bottom), beginning from the outside and working its way inward. As it digs, it ejects larger grains and triggers avalanches that cause large grains to fall inward. This leaves the walls of the final pit lined with small grains, which have a shallower angle of repose and will slip out from under any prey that wander in. The subsequent avalanche will carry the victim to the antlion lying in wait at the center of the pit. (Image credits: antlion larva – J. Numer; antlion digging – N. Franks et al.; research credit: N. Franks et al.; submitted by Kam-Yung Soh)

  • 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)

  • Flowing Flowers

    Flowing Flowers

    Granular mixtures with particles of different sizes will often segregate themselves when flowing. In this half-filled rotating drum large red particles and smaller white ones create a stable petal-like pattern. As the drum turns, an avalanche of small particles flows down, forming each white petal. When the avalanche hits the drum wall, a second wave – one of the larger, red particles – flows uphill toward the center of the drum. If the uphill wave has enough time to reach the center of the drum before the next avalanche of smaller particles, then the petal pattern will be stable. Otherwise, the small particles will tend to fall between the larger ones, disturbing the pattern. (Image and research credit: I. Zuriguel et al., source; via reprint in J. Gray)

  • The Catherine Wheel

    The Catherine Wheel

    When particles of different sizes fall in an avalanche, they separate out by size. Smaller particles form one layer with another layer of larger particles over the top. This happens because the smaller particles tend to fall in between the larger ones, similar to the percolation theory in the Brazil nut effect. In a slowly rotating drum, this size segregation during an avalanche forms a distinctive pattern (above) called a Catherine wheel pattern. Here, the gray layers form from smaller iron particles, while the white layers are large particles of sugar. Notice that the pattern starts to form during each avalanche, but it freezes in place after grains pile up against the drum wall and cause a shock wave to run back up the avalanche. (Image credit: J. Gray and V. Chugunov, reprinted in J. Gray, source)

  • Featured Video Play Icon

    Skiing, Avalanches, and Freezing Bubbles

    To wrap up our look at Olympic physics, we bring you a wintry mix of interviews with researchers, courtesy of JFM and FYFD. Learn about the research that helped French biathlete Martin Fourcade leave PyeongChang with 3 gold medals, the physics of avalanches, and how bubbles freeze. 

    If you missed any of our previous Olympic coverage, you can find our previous entries on the themed series page, and for more great interviews with fluids researchers, check out our previous collab video. (Video credit: T. Crawford and N. Sharp; image credits: GettyImages, T. Crawford and N. Sharp)