Feynman's Sprinkler Solved

In graduate school, my advisor introduced us to a particularly vexing fluid dynamical thought experiment known as the Feynman sprinkler. After observing an S-shaped sprinkler that rotated when water shot out its arms, physicist Richard Feynman wondered what would happen if the device were placed in a tank of water with the flow reversed. If the sprinkler was sucking in water, would it rotate and, if so, in what direction? (Image and research credit: K. Wang et al.; via APS Physics) Read the full article

Mixing the Immiscible

Immiscible liquids -- like oil and water -- do not combine easily. Typically, with enough effort, you can create an emulsion -- a mixture formed from droplets of one liquid suspended in the other -- like the one above. But a team of researchers have taken mixing immiscible liquids to a new level using their Vortex Fluid Device (VFD). (Image credit: pisauikan; research credit: M. Jellicoe et al.; video credit: Flinders University; submitted by Marc A.) Read the full article

Spinning Tops

What does the flow look like around a spinning top? Here, researchers used dye to visualize what happens in a Newtonian fluid (like air or water) as well as a viscoelastic fluid. (Image credit: B. Keshavarz and M. Geri) Read the full article

The Return of the Ice Disk

Maine's giant, spinning ice disk is taking shape again. In 2019, it reached about 91 meters across, rotating slowly in the Presumpscot River.  (Image credit: R. Bukaty/AP; via Gizmodo) Read the full article

A Colorful Fire Tornado

This one definitely belongs in the do-not-try-this-yourself category, but this Slow Mo Guys video of a colorful fire tornado is pretty spectacular.  (Video credit: The Slow Mo Guys) Read the full article

Leidenfrost droplets levitating over liquid baths induce a vortex ring beneath the surface. (Image credit: F. Cavagnon; research credit: B. Sobac et al.)

Enormous whirlpools are not simply the work of overactive imaginations. There are several spots in the world, including Japan's Naruto Strait, that regularly see these spectacular vortices. (Image credits: whirlpools - Mainichi/N. Yamada, Discover Tokushima; artwork: Hiroshige; via Mainichi; submitted by Alan M.)

So much of fluid dynamics comes down to finding the right way to observe a flow.  (Image credit: Expedition 59 Crew; via NASA Earth Observatory)

Reader embersofkymillo asks:

I enjoy doing that, too! So let’s talk a little about vortices. What Dan’s tea stirrer is doing is creating a low-pressure core for a vortex. We can see just how strong that low pressure region is by the way it sucks the air-water interface down toward the spinning arms. Eventually the interface and stirrer meet, and what was once a single, smooth(ish) surface gets torn into a myriad of bubbles. (As an aside, those bubbles get loud.) 

I also like the sequence of sugar cube drops because they make for some very cool splashes. Notice how the orientation of the cube’s edges as it hits determines the shape of the inital splash curtain. The asymmetry borne out of that impact actually follows through all the way through the seal of the cavity behind the cube. It reminds me of this oldie-but-goodie video on drops hitting different shapes. (Video and image credit: The Slow Mo Guys; submitted by embersofkymillo)

One of the challenges in studying tornadoes is being in the right place at the right time. In that regard, storm chaser Brandon Clement hit the jackpot earlier this week when he captured this footage of a tornado near Sulphur, Oklahoma from his drone. He was able to follow the twister for several minutes until it apparently dissipated. 

Scientists are still uncertain exactly how tornadoes form, but they’ve learned to recognize the key ingredients. A strong variation of wind speed with altitude can create a horizontally-oriented vortex, which a localized updraft of warm, moist air can lift and rotate to vertical, birthing a tornado. These storms most commonly occur in the central U.S. and Canada during springtime, and researchers are actively pursing new ways to predict and track tornadoes, including microphone arrays capable of locating them before they fully form. (Image and video credit: B. Clement; via Earther)

We’ve seen spinning ice disks before, but this month Westbrook, Maine has developed the largest one I’ve ever seen. A research paper from 2016 indicates that this seemingly alien formation spins due to an oddity of water. Water is at its densest around 4 degrees Celsius, so as the ice of the disk melts in the warmer waters of the river, it sinks. That downward plume sets up a vortex in the water beneath the disk. And as the water spins, it drags the ice with it, causing the disk’s rotation. The warmer the water is, the faster the disk spins. (Image credit: T. Radel/City of Westbrook; research credit: S. Dorbolo et al.; via Gizmodo; submitted by jpshoer)

For a fixed-wing aircraft, stall -- the point where airflow around the wing separates and lift is lost -- is an enemy. It’s the precursor to a stomach-turning freefall for the airplane and its contents. But the story is rather different when the wing is actively pitching through these high angles of attack. In this case, you get what’s known as dynamic stall, illustrated in three consecutive snapshots above.

In the top image, the flow has clearly separated from the upper surface of the wing, but this isn’t a cause for panic. As the middle image shows, there’s a vortex that’s formed in that separated region and it’s moving backward along the wing as the angle of attack continues to increase. That vortex causes a strong low-pressure region on the upper surface of the wing, allowing it to maintain lift. 

In the final image, the vortex is leaving the wing, taking its low-pressure zone with it. This is the point where the pitching wing loses its lift, but if the vortex’s departure is immediately followed by a pitch down to lower angles of attack, the aircraft will recover lift and carry on. (Image credit: S. Schreck and M. Robinson, source)

Although eyes are common at the center of large-scale cyclones, scientists are only now beginning to understand how they form. Since real-world cyclogenesis is complicated by many competing effects, researchers look at simplified model systems first. A typical one uses a shallow, rotating cylindrical domain in which heat rises from below. The rotation provides a Coriolis force, which shapes the flow. In particular, it causes a boundary layer along the lower surface of the domain, creating a thin region where the flow moves radially inward. (Its opposite forms at the upper surface of the domain, sending flow radiating outward.) Like an ice skater spinning, the flow’s vorticity intensifies as it approaches the central axis of rotation. When the conditions are right, this intensely swirling boundary layer flow lifts up into the main flow, forming an eyewall. The eye itself, it turns out, is merely a reaction to the eyewall’s formation. (Image credit: S. Cristoforetti/ESA; research credit: L. Oruba et al.)