Dust devils, like fire tornadoes and waterspouts, form from warm, rising air. As the sun heats the ground to temperatures hotter than the surrounding atmosphere, hot air will begin to rise. When it rises, that air leaves behind a region of lower pressure that draws in nearby air. Any vorticity in that air gets intensified as it gets pulled toward the low pressure area. It will start to spin faster, exactly like a spinning ice skater who pulls in his arms. The result is a spinning vortex of air driven by buoyant convection. On Earth, dust devils are typically no more than a few meters in size and can only pick up light objects like leaves or hay. On Mars, dust devils can be hundreds of meters tall, and, though they’re too weak to do much damage, they have helpfully cleaned off the solar panels of some of our rovers! (Image credit: T. Bargman, source; via Gizmodo)
Tag: buoyancy
Daily Fluids, Part 1
Just getting cleaned up and ready for the day involves a lot of fluid physics. Here are a few of the phenomena you may see daily without realizing:
Plateau-Rayleigh Instability
This behavior is responsible for the dripping of your faucet. More specifically, it’s the reason that a falling jet breaks up into droplets. It works on rain, too!Forced Convection
Everyone is familiar with a winter wind making them colder or hot air from a dryer getting the moisture off their hands. These are examples of forced convection – heat transfer by driving a fluid past a solid. Another common example? The fans in your computer!Liquid Atomization
This is the process of breaking a liquid into lots of tiny droplets. Aside from any aerosol can ever, this phenomenon is also key to your daily shower and internal combustion in your car.Archimedes Principle
This might be one of my favorite bits of the whole video because it hearkens back to some of my own earliest fluid dynamics exposure. Archimedes Principle says that buoyancy is equal to the weight of the fluid a body displaces. My mom (a science teacher) taught me about this one in the bathtub! It’s key to everything that ever floated, including us!Tune in all week for more examples of fluid dynamics in daily life. (Image credit: S. Reckinger et al., source)
Paper Marbling
Fluid dynamics and art have gone hand-in-hand for centuries. In this video, artist Garip Ay demonstrates one of the coolest fluids-based art techniques: paper marbling. In this technique, artists float ink or paints on a liquid surface, manipulate the colors as desired–in this case to recreate Van Gogh’s “Starry Night”–and then float a piece of paper atop the surface to transfer the image. Multiple cultures around the world developed marbling techniques, dating all the way back to the Middle Ages. Ay is an expert in ebru, a Turkish form of the art. For more of Ay’s art, check out his website and YouTube channel. (Video credit: G. Ay; via Gizmodo)
Mushrooms Make Their Own Breeze
Plants and other non-motile organisms have developed some clever methods to disperse their seeds and spores for reproduction. Some plants use vortex rings for dispersal; others make their seeds aerodynamic. Low ground-dwellers like mushrooms must contend with a lack of wind to lift their spores and carry them away. Instead, they use evaporative cooling to generate their own air currents.
Mushroom caps contain a lot of water and, as that water evaporates, it cools air near the mushroom, just as sweat evaporating off your skin cools you. That cooler, denser air tends to spread, carrying the spores outward. At the same time, the freshly evaporated water vapor is less dense than the surrounding air, so it rises. This combination of rising and spreading is capable of carrying spores tens of centimeters into the air, where the wind is stronger and able to carry spores further. (Image credit: New Atlantis, source; research credit: E. Dressaire et al.)
Icebergs and Caramel
What do icebergs and caramel have in common? Both have similar scalloped erosion patterns as they dissolve. When caramel dissolves in water, the denser caramel sinks in the buoyant water. An initially smooth surface will first form lines, then the flowing caramel and the uneven surface interact, forming chevrons, followed by larger scallops. A similar process happens with melting icebergs. The meltwater from an iceberg is less dense than the surrounding seawater, so it will rise as it melts. This causes variations in the salt concentration and temperature near the iceberg, which cause it to melt differently in different spots, ultimately leading to the same scallop shapes observed in the caramel. Check out the full-size PDF of the poster here. (Image credit: C. Cohen et al.)
Sandscapes
Many of us have played with sand art–the rotating frames filled with water, sand, and air. In this video, Shanks FX demonstrates some of the realistic and surrealistic landscapes you can create using this toy. It also makes for a neat fluid dynamics demonstration. The buoyancy of the trapped air bubbles lets the sand sift slowly down instead of falling immediately. And the sand descends in a variety of ways–sometimes laminar columns and other times wilder turbulent plumes. (Video credit and submission: Shanks FX/PBS Digital Studios)
Fire Tornadoes
Fire tornadoes, despite their name, are more closely related to dust devils or waterspouts than to true tornadoes. Though rarely documented, they are relatively common, especially in wildfires. The heat of the fire creates an updraft of warm, rising air that leaves behind a low-pressure region. Air from outside is drawn toward this low-pressure area, gets heated, and rises. As the outside air gets pulled in, any vorticity or rotation it had gets intensified via conservation of angular momentum–the same way a spinning ice skater speeds up when she pulls her arms in. The result is the tightly-spinning vortex at the heart of a fire tornado. (Video credit: C. Fleur; via NatGeo)
Convection Cells
This magnified photo shows Rayleigh-Benard convection cells in silicone oil. This buoyancy-driven convection occurs when a fluid is heated from below and cooled above. Inside the cells, fluid rises through the center and sinks along the edges; this motion is made apparent here thanks to aluminum flakes in the oil. The distinctive hexagonal shape of the cells is actually due to surface tension. Here, the upper surface of the fluid is left open to the air and this free surface boundary condition causes hexagonal shapes to form. If the fluid were instead covered by a solid surface, the convection cells that form would be shaped differently. (Image credit: M. Velarde et al.; via Van Dyke’s An Album of Fluid Motion)
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Convection from a Heat Source
Convection is a major driver in many flows in nature. In this film, the UCLA Spinlab demonstrates buoyant convection caused by a local heat source. They deposit dye on a submerged, continuously heated plate, then observe as the dye slowly rises with the heated (lower density) fluid. The surface forms a cap for the rising dye, which then spreads horizontally. Qualitatively similar flows can be seen in nature over volcanic eruptions or in thunderstorms when clouds reach the troposphere or a capping inversion. Be sure to check out the rest of the Spinlab’s videos. (Video credit: UCLA Spinlab; submitted by Jon B.)
