Tag: rotating flow

Spinning Water
If you spin a tank of water at a constant speed, it takes on a curved, parabolic shape–a demonstration often called Newton’s bucket. Here, a team from UCLA shows how it’s done, both in terms of the equipment needed and a concise explanation of the physics. In the rotating experiment, water is subjected to both gravity (which acts in a constant magnitude across the tank) and centrifugal force (which is stronger further from the axis of rotation). The shape that balances these forces is a paraboloid, which is why the water takes on that shape. (Video and image credit: UCLA SpinLab)
Fediverse Reactions
September 29, 2025
Bubbly Tornadoes Aspin
Rotating flows are full of delightful surprises. Here, the folks at the UCLA SpinLab demonstrate the power a little buoyancy has to liven up a flow. Their backdrop is a spinning tank of water; it’s been spinning long enough that it’s in what’s known as solid body rotation, meaning that the water in the tank moves as if it’s one big spinning object. To demonstrate this, they drop some plastic tracers into the water. These just drop to the floor of the tank without fluttering, showing that there’s no swirling going on in the tank. Then they add Alka-Seltzer tablets.
As the tablets dissolve, they release a stream of bubbles, which, thank to buoyancy, rise. As the bubbles rise, they drag the surrounding water with them. That motion, in turn, pulls water in from the surroundings to replace what’s moving upward. That incoming water has trace amounts of vorticity (largely due to the influence of friction near the tank’s bottom). As that vorticity moves inward, it speeds up to conserve angular momentum. This is, as the video notes, the same as a figure skater’s spin speeding up when she pulls in her arms. The result: a beautiful, spiraling bubble-filled vortex. (Video and image credit: UCLA SpinLab)
Fediverse Reactions
February 24, 2025
The Best of FYFD 2024
Welcome to another year and another look back at FYFD’s most popular posts. (You can find previous editions, too, for 2023, 2022, 2021, 2020, 2019, 2018, 2017, 2016, 2015, and 2014. Whew, that’s a lot!) Here are some of 2024’s most popular topics:
This year’s topics are a good mix: fundamental research, civil engineering applications, geophysics, astrophysics, art, and one good old-fashioned brain teaser. Interested in what 2025 will hold? There are lots of ways to follow along so that you don’t miss a post.
And if you enjoy FYFD, please remember that it’s a reader-supported website. I don’t run ads, and it’s been years since my last sponsored post. You can help support the site by becoming a patron, buying some merch, or simply by sharing on social media. And if you find yourself struggling to remember to check the website, remember you can get FYFD in your inbox every two weeks with our newsletter. Happy New Year!
(Image credits: dam – Practical Engineering, ants – C. Chen et al., supernova – NOIRLab, sprinkler – K. Wang et al., wave tank – L-P. Euvé et al., “Dew Point” – L. Clark, paint – M. Huisman et al., iceberg – D. Fox, flame trough – S. Mould, sign – B. Willen, comet – S. Li, light pillars – N. Liao, chair – MIT News, Faraday instability – G. Louis et al., prominence – A. Vanoni)
Fediverse Reactions
January 1, 2025
How Magnetic Fields Shape Core Flows
The Earth’s inner core is a hot, solid iron-rich alloy surrounded by a cooler, liquid outer core. The convection and rotation in this outer core creates our magnetic fields, but those magnetic fields can, in turn, affect the liquid metal flowing inside the Earth. Most of our models for these planetary flows are simplified — dropping this feedback where the flow-induced magnetic field affects the flow.
The simplification used, the Taylor-Proudman theorem, assumes that in a rotating flow, the flow won’t cross certain boundaries. (To see this in action, check out this Taylor column video.) The trouble is, our measurements of the Earth’s actual interior flows don’t obey the theorem. Instead, they show flows crossing that imaginary boundary.
To explore this problem, researchers built a “Little Earth Experiment” that placed a rotating tank (representing the Earth’s inner and outer core) filled with a transparent, magnetically-active fluid inside a giant magnetic. This setup allowed researchers to demonstrate that, in planetary-like flows, the magnetic field can create flow across the Taylor-Proudman boundary. (Image credit: C. Finley et al.; research credit: A. Pothérat et al.; via APS Physics)
November 21, 2024
Trapped in a Taylor Column
The world’s largest iceberg, A23a, is stuck. It’s not beached; there are a thousand meters or more of water beneath it. But thanks to a quirk of the Earth’s rotation, combined with underwater topology, A23a is stuck in place, spinning slowly for the foreseeable future. A23a is trapped in what’s known as a Taylor column, a rotating column of fluid that forms above submerged objects in a rotating flow. You can see the same dynamics in a simple tabletop tank.
