This photo from the Mars Reconnaissance Orbiter stares almost straight down a dust devil on Mars. Like their earthbound brethren, Martian dust devils form when the surface is heated by the sun, causing warm air to rise. The rising air causes a low pressure area that the surrounding air flows into. Any rotational motion of the air intensifies as it is entrained. This is a consequence of conservation of angular momentum. Just as a spinning ice skater spins faster when he pulls his arms in, the vorticity of the inward-flowing air increases, forming a vortex. In addition to dust devils, this same physical mechanism applies to waterspouts and fire tornadoes, although the heating source for those is different. (Photo credit: NASA; via APOD)
Search results for: “vorticity”
Below a Surfer’s Wave
From below a plunging breaking wave–the classic surfer’s wave–looks like a giant vortex tube. Smaller rib vortices, the rings around the main vortex in the photo above, can form where there are variations along the breaking wave. As the wave rolls on, it stretches the vorticity variations along the wave’s span. When stretched, vortices spin up and intensify; this is a result of conservation of angular momentum. Check out more amazing photos of waves in Ray Collins’ portfolio. (Photo credit: R. Collins; via The Inertia)
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)
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)
4th Birthday: Wingtip Vortices
Wingtip vortices are a result of the finite length of a wing. Airplanes generate lift by having low-pressure air travelling over the top of the wing and higher pressure air along the bottom. If the wing were infinite, the two flows would remain separate. Instead, the high-pressure air from under the wing sneaks around the wingtip to reach the lower pressure region. This creates the vorticity that trails behind the aircraft. I was first introduced to the concept of wingtip vortices in my junior year during introductory fluid dynamics. As I recall, the concept was utterly bizarre and so difficult to wrap our heads around that everyone, including the TA, had trouble figuring out which way the vortices were supposed to spin. A few good photos and videos would have helped, I’m sure. (Photo credits: U.S. Coast Guard, S. Morris, Nat. Geo/BBC2)
Vortex Ring Tricks
Vortex rings are wonderful at maintaining coherent vorticity while moving over significant distances. If you stand several meters from a foam cup and try blowing to knock it over, it’s not likely to budge. But move the air impulsively with a vortex cannon, and you can knock it over from the opposite side of the room. The same principle works underwater with added visual effect. Here an impulsive burst of air exhaled by the diver forms a bubble ring with vorticity strong enough to knock over a stack of rocks. It may look like a superpower, but this is science! Dolphins and whales are also known to play with this trick. For the non-scuba-divers among you, it’s also possible to learn to do it in a swimming pool. (Video credit: DjDeutchTv; h/t to coolsciencegifs)
The Bathtub Vortex
If you’ve ever watched a swirling vortex disappear down the drain of your bathtub and wondered what was happening, you’ll appreciate these images. This dye visualization shows a one-celled bathtub vortex, created by rotating a cylindrical tank of water until all points have equal vorticity before opening a drain in the bottom of the tank. A recirculating pump feeds water back in to keep the total fluid mass constant. Once a steady vortex is established, green dye is released from the top plate of the tank and yellow dye from the bottom. The green dye quickly marks the core of the vortex. Ekman layers–similar to the boundary layers of non-rotating flows–form along the top and bottom surfaces, and the yellow dye is drawn upward in a region of upwelling driven by Ekman pumping. (Photo credit: Y. Chen et al.)
Just a reminder for those at Texas A&M University: I will be giving a talk today Wednesday, October 2nd entitled “The Beauty of the Flow” as part of the Applied Mathematics Undergraduate Seminar series at 17:45 in BLOC 164.
Stingray Wakes
This numerical simulation shows a swimming stingray and the vorticity generated by its motion. Stingrays are undulatory swimmers, meaning that the wavelength of their motion is much shorter than their body length. Manta rays, in contrast, move their fins through a wavelength longer than their body length, making them oscillatory swimmers. Observe the difference in this video. To swim faster, stingrays increase the frequency of their undulation, not the amplitude. This is quite common among swimmers because increasing the amplitude also increases projected frontal area, which causes additional drag. Increasing the frequency of motion does not affect the projected area, making it the more efficient locomotive choice. (Video credit: G. Weymouth; additional research credit: E. Blevins; submitted by L. Buss)
Also, FYFD now has a Google+ page for those who prefer to follow along and share that way. – Nicole
Flow Over a Delta Wing
Fluorescent dye illuminated by laser light shows the formation and structure of vortices on a delta wing. A vortex rolls up along each leading edge, helping to generate lift on the triangular wing. As the vortices leave the wing, their structure becomes even more complicated, full of lacy wisps of vorticity that interact. Note how, by the right side of the photo, the vortices are beginning to draw closer together. This is an early part of the large-wavelength Crow instability. Much further downstream, the two vortices will reconnect and break down into a series of large rings. (Photo credit: G. Miller and C. Williamson)
H Booms
Holidays involving fireworks deserve high-speed videos of hydrogen explosions. Although Periodic Table of Videos focuses on the chemistry involved in setting hydrogen on fire, there are some lovely fluid dynamics on display, too. There’s turbulence, combustion (obviously), and, if you watch closely, you can even see the initial vorticity caused by the rubber’s burst twisting the growing flames. (Video credit: Periodic Table of Videos)