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)
Tag: vortices
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.)
Half Vortex Rings
Vortices are one of the most common structures in fluid dynamics. In this video, Dianna from Physics Girl explores an unusual variety of vortex you can create in a pool. Dragging a plate through the water at the surface creates a half vortex ring, which can be tracked either by the surface depressions created or by using food dye for visualization. Vortex rings are quite common, but a half vortex ring is not. The reason is that, ignoring viscous effects, a vortex filament cannot end in a fluid. The vortex must close back on itself in a loop, or, like the half vortex ring, the ends of the vortex must lie on the fluid boundary. It is possible to break vortex lines like those in smoke rings, but the lines will reattach, creating new vortex rings–just as they do in these vortex knots. (Video credit: Physics Girl; submitted by Tom)
Von Karman Vortex Streets
The wake of a cylinder is a series of alternating vortices shed as the flow moves past. This distinctive pattern is known as a von Karman vortex street. The speed of the flow and the size of the cylinder determine how often vortices are shed. Incredibly, this pattern appears at scales ranging from the laboratory demo all the way to the wakes of islands. Von Karman vortex streets can even be seen from space. (Image credit: R. Gontijo and W. Cerqueira, source video)
“Smoke”
Ethereal forms shift and swirl in photographer Thomas Herbich’s series “Smoke”. The cigarette smoke in the images is a buoyant plume. As it rises, the smoke is sheared and shaped by its passage through the ambient air. What begins as a laminar plume is quickly disturbed, rolling up into vortices shaped like the scroll on the end of a violin. The vortices are a precursor to the turbulence that follows, mixing the smoke and ambient air so effectively that the smoke diffuses into invisibility. To see the full series, see Herbich’s website. (Image credits: T. Herbich; via Colossal; submitted by @jchawner, @__pj, and Larry B)
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The Churning of Corals
Corals may appear static, but near the surface the tiny hair-like cilia of these polyps are churning the water. Although it has been known for some time that corals have cilia, scientists had previously assumed they only moved water parallel to the coral’s surface. Instead recent flow visualizations show that the cilia’s movements generate larger-scale vortical flows near the coral that can help draw fresh nutrients in as well as flush waste away. This means that, instead of being reliant on currents and tides, corals can exert some control on their environment in order to get what they need. This insight into coral cilia may shed some light on the micro- and macroscopic flows generated by other cilia, like those in our lungs. For a similar example of seemingly-passive organisms generating their own flows, check out how mushrooms create air currents to spread their spores. (Image credits: O. Shapiro et al. and MIT News; source video; h/t to Katie B)
Soap Film Grass
In the summer months, a breeze can set long grasses waving in an impressive display. Similar behaviors are seen in aquatic plants during tides. Researchers simulate the behavior in two-dimensions using a flowing soap film and nylon filaments. Flow visualization reveals the strong differences between flow above and between the grass. Vortices recirculate between the filaments at speeds much slower than the flow overhead. The instantaneous interaction of the high-speed freestream, the unsteady vortices, and the resistance of the grass results in familiar synchronous waves of grain. (Video credit: R. Singh et al.)
Island Vortex Street
Von Karman vortex streets are a pattern of alternating vortices shed in the wake of a bluff body. They’re commonly associated with cylinders and can be demonstrated in simulation and in the lab. (They even show up in supersonic flows.) But they also show up in nature quite frequently, like in this cloud pattern off Central America. Such wakes often occur downstream of rocky, volcanic islands that rise above the smooth ocean surface and disrupt the atmosphere’s boundary layer. The same phenomenon is responsible for the “singing” of electrical lines on a windy day, and I’ve even heard it make the spokes on my bicycle wheel sing in a crosswind. (Photo credit: R. Mastracchio; via @BadAstronomer; submitted by jshoer)
Growing Turbulence
Flow patterns can change dramatically as fluid speed and Reynolds number increase. These visualizations show flow moving from left to right around a circular plunger. The lower Reynolds number flow is on the left, with a large, well-formed, singular vortex spinning off the plunger’s shoulder. The image on the right is from a higher Reynolds number and higher freestream speed. Now the instantaneous flow field is more complicated, with a string of small vortices extending from the plunger and a larger and messier area of recirculation behind the plunger. In general, increasing the Reynolds number of a flow makes it more turbulent, generating a larger range of length scales in the flow and increasing its complexity. (Image credit: S. O’Halloran)
Cylinder Wakes
A simple cylinder in a steady flow creates a beautiful wake pattern known as a von Karman vortex street. The image above shows several examples of this pattern. Flow is from bottom to top, and the Reynolds number is increasing from left to right. In the experiment, this increasing Reynolds number corresponds to increasing the flow velocity because the cylinder size, fluid, and temperature were all fixed. As the Reynolds number first increases, the cylinder begins to shed vortices. The vortices alternate the side of the cylinder from which they are shed as well as alternating in their sense of rotation (clockwise or counterclockwise). Further increasing the Reynolds number increases the complexity of the wake, with more and more vortices being shed. The vortex street is a beautiful example of how fluid behavior is similar across a range of scales from the laboratory to our planet’s atmosphere. (Image credit: Z. Trávníček et. al)
