FYFD

Year: 2019

  • “Transient 2”

    “Transient 2”

    Where cold and warm air meet, our atmosphere churns with energy. From the turbulence of supercell thunderclouds to the immense electrical discharge of lightning, there’s much that’s breathtaking about stormy skies. Photographer Dustin Farrell explores them, with a special emphasis on lightning, in his short film, “Transient 2″. 

    As seen in high-speed video, lightning strikes begin with tree-like leaders that split and spread, searching out the path of least resistance. Once that line from cloud to ground is discovered, electrons flow along a plasma channel that arcs from sky to earth. The estimated temperatures in the core of this plasma reach 50,000 Kelvin, far hotter than the Sun’s surface. It’s this heating that generates the blue-white glow of a lightning bolt. The heating also expands the air nearby explosively, producing the shock wave we hear as a crash of thunder. (Images and video credit: D. Farrell et al.; via Colossal)

  • Nighttime Streets

    Nighttime Streets

    Clouds spiral behind the islands of Tenerife and Gran Canaria in this nighttime satellite imagery. Although it’s not entirely unusual to see these von Karman vortex street clouds in the wakes of islands, this is the first time I’ve seen them at night. They form when winds off the ocean are forced up and around rocky islands. Like air moving past a cylinder, the flow forms a swirling vortex off one side of the island, which separates and moves downstream while another forms on the island’s opposite side. When the resulting flow mixes with a cloud layer, we can see the pattern from space. (Image credit: J. Stevens; via NASA Earth Observatory)

  • The Disappearing Cotton Candy

    The Disappearing Cotton Candy

    Moisture is cotton candy’s natural enemy. The spun sugar dissolves incredibly quickly under the influence of even a couple drops of water. Why that’s so is clearer when looking at a single fiber. Inside the droplet there’s a gradient in the sugar concentration. The more sugary water sinks, and the sugar fiber dissolves more quickly in the upper part of the droplet, where the less sugary water can more easily take up new sugar. 

    Once the fiber breaks, capillary forces draw the droplet upward, giving it a fresh section of fiber to dissolve. In a web of fibers, this process can pull droplets apart and together as they quickly eat through the spun sugar. (Image and video credit: S. Dorbolo et al.; submitted by Alexis D.)

  • Reader Question: Cross Sea

    Reader Question: Cross Sea

    Reader Matt G asks:

    [What’s] going on here?

    Why’s the pattern square? Just a special case of waves traveling in different directions, and this photo happened to catch some at right angles to one another?

    You’re not far off, Matt! This is an example of cross sea, where wave trains moving in different directions meet. Like most ocean waves, these waves originated from wind moving over the water. As the wind blows, it transfers energy to the water, disturbing what would otherwise be a smooth surface and setting up a series of waves. Oftentimes, these waves can outlast the wind that generates them and travel over long distances of open water as a swell.

    Cross seas occur when two of these wave systems collide at oblique angles. They’re most obvious in shallow waters like those seen here, where the depth makes their criss-cross pattern clearer. Another name for them is square waves, and although the pattern isn’t a perfect square, it’s usually fairly close. If the waves aren’t separated by a large angle, they’re more likely to merge than to create this sort of pattern.

    Neat as cross seas look, they’re quite dangerous, both to ships and swimmers. Ships are built to tackle waves head-on and don’t fare well when they’re forced to take waves from the side. For swimmers, the danger is a little different. Cross seas create intense vorticity under the surface and can generate stronger than usual riptides that sweep the unwary out to sea. (Image credit: M. Griffon)

  • Galileo’s Descent

    Galileo’s Descent

    In December 1995, the Galileo probe made its dramatic descent into Jupiter’s atmosphere at a velocity of more than 47 km/s. In 30 seconds, it decelerated from Mach 50 to Mach 1, undergoing incredible heating as it did so. Anytime an object moves through a fluid faster than the local speed of sound, it creates a leading shock wave that compresses the fluid, heats it, and redirects it around the object. The faster the speed, the hotter the fluid will be after passing through the shock wave. 

    Above about five times the speed of sound, the heating effect is so strong that it’s able to rip molecules apart, creating a chemically reactive mixture that will ablate away material from the object. For this reason, Galileo and other planetary entry vehicles carry heat shields made to sacrifice themselves while protecting the cargo and (in some cases) crew onboard. Data from Galileo showed that, although the heat shield survived the brunt of its descent, it experienced worse conditions than expected. Near the heat shield’s shoulder, almost all of its material ablated away. 

