FYFD

Category: Research

  • 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)

  • Overheating Slows Large Animals

    Overheating Slows Large Animals

    As climate change and human development continue to encroach on animals’ territories, mass migrations will become more and more common. But animals aren’t all equally able to travel long distances at speed. In general, larger animals are faster than smaller ones. But a new study shows that there’s another important factor in an animal’s top speed: heat dissipation.

    By studying the characteristics of over 500 animals that walk, fly, and swim, the team found that animals were limited in their speed by how well they could dissipate heat. This makes sense, even from a human perspective; we may be able to run long distances, but once we’re too hot, we have to slow down. The same principle holds for animals, and the bigger the animal, the longer it takes to dissipate heat. As a result, the team found that the fastest animals over long distances all have intermediate body mass. At their size, they can balance the mechanical ability to produce speed with the thermodynamic requirement to dissipate heat. (Image credit: N. and Z. Scott; research credit: A. Dyer et al.; via APS Physics)

  • Getting Water Out of Your Ear

    Getting Water Out of Your Ear

    Swimming often results in water getting stuck in our ear canals. The narrow space, combined with the waxy surface, is excellent at trapping small amounts of water. If left in place, that excess fluid distorts hearing, can cause pain, and may eventually lead to an ear infection. So most people’s common response is to tilt their head sideways and shake it or jump to knock the water out. This recent study looks at just how much acceleration is needed to dislodge that water.

    An acceleration of 7.8g isn't enough to remove the water from this artificial ear canal.
    An acceleration of 7.8g isn’t enough to remove the water from this artificial ear canal.

    The team built an artificial ear based on the shape of a human’s ear canal and observed how much acceleration was needed to knock the water out. The answer? Quite a bit. As seen above, nearly 8g of acceleration was enough to distort the interface of the water in the ear canal, but it didn’t move the water out.

    At higher accelerations — above 20 times the acceleration due to gravity – the air-water interface distorts enough to get the water to flow. But accelerations that large are enough to potentially damage brain tissues.

    At over 24g, the acceleration is enough to dislodge the water from this artificial ear canal. But accelerations this high can cause brain damage.
    At over 24g, the acceleration is enough to dislodge the water from this artificial ear canal. But accelerations this high can cause brain damage.

    The problem is worse for children and babies, whose tiny ear canals necessitate even larger accelerations. For them, shaking hard enough to remove water could cause real damage. Instead, a couple drops of vinegar or alcohol in the ear will lower the surface tension and make the fluid easier to remove. (Image credit: top – J. Flavia, others – S. Kim et al.; research credit: S. Kim et al.; submitted by Sunny J.)

  • Shouting Into the Wind is Easier Than You Think

    Shouting Into the Wind is Easier Than You Think

    “Shouting into the wind” usually means a failure to communicate, but it turns out that shouting into the wind doesn’t work the way people usually think. In fact, it’s easy for people upstream to hear your shouting, thanks to an acoustical effect called convective amplification. You’ve likely experienced it firsthand as an ambulance approaches. With its sirens blaring, the ambulance sounds louder as it comes toward you and quieter after it’s past. (This is separate from the Doppler effect, which changes the pitch of the approaching and receding vehicle.)

    So why does shouting into the wind seem so hard? It’s because your ears are downstream of your mouth. Like the ambulance that’s already gone by, your voice comes from ahead of your ears and therefore sounds quieter to you than it does to your audience upstream. (Image credit: I. Huhtakallio; research credit: V. Pulkki et al.; via Science News; submitted by Kam-Yung Soh)

  • Fixing Reverse Osmosis

    Fixing Reverse Osmosis

    Desalination and water treatment plants both rely on reverse osmosis to generate clean water for human use. The standard theory behind reverse osmosis for the last half century suggested that the membranes separated water and other chemicals by forcing water molecules, driven by chemical gradients, to travel one-by-one through a dense membrane forest. But over the years, researchers saw signs that this theory didn’t hold up; for one, the membranes water travels through have pores in them that are larger than individual water molecules.

    A new study examines the underlying assumptions of the prevailing model and finds instead that water moves through reverse osmosis membranes by pore flow. Instead of individual molecules pushed by concentration, flow takes place through pores and is driven by a pressure gradient. The difference is important because it enables engineers to design more efficient membranes according to real-world physics. By understanding the underlying mechanism, designers can tweak the pore size, density, and other features of reverse osmosis membranes to better filter unwanted chemicals and to remove salt from water with less energy input. (Image credit: Florida Water Daily; research credit: L. Wang et al.; via Wired; submitted by Kam-Yung Soh)

  • Bubble Trails – Straight or Wonky?

    Bubble Trails – Straight or Wonky?

    Watch the bubbles rising in a glass of champagne and you’ll see them form tiny straight lines, with each bubble following its predecessor. But in a carbonated soda, the bubbles rise all over the place, each following its own zig-zaggy line. Why the difference? A recent study points out the culprits: bubble size and surfactants.

    As bubble size increases from left to right, the bubble trail straightens.
    As bubble size increases from left to right, the bubble trail straightens.

    Looking at a variety of beverage scenarios, researchers found that both a bubble’s size and its surfactant concentration affected what sort of path it followed. For clean (surfactant-free) bubbles, small bubbles take a winding path, but bigger ones move in a straight line. Simulations show that bubbles can only form a straight path if they produce enough vorticity on their surface. Small bubbles just can’t deform enough to do that.

    For bubbles of the same size, increasing the surfactant on the bubbles straightens their path.
    For bubbles of the same size, increasing the surfactants on the bubbles straightens their path.

    When surfactants get added, though, the story changes. For bubbles of a set size, adding surfactants made their paths straighter. This was due, the team found, to a bump in vorticity provided by the stabilizing effect of the surfactants. Champagne, they concluded, has straight bubble paths despite its tiny bubbles because of the drink’s high number of flavorful surfactants. (Image credit: top – D. Cook, experiments – O. Atasi et al.; research credit: O. Atasi et al.; via APS Physics)

  • Lanes in Crowds

    Lanes in Crowds

    In nature — from atoms to human crowds — two groups moving in opposite directions often spontaneously organize into interwoven lanes flowing in their respective directions. Now researchers have built a mathematical model for this behavior, building on Einstein’s observations of Brownian motion.

    To test their model, the researchers performed numerical simulations and experiments with pedestrians. Intriguingly, they found that introducing rules like “always pass on the right” created unexpected results, such as tilted lanes. With their model verified — at least for low-density crowds — the group hope to uncover other hidden patterns within crowds. (Image and research credit: K. Bacik et al.; via Physics World)

    An animation showing one pedestrian experiment.
    In their validation experiments, the researchers filmed groups of pedestrians walking past one another under different conditions. Note the lanes that form as the two groups interleave.