This schlieren image shows a sphere traveling at Mach 3 over a perforated plate. The bow shock in front of the sphere is clearly visible, as is its reflection off the plate. The pressure caused by the bow shock produces a series of spherical acoustic waves below the plate. A tiny vortex ring moves downward from each hole, followed at the right by a secondary ring moving upward from the holes in the plate. (Photo credit: U.S. Army Ballistic Research Laboratory; reprinted in Van Dyke’s An Album of Fluid Motion)
Tag: vortex rings
Shark Wakes
Volumetric imaging of swimming spiny dogfish, a type of shark, shows that their distinctively asymmetric tails produce a set of dual-linked vortex rings with every half beat of their tail. The figure above shows data from the actual shark on the right (b,d,f) and a similarly shaped robotic tail on the left (a,c,e). The second row contains lateral views (c,d) and the bottom row contains dorsal views (e,f) of the vorticity isosurfaces measured. The robotic tail does not demonstrate the same double vortex structure, leading scientists to suspect that the shark may be actively stiffening its tail mid-stroke to control its wake. The finding could help engineers design aquatic robots whose morphing fins help it swim more efficiently. For more, see Wired.
The Spinning Underwater Vortex
Vortex rings are a topic we’ve covered before with dolphins, whales, humans, volcanoes and even moss, but this video is particularly fun thanks to the addition of a bottle cap. By sticking the bottle cap next to the ring, these swimmers are able to demonstrate the forceful spinning of the fluid near the vortex. This spinning is what helps the vortex hold its shape over distances much larger than its diameter. As you can also see, though, sticking a bottle cap in the ring causes it to break up faster than it would otherwise! (submitted by Kris S)
Beluga Whale Bubble Rings
Beluga whales and dolphins in captivity have taken to blowing bubble rings to entertain themselves. You can learn how to do the same in the pool. #
Computational Vortex Rings
Computational fluid dynamics (CFD) sometimes gets a bad rep as “colorful fluid dynamics”, but as computers get faster and faster, more complicated and physically accurate simulations are possible. Shown here are simulations of vortex rings and wingtip vortices in stunningly gorgeous detail. Understanding the evolution of these vortices from a fundamental level helps fluid mechanicians design better methods of controlling them. As mentioned in the video, wingtip vortices are a particularly hazardous everyday example; the time it takes for one plane’s wingtip vortices to disperse determines how quickly the next airplane can take-off or land on that same runway. Being able to break down these vortices faster would allow more frequent use of existing facilities.
Oil Chandeliers
What you see above is a composite of images of an oil droplet falling into alcohol from two different heights. The top row of images is from a height of 25 mm and the bottom from a height of 50 mm. The first droplet forms an expanding vortex ring which breaks down via the Rayleigh-Taylor instability due to its greater density than the surrounding alcohol. The second droplet impacts the alcohol with greater momentum and is initially deformed by viscous shear forces. Eventually it, too, breaks down by the Rayleigh-Taylor mechanism. This image is part of the 2010 Gallery of Fluid Motion. # (PDF)
How to Blow Bubble Rings
Next time you’re at the pool, join the dolphins, moss, and volcanoes in blowing vortex rings. Here’s how: first, squeeze your lips like you’re going to give someone a kiss. Second, increase the air pressure in your mouth. Then quickly open and close your lips so a small amount of air pops out. It can require some practice, not even dolphins learn the trick right away. #
Volcanic Vortex Rings
Plants and dolphins are not the only ones in nature creating vortex rings. Volcanoes are known to produce them as well. The vortex ring forms when gas is rapidly expelled from the volcano (much the same way as with a vortex cannon); the rings are visible in the video above because smoke has been entrained into the vortex.
Spore-Spreading via Vortex
As it turns out, animals aren’t the only ones to have figured out the usefulness of vortex rings. A team of physicists and biologists have captured peat moss using vortex rings to project their spores to a height where the wind will catch and carry them further afield. #
Dolphins Making Bubble Rings
Marine animals know more about fluid dynamics than we might think. Dolphins make games out of blowing vortex rings (a.k.a. bubble rings) and tossing them around. You can try, too, next time you’re at the pool!
