One of my favorite aspects of fluid dynamics is watching how patterns repeat at all kinds of scales. The cotton-candy-colored image above is a false-color satellite image of the island Tristan da Cunha (left), a volcanic island group in the South Atlantic. The prevailing winds, oriented roughly left to right in the image, flow over the rocky island and part in a series of swirls that alternate in their direction of rotation: clockwise for the upper set and counter-clockwise for the lower ones. This pattern is called a von Karman vortex street, named for an aerodynamicist who studied the mechanism. Von Karman vortices are frequently observed in satellite images of remote islands, but they are also common behind spherical and cylindrical objects of all sizes. Sometimes they even show up in sci-fi! (Image credit: NASA Earth Observatory; submitted by Steve G.)
Search results for: “vortex”
Farewell, Cassini!
Tomorrow one of the most prolific and beloved spacecraft missions will come to an end when the Cassini spacecraft makes its final plunge into Saturn. After nearly 20 years in space and 13 years orbiting Saturn, the Cassini mission is close to running out of fuel. To prevent the craft from contaminating one of Saturn’s moons – which its mission revealed may harbor the ingredients for life – mission operators are instead sending it on a fatal dive into the gas giant.
Cassini has and will continue to provide a trove of scientific insights about Saturn and its environs. It has given us front-row seats to a storm that wrapped around the entire planet. It shed new light on Saturn’s spectacular hexagonal polar vortex and showed us the beauty of auroras on other planets. Cassini also showed us that Saturn’s moon Titan has stable hydrocarbon lakes at its surface, fed by methane rains and driven by processes unfamiliar to terrestrial ones. It also gave us paths for future exploration by documenting plumes of water ejected from Enceladus’ icebound oceans.
Cassini also holds a special place in my heart. It launched while I was in middle school, reached Jupiter while I was in college, and collected data throughout my postgraduate research career. It was an inspiration for my undergraduate spacecraft mission design projects, and it provided fun and exciting fluid dynamical discoveries throughout my time writing FYFD. It’s my favorite mission (sorry, Mars rovers, New Horizons, Dawn, and Juno!) and likely to remain so for years to come.
So thank you, Cassini, and many thanks to all the scientists, engineers, and operators who’ve worked on the mission during the decades from its conception to completion. You did a hell of a job. Godspeed, Cassini! (Photo credits: NASA/JPL)
P.S. – Tonight I’ll be helping kick off the Ig Nobel Prize ceremony. You can tune into the live webcast here. The ceremony officially starts at 6 PM Eastern time, but I recommend tuning in early, especially if you want to catch my full spiel. – Nicole
Fluid Black Holes
Fluid systems can sometimes serve as analogs for other physical phenomena. For example, bouncing droplets can recreate quantum effects and a hydraulic jump can act like a white hole. In this work, a bathtub vortex serves as an analog for a rotating black hole, a system that’s extremely difficult to study under normal circumstances. In theory, the property of superradiance makes it possible for gravitational waves to extract energy from a rotating black hole, but this has not yet been observed. A recent study has, however, observed superradiance for the first time in this fluid analog.
To do this, the researchers set up a vortex draining in the center of a tank. (Water was added back at the edges to keep the depth constant.) This served as their rotating black hole. Then they generated waves from one side of the tank and observed how those waves scattered off the vortex. The pattern you see on the water surface in the top image is part of a technique used to measure the 3D surface of the water in detail, which allowed the researchers to measure incoming and scattered waves around the vortex. For superradiance to occur, scattered waves had to be more energetic after interacting with the vortex than they were before, which is exactly what the researchers found. Now that they’ve observed superradiance in the laboratory, scientists hope to probe the process in greater detail, which will hopefully help them observe it in nature as well. For more on the experimental set-up, see Sixty Symbols, Tech Insider UK, and the original paper. (Image credit: Sixty Symbols, source; research credit: T. Torres et al., pdf; via Tech Insider UK)
Chains of Salps
Salps are small, jellyfish-like marine invertebrates that swim by ejecting a pulsatile jet. They are unusual creatures whose lives have two major stages: one in which salps swim individually and one in which they link together and swim in large chains. In the chain, salps don’t synchronize their jetting; each salp jets with its own phase and frequency. A new study suggests that, in spite of this lack of synchronicity, the salp chain’s swimming reduces the animals’ drag. There are several factors that contribute to this result. One is that drag is generally lower on a body moving at constant speed compared to one moving in bursts. When linked together and firing randomly, all the individual jets tend to average out into one continuous swimming speed. There’s even a benefit to being out of sync: previous work showed that synchronized jets lose some of their thrust when they are too close together. Salps avoid that loss by keeping to their own beat. (Image and research credit: K. Sutherland and D. Weihs, source; via Gizmodo)
Wrinkling Drops
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)
Songs in Soap
There are many beautiful ways to visualize sound and music – Chris Stanford’s fantastic “Cymatics” music video comes to mind – but this is one I haven’t seen. This visualization uses a soap film on the end of an open tube with music playing from the other end. You can see the set-up here. The result is a fascinating interplay of acoustics, fluid dynamics, and optics. As sound travels through the tube, certain frequencies resonant, vibrating the soap film with a standing wave pattern (3:20). At the same time, interference between light waves reflecting off the front and back of the soap film create vibrant colors that show the film’s thickness and flow.
When the frequency and amplitude are just right, the sound excites counter-rotating vortex pairs in the film (0:05), mixing areas of different thicknesses. With just a single note, the vortex pairs appear and disappear, but with the music, their disappearance comes from the changing tones. Watching the patterns shift as the film drains and the black areas grow is pretty fascinating, but one of the coolest behaviors is how the acoustic interactions are actually able to replenish the draining film (2:15). Because the tube was dipped in soap solution, some fluid is still inside the tube, lining the walls. With the right acoustic forcing, that fresh fluid actually gets driven into the soap film, thickening it.
There are several more videos with different songs here – “Carmen Bizet” is particularly neat – as well as a short article summarizing the relevant physics for those who are interested. (Video and research credit: C. Gaulon et al.; more videos here)
Flow Around a Cylinder
A cylinder standing upright in a flow creates a complicated system of vortices and recirculation. In the photo above, the flow is left to right. The cylinder itself is somewhat hard to see but is located in the center of the image; we see it from above. The colored streaks of dye show the flow path around the cylinder. In yellow, we see a spiraling vortex that forms just ahead of the cylinder and stretches downstream on either side. Because of its shape, this is called a horseshoe vortex. Its sense of rotation is such that it tends to pick up loose material in front of the cylinder; in other words, it can erode that area. This is often seen around the pilings of bridge supports and must be accounted for in designs. You also see the effects of this horseshoe vortex digging out material at the base of trees after snowfalls in areas with a dominant wind direction, and here’s an example with a snow roller. (Image credit: H. Werlé; via eFluids)
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)
The Japanese Pufferfish
[original media no longer available]
If you’ve ever dived or snorkeled over a sandy lake or ocean bottom, you’ve probably seen some neat patterns there. But it’s hard to compete with the Japanese pufferfish for pure artistry. This small fish creates enormous and elaborate designs in the sand in order to attract a mate. The male fish moves the sand into place by flapping his fins very close to the surface. Above a critical flapping velocity, his fins generate vortices capable of picking up sand, as seen below. With repeated passes, the fish is able to excavate the trough that is key to his creation. It’s a constant fight against the current, though.
Puffers aren’t the only ones who flap their fins to move the sands. Rays and flounders use this technique to bury themselves and hide (Video credit: BBC Earth; image credit: A. Sauret, source; research credit: A. Sauret et al.)