FYFD

Tag: science

  • Convection

    Convection

    Blue paint in alcohol forms an array of polygonal convection cells. We’re accustomed to associating convection with temperature differences; patterns like the one above are seen in hot cooking oil, cocoa, and even on Pluto. In all of those cases, temperature differences are a defining feature, but they are not the fundamental driver of the fluid behavior. The most important factors – both in those cases and the present one – are density and surface tension variations. Changing temperature affects both of these factors, which is why its so often seen in Benard-Marangoni convection.

    For the paint-in-alcohol, density and surface tension differences are inherent to the two fluids. Because alcohol is volatile and evaporates quickly, its concentration is constantly changing, which in turn changes the local surface tension. Areas of higher surface tension pull on those of lower surface tension; this draws fluid from the center of each cell toward the perimeter. At the same time, alcohol evaporating at the surface changes the density of the fluid. As it loses alcohol and becomes denser, it sinks at the edges of the cell. Below the surface, it will absorb more alcohol, become lighter, and eventually rise at the cell center, continuing the convective process. (Image credit: Beauty of Science, 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)

  • Self-Healing Bubbles

    Self-Healing Bubbles

    Soap films have the remarkable property of self-healing. A water drop, like the one shown above, can pass through a bubble (repeatedly!) without popping it. This happens thanks to surfactants and the Marangoni effect. Surfactants are molecules that lower the surface tension of a liquid and congregate along the outermost layer of a soap film. When water breaks through the soap film, its lack of surfactants causes a higher surface tension locally. This triggers the Marangoni effect, in which flow moves from areas of low surface tension toward ones of high surface tension. That carries surfactants to the region where the drop broke through and helps stabilize and heal the soap film. Incidentally, the same process lets you stick your finger into a bubble without popping it as long as your hand is wet! (Image credit: G. Mitchell and P. Taylor, source)

  • Creating Clouds

    Creating Clouds

    Despite their ubiquity and importance, we know surprisingly little about how clouds form. The broad strokes of the process are known, but the details remain somewhat fuzzy. One challenge is understanding how nucleation – the formation of droplets that become clouds or rain – works. A recent laboratory experiment in an analog cloud chamber suggests that falling rain drops may help spawn more rain drops.

    The experiment takes place in a chamber filled with sulfur hexafluoride and helium. The former acts like water in our atmosphere, appearing in both liquid and vapor forms, while the latter takes the place of dry components of our atmosphere, like nitrogen. The bottom of the chamber is heated, forming a liquid layer of sulfur hexafluoride, seen at the bottom of the animation above. The top of the chamber is cooled, encouraging sulfur hexafluoride vapor to condense and form droplets that fall like rain. A top view of the same apparatus during a different experiment is shown in this previous post.

    When droplets fall through the chamber, their wakes mix cold vapor from near the drop with warmer, ambient vapor. This changes the temperature and saturation conditions nearby and kicks off the formation of microdroplets. These are the cloud of tiny black dots seen above. Under the right conditions, these microdroplets grow swiftly as more vapor condenses onto them. In time, they grow heavy enough to fall as rain drops of their own. (Image credits: P. Prabhakaran et al.; via APS Physics; submitted by Kam-Yung Soh)

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    Flames in Freefall

    Gravity is such an omnipresent force in our lives that we frequently forget how strongly it affects our daily experiences and how differently nature behaves without it. A wonderful example of this is the simple flame of a candle. On Earth, a candle flame is tear-drop-shaped and elongated, burning hotter near the bottom and glowing yellow from soot at the top. But, as Dianna demonstrates with her free-fall experiment, this shape is due entirely to the effects of gravity. Buoyant forces make the hot air near the candle rise, pulling in cooler air and fresh oxygen at the base while stretching out the flame. In microgravity – or free-fall – flames are instead spherical, their shape driven by molecular and chemical diffusion. Check out the full video to see more effects of acceleration on flames. (Video credit: Physics Girl)

  • Island Wakes

    Island Wakes

    One of my favorite aspects of fluid dynamics is watching how patterns repeat at all kinds of scales. The cotton-candy-colored image above is a false-color satellite image of the island Tristan da Cunha (left), a volcanic island group in the South Atlantic. The prevailing winds, oriented roughly left to right in the image, flow over the rocky island and part in a series of swirls that alternate in their direction of rotation: clockwise for the upper set and counter-clockwise for the lower ones. This pattern is called a von Karman vortex street, named for an  aerodynamicist who studied the mechanism. Von Karman vortices are frequently observed in satellite images of remote islands, but they are also common behind spherical and cylindrical objects of all sizes. Sometimes they even show up in sci-fi! (Image credit: NASA Earth Observatory; submitted by Steve G.)