Pirie Bank sticks up from the seafloor, which sets up a stationary column of rotating water that iceberg A23a is now stuck in.
When a tank (or planet) is rotating steadily, there’s little variation in flow with depth. With an obstacle at the deepest layer — in this case, an underwater rise known as the Pirie Bank — water cannot pass through that lowest layer. And that deflection extends to all the layers above. The water above Pirie Bank just stays there, as if the entire column is an independent object. Caught inside this region, A23a will remain imprisoned there. How long will that last? There’s no way to know for sure, but a scientific buoy in another nearby Taylor column has been hanging out there for 4 years and counting. (Image credit: A23a – D. Fox/BAS, diagram – IBSCO/NASA; via BBC News; submitted by Anne R.)
August 21, 2024
Stopping a Bottle’s Bounce
A few years ago, the Internet was abuzz with water bottle flips. Experimentalists are still looking at how they can arrest a partially fluid-filled container’s bounce, but now they’re rotating the bottles vertically rather than flipping them end-over-end. Their work shows that faster rotating bottles have little to no bounce after impacting a surface.
This image sequence shows how water in a rotating bottle moves during its fall (top row) and after impact (bottom row). Water climbs the walls during the fall, creating a shell of fluid that, after impact, forms a central jet that arrests the bottle’s momentum.
The reason for this is visible in the image sequence above, which shows a falling bottle (top row) and the aftermath of its impact (bottom row). When the bottle rotates and falls, water climbs up the sides of the bottle, forming a shell. On impact, the water collapses, forming a central jet that shoots up the middle of the bottle, expending momentum that would otherwise go into a bounce. It’s a bit like the water is stomping the landing.
The authors hope their observations will be useful in fluid transport, but they also note that this bit of physics is easily recreated at home with a partially-filled water bottle. (Image and research credit: K. Andrade et al.; via APS Physics)
September 28, 2023
Spinning Liquids With Lego
One way to explore the effects of spinning liquids at high-speeds is to build an expensive and precise lab apparatus. Another method is to raid the Lego bin. Here, a YouTuber builds ever-more-elaborate Lego constructions to spin a sphere of water. He begins with a relatively straightforward magnetic stirrer that creates a bathtub vortex in his sphere, but as the set-up grows, he eventually encases the sphere to spin the entire thing at high-speed. It’s a cool way to see how spinning liquids react, from forming a vortex to spin coating the interior of the sphere and to generating a parabolic interface between air and liquid. Set-ups like these are not merely for fun, though; scientists use them to simulate the interiors of planets. (Image and video credit: Brick Technology; submitted by clogwog)
January 23, 2023
The Tea Leaves Effect
If you’ve ever stirred a cup of tea with loose leaves in it, you’ve probably noticed that the leaves tend to swirl into the center of the cup in a kind of inverted whirlpool. At first, this behavior can seem counter-intuitive; after all, a spinning centrifuge causes denser components to fly to the outside. In this video, Steve Mould steps through this phenomenon and how the balance of pressures, velocities, densities, and viscosity cause the effect. (Note that Mould uses the term “drag,” but what he’s really referring to is the boundary layer across the bottom of the container. But who wants to explain a boundary layer in a video when they can avoid it?) (Video and image credit: S. Mould)
When liquid in a cup is stirred, the densest layers move to the center.
August 10, 2022
Rotating Waves of Grains
Rotating drums are a popular way to explore granular dynamics. Here, researchers fill a cylinder (seen below) with heavy grains and a low-viscosity fluid, then rotate the mixture about a horizontal axis. This sets up a contest between centrifugal forces and gravitational forces on the grains. At the right rotation rates, the grains form annular rings around the outside of the cylinder, where they rotate at a different speed than the fluid. This difference in speed between the two layers can trigger a Kelvin-Helmholtz instability and cause waves along the interface between the grains and the fluid, as seen in the examples above. (Image and research credit: V. Dyakova and D. Polezhaev; top image adapted by N. Sharp)
August 4, 2022
Spinning Off-Axis
To make a vortex in the laboratory, researchers typically set a tank on a rotating platform and allow the water to drain out a hole in the center of the tank. In that case, a vortex forms over the drain (like in your bathtub!) and remains centered over the hole. In nature, though, vortices rarely follow such a simple path.
In this experiment, researchers moved the drainage hole so that it is not aligned with the tank’s axis of rotation. Although the vortex forms over the drain (marked by a yellow dot in the lower image), it quickly moves away, following a roughly circular path around the axis until it comes to a stop. Green dye marks fluid from the tank’s bottom boundary layer, which eventually gets entrained up into the vortex. (Image and research credit: R. Munro and M. Foster; via Physics Today)
Timelapse animation showing the development of the vortex. The yellow dot marks the location of the drain.
June 30, 2022