    Scientists continue to study Galileo’s descent even now, using it to test and inform their models of the flow and chemistry that occurs at these hypersonic speeds. The better we can understand and predict these flows, the better our designs will become. Mass that’s currently spent on overly-conservative heat shields can instead go toward additional instruments or supplies. (Image credit: Chop Shop Studio; research credit: L. Santos Fernandes et al.; via AIP)

  • Featured Video Play Icon

    Supercooling Thermodynamics

    In the latest Gastrofiscia episode, Tippe Top Physics takes on thermodynamics and the complicated truth behind certain phase changes. Although we’re accustomed to thinking of water freezing at 0 degrees Celsius and boiling at 100 degrees Celsius, reality is more complex, and temperature is only one of the factors that goes into a change of phase. Pressure and purity also play an important role. 

    This is why it’s possible, for instance, to supercool purified water to below 0 degrees Celsius without freezing it. Liquid water needs a nucleus to serve as a seed for its freezing. Without dust or other impurities, it takes a lot of energy for water to spontaneously generate its own nucleus. Check out the full video to see how and why that’s so. (Image and video credit: Tippe Top Physics)

  • Sliding Down a Pitcher Plant

    Sliding Down a Pitcher Plant

    Carnivorous pitcher plants supplement their nutrient-poor environments by capturing and consuming insects. The viscoelastic fluid inside them helps trap prey, but fluid dynamics plays a role elsewhere on the plant as well. The inner and outer surfaces of the pitcher are covered in macroscopic and microscopic grooves, seen above, oriented toward the interior of the plant. 

    Researchers found that these grooves trap droplets on the slippery plant through capillary action. Once adhered, the droplet cannot easily move across the grooves, but it can slip along them, carrying the droplet and any insect stuck to it, into the plant. By replicating pitcher-plant-inspired grooves on manmade surfaces, researchers found they were able to better control droplet motion on slippery, lubricant-infused surfaces than in previous work. (Image and research credit: F. Box et al.; via Royal Society; submitted by Kam-Yung Soh)

  • Bay of Fundy Tides

    Bay of Fundy Tides

    Canada’s Bay of Fundy has some of the wildest tidal flows in the world. Every six hours, the flow direction through the strait shifts and tidal currents rise to several meters per second. This creates distinct jets a couple kilometers long that pour from one side of the strait to the other. 

    What you see here is a numerical simulation of the flow using a technique called Large Eddy Simulation (or LES, for short). It’s one method used by fluid dynamicists to model turbulent flows without taking on the complexity of the full Navier-Stokes equations. At large lengthscales, like those of the jets and eddies we see above, LES uses the exact physics. But when it comes to the smaller scales – like the flow nearest the shores or the bottom of the strait – the simulation will approximate the physics in order to make calculations quicker and easier. Models like these make large-scale problems – including modeling our daily weather patterns – possible. (Image credit: A. Creech, source)

  • Reader Question: Exoplanetary Life

    Reader orbiculator asks:

    I’ve been having this thought regarding biological adaptations to viscous mediums. In a hypothetical exoplanet where the ocean is this thick, aqueous gel – could we assume that the native macroscopic species would have morphologies similar to Earth’s plankton despite their large sizes? That is, instead of being propelled by fins like our fish and whales, they’d go around using large ciliar or flagella?

    Propulsion-wise, that’s a reasonable theory. If the ambient environment were viscous enough that macroscopic creatures would still be limited to laminar flow, then, yes, you could expect them to use something like cilia or flagella to move. They’d be restricted by the same reversibility that microscopic species are here on Earth.

    But there are other factors that could come into play. Many microscopic species rely on diffusion for survival, whether that’s chemical diffusion across their exterior or diffusion within their body. As a species gets larger, the distance diffusion has to occur across grows, and diffusion becomes harder and harder to sustain. 

    So while hydrodynamic constraints might result in an exoplanet’s fauna having features similar to Earth’s microscopic life, it probably wouldn’t be as simple as merely enlarging the species we see here on Earth. Some of the key biophysics that goes on inside cellular life as we know it just doesn’t hold at larger scales.

  • Featured Video Play Icon

    A Broken Monitor’s Fingers

    In this short video, the artists of Chemical Bouillon explore a broken LCD monitor and its liquid crystals. By sandwiching the fluid between thin, transparent sheets, they create dendritic shapes as the liquid crystals and other fluids (air? ink?) push into one another. There’s a lot here that’s likely connected to the Saffman-Taylor instability, but without knowing more details on the ingredients and set-up, it’s hard to speculate beyond that. (Video and image credit: Chemical Bouillon)