When a viscous drop falls into a pool of a less viscous liquid, the drop can deform into some beautiful and complex shapes. Typically, shear forces between the drop and its surroundings cause a vortex ring to roll up and advect downward, thereby stretching the remainder of the drop into thin sheets that can buckle and wrinkle. Here the drop is about 150 times more viscous than the pool and impacts at 1.45 m/s, making a rather energetic entry. The vortex ring (not visible) has stretched the drop’s remains downward while a buoyant bubble caught by the impact pulls some of the drop back toward the surface. As a result, the thin sheets of the drop’s fluid are buckling and folding back on themselves like an elaborate and delicate glass sculpture. This entire paper is full of gorgeous images and videos. Be sure to check them out! (Image and research credit: E. Q. Li et al.; see supplemental info zip for videos)
Tag: vortex rings
Vortex Reconnection
In slow motion, vortex rings can be truly stunning. This video shows two bubble rings underwater as they interact with one another. Upon approach, the two low-pressure vortex cores link up in what’s known as vortex reconnection. Note how the vortex rings split and reconnect in two places – not one. According to Helmholtz’s second theorem a vortex cannot end in a fluid–it must form a closed path (or end at a boundary); that’s why both sides come apart and together this way. After reconnection, waves ripple back and forth along the distorted vortex ring; these are known as Kelvin waves. Some of those perturbations bring two sides of the enlarged vortex ring too close to one another, causing a second vortex reconnection, which pinches off a smaller vortex ring. (Image source: A. Lawrence; submitted by Kam-Yung Soh)
Note: As with many viral images, locating a true source for this video is difficult. So far the closest to an original source I’ve found is the Instagram post linked above. If you know the original source, please let me know so that I can update the credit accordingly. Thanks!
How Smoke Rings Work
Vortices are a ubiquitous part of life, whether they’re draining down your bathtub or propelling underwater robots. In the latest video from the Lib Lab project, you can learn about how vortex rings form, what makes them last so long, and even make a vortex generator of your own. I can personally attest that vortex cannons are good for hours of entertainment, no matter your age. They’re even more fun with friends, as the Oregon State drumline demonstrates in the video. Want even more vortex fun? Check out leapfrogging vortices, vortex rings colliding head-on, and a giant 3 meter wide vortex cannon in action. (Video and image credit: Lib Lab)
Reconnecting
Vortices are a common feature of many flows. Here we see a helical vortex tube spinning in a swirling flow. The vortex itself is visible thanks to air trapped in its low-pressure core. As the vortex spins, two sections of it come together. This results in what’s known as vortex reconnection: the vortex lines break apart and rejoin in a new configuration – as a small independent vortex ring and a shorter section of helical vortex. Events like this are common but usually hard to observe directly. They’ve been previously visualized using vortex knots and have even been sighted in the quantum vortices of superfluid helium. (Image credit: S. Skripkin, source; research credit: S. Alekseenko et al., pdf)
A Drip’s Vortex
Drip food coloring into water and you can often see a torus-shaped vortex ring after the drop’s impact. That vortex rings form during droplet impact has been well known for over a century, but only recently have we begun to understand the process that leads to that vortex ring. Part of the challenge is that the vortex formation is very small and very fast, but recent work with x-ray imaging has allowed experimentalists to finally capture this event.
When a drop impacts a pool, surface tension draws some of the pool liquid up the sides of the drop. At the same time, the impact causes ripple-like capillary waves down the sides of the drop. This causes pool liquid to penetrate sharply into the drop, triggering the spirals that mark the forming vortex ring. When drops impact with even higher momentum, multiple vortex spirals can form, as seen on the lower right image. The authors observed as many as four rings during an impact. For more, check out the (open access) article. (Image and research credit: J. Lee et al., source)
When Vortices Collide
In a new ad campaign for paint manufacturer Sherwin-Williams, the production team at Psyop show off some awesome fluid dynamics by swirling and injecting paint underwater. You can see one sequence above, where red and blue paint vortex rings collide head-on before breaking down into a purple turbulent cloud. (What a great way to demonstrate the mixing power of turbulence, right?) Here’s the full 30-second ad clip. Impressively, everything in the video is a practical effect, even the segment that flies past multicolored turbulent plumes. You can see how they filmed everything in their behind-the-scenes featurette below. In the meantime, enjoy the mesmerizing beauty of real-world physics and check out FYFD’s “fluids as art” tag for more examples. (Image and video credit: Psyop for Sherwin-Williams; submitted by Alan B.)
Breaking Down Vortices
Vortex rings are ubiquitous in nature, showing up in droplet impacts, in propulsion, and even in volcanic eruptions. Understanding the interaction and breakdown of multiple vortices with one another is therefore key. The image above shows a circular disk that’s being oscillated up and down (in and out of the page). As the disk moves and changes direction, it generates vortices that interact with one another. Here some of those interactions are visualized with fluorescent dye. The overlapping vortices form complex and beautiful shapes on their way to breakdown. (Image credit: J. Deng et al., poster, paper)
Vortex Impact
When a vortex ring impacts a solid wall (or a mirrored vortex ring), it expands and quickly breaks up. The animations above show something a little different: what happens when a vortex ring hits a water-air interface. As seen in the side view (top image), the vortex starts to expand, but its shear at the interface generates a stream of smaller vortices that disrupt the larger vortex. (They even look like a little string of Kelvin-Helmholtz vortices!) When viewed from above (bottom image), the vortex ring impact and breakdown look even more complicated. Mushroom-like structures get spat out the sides as those secondary vortices form, and the entire structure quickly breaks up into utter turbulence. There’s some remarkable visual similarities between this situation and some we’ve seen before, like a sphere meeting a wall and drop hitting a pool. (Image credit: A. Benusiglio et al., source)
Leapfrogging Vortices
Two vortex rings travelling along the same line can repeatedly leapfrog one another. During my recent visit to the University of Chicago, PhD student Robert Morton of the Irvine Lab demonstrated this leapfrogging in the same apparatus they use to study knotted vortices. Leapfrogging works because of the mutual interaction of the flow fields of the two vortex rings. Their influence on one another causes the front vortex ring to slow down and widen while the trailing vortex narrows and speeds up. Once the vortices have switched places, the process repeats. In a real fluid, viscosity eventually breaks things down and causes the vortex rings to merge, but in simulation, inviscid vortex rings can leapfrog indefinitely. Our friend Physics Girl even showed that half-vortex-rings can leapfrog. (Image credit: N. Sharp; thanks to R. Morton for the demo)
Crow Instability
Watching airplane contrails overhead, you may have noticed them transform into a daisy chain of distorted rings. This is an effect known as the Crow instability. The contrails themselves are the airplane’s wingtip vortices, made visible by water vapor condensed out of the engine exhaust. These two initially parallel vortex lines spin in opposite directions. A slight crosswind can disturb the initially straight lines, causing them to become wavy. This waviness increases over time until the vortex lines almost touch. Then the vortices pinch off and reconnect into a line of vortex rings that slowly dissipate. Be sure to check out the full-resolution version of this animation for maximum effect. (Image credit: J. Hertzberg, source)
