FYFD

Category: Research

  • Modeling Wildfires With Water

    Modeling Wildfires With Water

    Turbulence over a burning forest can carry embers that spread the wildfire. To understand how wildfire plumes interact with the natural turbulence found above the forest canopy, researchers modeled the situation in a water flume. Dowel rods acted as a forest, with turbulence developing naturally from the water flowing past. For a wildfire, the researchers used a plume of warmer water, which buoyancy lofted into the turbulence over their model forest.

    The experiment used to model wildfire flows. Dowel rods represent the forest and a plume of warm water (right side; distorting the background) represents the wildfire. The dark device in the foreground is a probe used to measure turbulence.
    The experiment used to model wildfire flows. Dowel rods represent the forest and a plume of warm water (right side; distorting the background) represents the wildfire. The dark device in the foreground is a probe used to measure turbulence.

    The flow over the forest canopy naturally forms side-by-side rolls of air rotating around a horizontal axis. As the buoyant plume rises, it can be torn apart by these rollers, as well as carried downstream. Varying the turbulence, they found, did not affect the average trajectory of the plume. But the more intense the turbulence, the greater the vertical fluctuations in the plume. Those large variations, they concluded, could lift more embers into stronger winds that distribute them further and spread a fire faster. (Image credit: wildfire – M. Brooks, experiment – H. Chung and J. Koseff; research credit: H. Chung and J. Koseff; via APS Physics)

  • Predicting Landslides

    Predicting Landslides

    Landslides can cause catastrophic damage, but historically it’s been difficult to monitor susceptible slopes and predict when they’ll fail. But a recent study looking at the 2017 Mud Creek landslide in California shows that new methods could provide a heads up.

    The researchers used satellite data from the months preceding the landslide to study how areas on the slope moved relative to one another. Within their survey region, they found sub-regions where ground locations largely moved together. These sub-regions, called communities in the researchers’ parlance, were remarkably persistent, showing little variation over long periods. But 56 days before the landslide, the researchers saw a sudden change between the communities on the slope. They believe their methodology could help identify slopes in danger of imminent slides.

    So far, though, they’ve only applied this method to the Mud Creek landslide. It’s a promising start, but they’ll need to show that the technique works for other slides as well. If so, it will be a major step forward in landslide prediction. (Image credit: USGS; research credit: V. Desai et al.; via APS Physics)

  • Forming Zigzags

    Forming Zigzags

    Scientists are fascinated by the organized patterns that can emerge from non-living systems. Here, researchers study micron-sized magnetic particles, immersed in a viscoelastic fluid and subjected to an oscillating magnetic field. The peanut-shaped particles roll around their long axis and assemble to form millimeter-sized bands of zigzags. These patterns, the researchers found, do not depend on the particles’ specific shape or on the details of the applied magnetic field. Instead, the zigzags depend only on the symmetry of the flow generated around each particle. In their system, illustrated above, each particle pushed fluid away along their long axis and drew in fluid toward their waist; as a result, particle pairs would attract or repel, depending on their relative orientation. That interparticle force ultimately caused the particles to self-organize into zigzags. (Image, video, and research credit: G. Junot et al.; via APS Physics)

    This sped-up animation shows the zig-zag pattern that the particles self-organization into.
    This sped-up animation shows the zigzag pattern that the particles self-organization into.
  • Uranus’s Polar Cyclone

    Uranus’s Polar Cyclone

    Uranus is an oddity among the planets of our solar system. Where other planets spin around an axis roughly in line with their orbital axis, Uranus spins on its side, placing its poles in line with the sun. On Earth, the polar regions are naturally colder the equator, but that doesn’t hold true for Uranus. Yet new observations of the ice giant show that it, like the other planets with atmospheres in our solar system, has a polar cyclone.

    Those observations are thanks to improvements in radio astronomy over the past couple decades. Uranus’s odd orbital geometry means that each of its poles are hidden from Earth for 42 years at a time; the current northern-hemisphere spring marks our first view of Uranus’s northern pole since 1965. In the recent observations, researchers saw a bright spot on the pole, surrounded by a faint darker ring. The team modeled the temperature and gas composition necessary to match their observations and found that those patterns were consistent with a cyclone sitting at the northern pole. (Image credit: NASA/JPL-Caltech/VLA; research credit: A. Akins et al.; via Physics Today)

  • Fish Fins Work Together

    Fish Fins Work Together

    Researchers studying how fish swim have long focused on their tail fins and the flows created there. But a fish’s other fins have important effects, too, as seen in this recent study. Researchers built a CFD simulation based on observations of a swimming rainbow trout, focusing on the flow from its back and tail fins. They found that the vortex created by the back fin stabilizes and strengthens the one generated by the tail. It also played a role in reducing drag on the fish by maintaining the pressure difference across the body. When they tried changing the size and geometry of the fins, the fish’s efficiency suffered, indicating that evolution has already optimized the trout’s fins for swimming efficiency. (Image credits: top – J. Sailer, simulation – J. Guo et al.; research credit: J. Guo et al.; via APS Physics)

    Visualization of flow around a digitized rainbow trout.
    Visualization of flow around a digitized rainbow trout.
  • Gravity Changes Droplet Shapes

    Gravity Changes Droplet Shapes

    With small droplets, gravity usually has little effect compared to surface tension. An evaporating water droplet holds its spherical shape as it evaporates. But the story is different when you add proteins to the droplet, as seen in this recent study.

