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)
Tag: buoyancy
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.)
“Jack and the Giant”
This fantastic music video by Kim Pimmel is a beautiful merger of art and fluid dynamics. Using household goods (and some slightly more exotic ferrofluid), the video shows how mesmerizing diffusion, buoyancy, Marangoni flow, and other fluid effects can be up close. It may also be the first time I’ve ever seen fluid dynamics–specifically bubbles–used as characters! Also be sure to check out some of his previous videos, many of which also feature cool fluid dynamics. (Video credit and submission: K. Pimmel)
How Rain Gets Its Smell
Light rain after a dry spell often produces a distinctive earthy scent called petrichor that is associated with plant oils and bacteria products. How these chemicals get into the air has been unclear, but new research suggests that the mechanism may come from the rain itself. When water falls on a porous surface like soil, tiny air bubbles get trapped beneath the drop. These bubbles rise rapidly due to buoyancy and, upon reaching the surface, burst and release tiny droplets known as aerosols. Depending on the surface properties and the drop’s impact speed, a single drop can produce a cloud of aerosol droplets. The research team is now investigating how readily bacteria or pathogens in the soil can spread through this mechanism. Other human-focused research has already shown that these tiny aerosol droplets can persist in the air for remarkably long periods and may help spread diseases. (Video credit: Massachusetts Institute of Technology; research credit: Y. Joung and C. Buie; submitted by Daniel B and entropy-perturbation)
Lava-Driven Waterspouts
Seven waterspouts align as lava from the Hawaiian volcano Kilauea pours into the ocean in this striking photo from photographer Bruce Omori. Like many waterspouts–and their landbound cousins dust devils–these vortices are driven by variations in temperature and moisture content. Near the ocean surface, air and water vapor heated by the lava create a warm, moist layer beneath cooler, dry air. As the warm air rises, other air is drawn in by the low pressure left behind. Any residual vorticity in the incoming air gets magnified by conservation of angular momentum, like a spinning ice skater pulling her arms in. This creates the vortices, which are made visible by entrained steam and/or moisture condensing from the rising air. (Photo credit: B. Omori, via HPOTD; submitted by jshoer)
A Toast!
When you lift a glass of champagne or sparkling wine at midnight tonight, your nose and mouth will be greeted by a plethora of aromas, flavors, and sensations propagated by the tiny bubbles in the drink. Carbon dioxide dissolved in the wine gathers in a stream of tiny bubbles that rise at the center of the glass. (The bubbles form at the center because champagne glasses are often etched in a ring there to provide nucleation points where the bubbles can grow.) This stream of rising bubbles generates vortical motion in the glass that helps carry the carbon dioxide to the surface, where it is released when the bubbles burst. In the tall, thin champagne flute these vortices mix the entire contents of the glass, but, in a wider coupe, the vortices are confined to the center, leaving a stiller region along the glass’s edges. For those who find that a freshly poured flute of champagne stings their noses–a side effect of the high gaseous carbon dioxide concentration just after decanting–the wider coupe lowers the concentration at the glass’s lip and may provide a more pleasant experience for toasting the new year. (Image credit: F. Beaumont et al.)
Plume Stratification
Clean-up of accidents like the 2010 Deepwater Horizon oil spill can be complicated by what goes on beneath the ocean surface. Variations in temperature and salinity in seawater create stratification, stacked layers of water with differing densities. When less dense layers are on top, the fluid is said to be stably stratified. Since oil is less dense than water, one might assume that buoyancy should make an oil plume should rise straight to the ocean surface. But the presence of additives or surfactants in the oil mixture plume can prevent that. With surfactants present, an oil mixture tends to emulsify, breaking into tiny droplets like a well-mixed salad dressing. Even if the density of the emulsion is smaller than the surrounding fluids, such a plume can get trapped at a density boundary, as seen in the photo above. Researchers report a critical escape height, which depending on the plume’s characteristics and stratification boundary, determines whether a plume escapes or becomes trapped. (Image credit: R. Camassa et al.)
Freediving
The freediving del Rosario brothers have created a real treat with this underwater film. There are no computer-generated special effects, just some clever tricks with camera angles, perspective, and buoyancy. The end result is slightly surrealistic and captures some of the fluid beauty of the ocean. And don’t miss the excellent bubble ring vortices. (Video credit: The Ocean Brothers; via Gizmodo; submitted by jshoer)
Volcanic Vortex
This infrared image shows a kilometer-high volcanic vortex swirling over the Bardarbunga eruption. The bright red at the bottom is lava escaping the fissure, whereas the yellow and white regions show rising hot gases. Although the vortex looks similar to a tornado, it is actually more like a dust devil or a so-called fire tornado. All three of these vortices are driven by a heat source near the ground that generates buoyant updrafts of air. As the hot gases rise, cooler air flows in to replace them. Any small vorticity in that ambient air gets amplified as it’s drawn to the center, the same way an ice skater spins faster when she pulls her arms in. With the right conditions, a vortex can form. Unlike a harmless dust devil, though, this vortex is likely filled with sulphur dioxide and volcanic ash and would pose a serious hazard to aviation. (Image credit: Nicarnica Aviation; source video; via io9)