  • 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)

  • Jupiter’s Atmosphere

    Jupiter’s Atmosphere

    Jupiter’s atmosphere is fascinatingly complex and stunningly beautiful. This close-up from the Juno spacecraft shows a region called STB Spectre, located in Jupiter’s South Temperate Belt. The bluish area to the right is a long-lived storm that’s bordering on very different atmospheric conditions to the left. Shear from these storms moving past one another creates many of the curling waves we see in the image. These are examples of the Kelvin-Helmholtz instability, which generates ocean waves here on Earth, creates spectacular clouds in our atmosphere, and is even responsible for waves in galaxy clusters. Check out some of the other amazing images Juno has sent back of our solar system’s largest planet. (Image credit: NASA/JPL-Caltech/SwRI/MSSS/R. Tkachenko; via Gizmodo)

  • Galapagos Week: Sea Turtles

    Galapagos Week: Sea Turtles

    It’s easy to imagine sea turtles as slow and awkward given our familiarity with their terrestrial cousins, tortoises, but this could hardly be further from the truth. There are currently seven living species of sea turtles and all use a mode of locomotion known as aquatic flight. As the name suggests, swimming sea turtles share a lot in common with birds and other fliers. They generate most of their propulsion by flapping their forelimbs. Like birds, they change the angle of attack of their flippers over the course of both their upstroke and downstroke. 

    Of course, a cruising sea turtle is more interested in thrust than lift, but the efficiency of flapping is far higher than that of a rowing motion. That holds true across a range of speeds and is probably why marine turtles, known for their vast migrations, predominantly use flapping. It’s also remarkable how fast they can move when they want to. The animations above show two species of sea turtles cruising casually at a speed where a snorkeler in fins could follow along. But when the turtles wanted to, they could take off at a clip no human could hope to match! (Image credit: N. Sharp; research credit: J. Davenport et al., J. Walker and M. Westneat, H. Prange, E. Dougherty et al.)

    Today’s post wraps up Galapagos Week here at FYFD, but there’s plenty more Galapagos-relevant fluid dynamics to go around. Here are some previous, related posts: how frigatebirds cruise the seas without getting wet;  aerodynamics of flying fish; hydrodynamics of humpback whales; incredible bioluminescent plankton; and leaping mobula rays. 

  • Galapagos Week: Diving Birds

    Galapagos Week: Diving Birds

    One of my favorite things to do while we were sailing along the Galapagos was watching the blue-footed boobies hunt. Like the gannets shown above, boobies are plunge divers. They circle overhead until they spot their prey, then they fold their wings and dive headfirst into the water, impacting at speeds of more than 20 m/s (~45 mph). It’s absolutely incredible to watch. The physics involved are impressive, too, especially considering how badly a human would be injured diving at their speeds! 

    Fluid dynamically speaking, there are three important phases to the birds’ entry. The first is the impact phase, which lasts from initial contact until the bird’s head is underwater. In the second phase, an air cavity forms behind the head and around the neck as it enters the water. Finally, when the chest – the widest point of the bird – hits the water, the bird reaches the submerged phase. 

    Mechanically, the most interesting part is the air cavity phase. During this time, the bird’s head is slowing down due to high hydrodynamic drag from the water, but the rest of the bird is still moving fast. That means the bird’s slender neck experiences strong compressive forces, which would tend to make it buckle. Researchers at Virginia Tech examined this very problem and found that the birds’ sizing – its head shape, neck length, and so forth – combined with their typical diving speeds kept these birds well away from the conditions that would cause their necks to buckle. With the added stabilization from the birds’ neck muscles, they estimated that gannets and other plunge divers might be able to safely dive at speeds twice what would kill a human! Check out the BBC video below to see high-speed footage of gannets diving. (Image credits: G. Lecoeur; B. Chang et al.; research credits: B. Chang et al., pdf; video credit: BBC)

    Tomorrow will be the final day of Galapagos Week. Catch up on previous posts here.