    The protein-filled sessile drop starts out largely spherical, but as the drop evaporates, the concentration of proteins reaches a critical point and an elastic skin forms over the drop. From this point onward, the drop flattens.
    The protein-filled sessile drop starts out largely spherical, but as the drop evaporates, the concentration of proteins reaches a critical point and an elastic skin forms over the drop. From this point onward, the drop flattens.

    As a protein-doped droplet sitting on a surface evaporates, it starts out spherical, like its protein-free cousin. But, as the water evaporates, it leaves proteins behind, gradually increasing their concentration. Eventually, they form an elastic skin covering the drop. As water continues to evaporate, the droplet flattens.

    For a hanging droplet, the shape again starts out spherical. But as the drop's water evaporates and the proteins concentrate, it also forms an elastic skin. As the drop evaporates further, the skin wrinkles.
    For a hanging droplet, the shape again starts out spherical. But as the drop’s water evaporates and the proteins concentrate, it also forms an elastic skin. As the drop evaporates further, the skin wrinkles.

    In contrast, a hanging droplet with proteins takes on a wrinkled appearance once its elastic skin forms. The key difference, according to the model constructed by the authors, is the direction that gravity points. Despite these droplets’ small size, gravity makes a difference! (Image, video, and research credit: D. Riccobelli et al.; via APS Physics)

  • Testing Turbulence’s Limits

    Testing Turbulence’s Limits

    Understanding chaotic, turbulent flows has long challenged scientists and engineers due to their sheer complexity. In turbulent flows, energy cascades from the largest scales — like the kilometer-size cross-section of a cloud — to the very smallest scales, less than a millimeter in size, where viscosity transforms the flow’s motion to heat. For nearly a century, our theoretical understanding of turbulence has posited that there are certain universal behaviors in the statistics of a turbulent flow — essentially that, due to this energy cascade, some aspects of every turbulent flow are the same from clouds to ocean currents to your coffee cup.

    Accordingly, experimentalists have tried for decades to measure this expected universality. Often, there are some signs of agreement, and any deviation was attributed to the finite difference between the large and small scales of the flow. (The theory assumes the difference in these scales’ size is effectively infinite.) But now researchers have achieved the largest range of scales yet — comparable to those found in the atmosphere — and the gaps between theory and experiment remain. The new study does show signs of universality but in a different way than existing theory predicts. As the authors point out, we’ll need new theories to explain these findings. (Image credit: D. Páscoa; research credit: C. Küchler et al.; via APS Physics)

  • 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)

  • Bubble Growth, Inspired By Art

    Bubble Growth, Inspired By Art

    Eighteenth- and nineteenth-century French painters like Chardin and Manet had a certain fascination with bubble-blowing physics. Both left behind artwork depicting children blowing soap bubbles through straws. Now researchers are exploring this bubble-making method in a recent study.

    To blow a bubble from a straw or other narrow constriction, there are three basic stages. In the first, the soapy interface bulges and takes on a spherical shape. That’s followed by a period of rapid growth in less than 100 milliseconds. And, finally, the bubble will pinch off and detach from the straw. So far, most studies have focused on that third phase. Instead, this team focused on those early stages.

    In that first stage, the bubble’s growth depends on air getting forced out of an attached reservoir. For children, that’s their lungs reducing in volume as they blow air into the straw. In their experiments, the team found that the initial volume of the air reservoir is an important (and previously overlooked) factor in controlling bubble growth. (Image credit: J. Chardin; research credit: M. Grosjean and E. Lorenceau; via Ars Technica)

  • Puddle Depth Matters for Stalagmites

    Puddle Depth Matters for Stalagmites

    In a cave, mineral-rich water drips from the ceiling, spreading ions used to build stalagmites. A recent study considers how the depth of a pool affects the droplet’s splash and how material from the droplet spreads. The authors found several scenarios that vary widely depending on pool depth.

    A droplet falling into a shallow pool creates a splash that quickly breaks up into droplets. This flings the red droplet material in many directions.
    A droplet falling into a shallow pool creates a splash that quickly breaks up into droplets. This flings the red droplet material in many directions.

    A drop falling into a shallow pool had a splash that quickly broke up into droplets (above). By dyeing the pool green and the droplet red, they could track where the droplet’s material wound up. The spray of small droplets carried fluid far, but the main point of impact had a strong concentration of the drop’s fluid.

    With a deeper pool, the drop's impact creates a thick crown splash that collapses in on itself. The drop's fluid is quickly mixed into the pool.
    With a deeper pool, the drop’s impact creates a thick crown splash that collapses in on itself. The drop’s fluid is quickly mixed into the pool.

    In contrast, a deeper pool sent up a thick-walled splash crown that collapsed in on itself. This droplet’s material saw lots of mixing with the pool, but only near the point of impact. From their work, the authors concluded that models of stalagmite growth should incorporate pool depth in order to capture how minerals actually concentrate and move. (Image credit: cave – H. Roberson, others – J. Parmentier et al.; research credit: J. Parmentier et al.; via APS Physics; submitted by Kam-Yung Soh)