Category: Phenomena

Aerated Faucets
So much goes on in our daily lives that we never see. But with the power of the smartphones in our pockets, we can catch more than ever before, as illustrated in this video. Here a researcher uses the standard “slo-mo” (240 fps) video mode on a smartphone to look at the flow from a typical kitchen faucet. Household faucets often have an aerator that adds air bubbles to the flow, something that’s particularly visible in slow motion at high flow rates. What you can see depends on more than just the frame rate, though. Without strong illumination — provided in this case by sunlight — you could easily miss the cloud of droplets ejected by the faucet. (Image and video credit: M. Mungal)
September 12, 2022
Fish-Scale Tides
On 31 July 2022, an unusual tidal phenomenon, a fish-scale tide, took place on the Qiantang River’s estuary in Zhejiang Province, China. Here are a couple videos. I’ve not found any explanations for it thus far, so I’m assembling my own. The Qiantang River and its estuary, Hangzhou Bay, are home to the world’s largest tidal bores, reaching 9 meters in places. That means the area regularly sees trains of large waves moving upstream against the normal current.
The area is also known to have rotating currents, meaning that the tide does not simply move inland and then smoothly reverse direction. Instead, a rotating current can change its direction of flow over the course of a tidal cycle without changing its speed. Taken together, this makes the Qiantang River region perfect for winding up with groups of waves colliding at oblique angles, similar to a cross sea. I believe that’s what’s going on here with the fish-scale tide. Two sets of tidal-bore-induced waves are colliding at an angle, creating some gnarly conditions and a very cool pattern. (Image credit: VCG; submitted by Antony B.)
September 7, 2022
Groundwater-Structure Interactions
Groundwater can sometimes wind up in unexpected places, given the way it interacts with subsurface structures. In this Practical Engineering video, Grady discusses the paths that groundwater takes around structures and how civil engineers account for groundwater-related forces on dams and other buildings. As always, he illustrates with excellent model demos, allowing viewers to see groundwater interactions for themselves. (Image and video credit: Practical Engineering)
September 5, 2022
Seeing the Flow
Experimentalists often need a sense for the overall flow before they can decide where to measure in greater detail. For such situations, flow visualization techniques are a powerful tool since they provide quick ways to see and compare flows.
Here, researchers paint a viscous oil atop their flying wing model and observe how the oil moves once the air flow starts up. This oil flow visualization shows the large-scale shifts in how air flows over the craft as the angle of attack increases. The disadvantage is that these techniques often give only a qualitative sense of the flow. But they can allow experimentalists to test many different conditions to decide which specific cases they should examine quantitatively. (Image and video credit: V. Kumar et al.)
August 31, 2022
Reefs Along New Caledonia
Brown reefs edge a turquoise lagoon in this astronaut snapshot of the New Caledonian coastline. Reefs like these form a natural barrier that protects coastlines from storms by breaking up waves (seen here as those white edges) before they reach the shore. The lagoon is streaked with lines of tan where sediment flows from the uplands into the water. Similarly, the color variations from green to blue in water hint at changes in depth, organic content, and more. (Image credit: NASA; via NASA Earth Observatory)
August 29, 2022
A Levitated Boil
When acoustically levitated, objects tend to clump together and move like a single, large solid. But researchers found more fluid-like states for their levitated particles when the particles were smaller. At low acoustic power, the particles behave like a liquid and shift primarily within a plane. But as the acoustic power increases, the granular liquid begins to “boil” and transition into a gaseous state, with particles moving in all directions. It’s amazing how often these metaphors (e.g., treating a group of particles as a “liquid”) hold true when observing different physical systems! (Image and video credit: B. Wu et al.)
August 24, 2022
Perturbations
At first glance, today’s video appears to have little to do with fluid dynamics since it’s a demonstration of interactions between magnets. But for those who’ve delved into the mathematics of fluid dynamics — especially subjects like perturbation theory — there’s a lot to appreciate here. In the video, we see systems of magnets constructed and then manipulated, often by moving a single magnet and watching how the rest respond. Visually, this demonstrates how disturbances move in complex, interconnected damped systems. The auditory component — definitely turn the sound on for this video — is an extra layer of fluids-related goodness that also shows how reconfiguring a system changes its resonant frequencies. (Image and video credit: Magnetic Tricks and Magnetic Games; via Colossal)
August 22, 2022
Florida’s Keys
Stretching from the southern tip of Florida, a chain of low-lying islands, known as keys or cays, formed underwater during a warm interglacial period some 125,000 years ago. Originally coral reefs and sand bars, the islands hardened and fossilized when sea levels dropped during an ice age. These natural-color satellite images hint at the keys’ impressive ecosystems. The bright blue streak is a giant coral reef separating the deeper waters of the Atlantic from the shallow waters and sea-grass beds lying between the islands. Formations like these, along with mangrove forests, are part of nature’s way to mitigate the damage and flooding caused by hurricanes. Unfortunately, warmer seas and rising sea levels now threaten the keys. (Image credit: L. Dauphin/USGS; via NASA Earth Observatory)
August 17, 2022
When Reservoirs Run Dry
With the ongoing megadrought in the U.S. Southwest, more and more reservoirs are reaching historic low water levels. So it’s worth asking: what happens when a reservoir runs dry? And what, exactly, does a reservoir do in the first place? In this Practical Engineering video, Grady tackles both questions and takes a look at the many disciplines — beyond just civil engineering — that go into making a functional reservoir. (Image and video credit: Practical Engineering)
August 15, 2022
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