If you’ve ever left a sealed container of Playdoh untouched for months, you know that there’s a big difference between the fresh stuff and what’s left in that can. Aging can have big effects on non-Newtonian fluids. In this video, we see drops of a synthetic clay impacting at different speeds. In the top row of images, the clay is fresh and unaged; on impact, the clay forms large crown-like splashes. In the bottom row, however, the aged clay behaves quite differently. Instead of a splash, the drops make more of a splat. (Image and video credit: R. Ewoldt et al.)
Tag: science
Hammerhead Hydrodynamics
Hammerhead sharks have some of the most distinctive craniums in the ocean, which begs the question: how do they swim with that head? New computational fluid dynamics studies suggest that their long foil-shaped heads help the sharks maneuver swiftly, but they come at the cost of substantially higher drag. The researchers found that drag on the hammerhead’s cranium required energy expenditures more than 10 times higher than other sharks, but since the study looked at heads only, it’s possible that the rest of the shark’s positioning helps mitigate that cost. (Image credit: shark – J. Allert, CFD – M. Gaylord et al.; research credit: M. Gaylord et al.; via NYTimes; submitted by Kam-Yung Soh)
Rolling Off a Duck’s Back
Ducks and other water fowl need protection from the elements. Fortunately for them, the structure of their feathers cleverly helps them shed water. As seen in this video, feathers have tiny hooks, called barbicels, that act like Velcro, zipping the individual barbs of a feather together to keep water out. When birds preen, they’re using their bills to rezip any sections that came loose. They also use their bills to spread a waxy substance onto the feathers to give them even more waterproofing. All together, these measures help the birds keep out cold water and trap warm air in the down near their skin. (Image and video credit: Deep Look)
Density Drift
This colorful photo shows three fluids — oil, water, and dish soap — illuminated by the rainbow reflection of a CD. The differing densities of each fluid creates a stratification with water sandwiched between dish soap on the bottom and oil on the top. Because the dish soap is miscible in water, it leaves a smudgy blur against the background, whereas the immiscible oil creates bubble-like lenses at the top. (Image credit: R. Rodriguez)
The Structure of the Blue Whirl
Several years ago, researchers discovered a new type of flame, the blue whirl. Now computational simulations have helped them untangle the complex structure of this clean-burning flame. Their work shows that the blue whirl is made up of three types of flames, which meet to form a fourth.
The conical base of the whirl is a fuel-rich flame in which the fuel and oxygen are initially well-mixed. Above that is a diffusion flame, where the fuel and oxygen are initially separate and the flame’s ability to burn is limited by how readily the two mix. Along the sides of the blue whirl is a third flame type, visible only as a faint wisp. Like the first flame, this one is premixed, but it contains much less fuel than oxygen. Finally, those three flames meet in the bright blue ring of the whirl, where the ratio of fuel and oxygen is just right to burn the fuel completely. (Image and research credit: J. Chung et al.; via Science News; submitted by Kam-Yung Soh)
The Greedy Cup in Your Washing Machine
A Pythagorean, or “greedy” cup, is one that automatically drains itself once filled to a certain level. In other words, it’s a self-starting siphon – one that triggers only at certain fill level. And chances are you have an example of this mechanism close at hand: inside your washing machine’s soap tray. That’s why the tray has such a clearly marked maximum fill line; if you were to put more soap than that in the tray, it would automatically drain! (Image and video credit: S. Mould)
Wrinkles on Bubble Collapse
A viscous bubble wrinkles when it collapses, and scientists long assumed this behavior was caused by gravity. But a new experiment shows that the buckling is, instead, driven by surface tension.
To test gravity’s influence on bubble collapse, the researchers popped bubbles in three orientations: the (normal) upright orientation (Images 1 and 2), upside-down (Image 3), and sideways (Image 4). In all cases, the bubble’s thin film wrinkled as it collapsed, indicating that gravity had little influence on the process. Instead the authors concluded that surface-tension-driven collapse causes the dynamic buckling of the film. (Image and research credit: A. Oratis et al.; submitted by Zander B.)
Curls Past the Canaries
When winds flow past a solitary peak, like an island in the ocean, they’re disrupted into a series of counter-rotating curls. That’s what we see here stretching to the southwest of Madeira Island. The official name for this flow is a von Karman vortex street, and it can be found anywhere from a soap film to a starship. (Image credit: J. Stevens; via NASA Earth Observatory)
Precipitation
Chemistry and fluid dynamics often go hand-in-hand. Here chemical reactions produce visible precipitates as one chemical drops into the other. The shapes that form are distinctly fluid dynamical, with vortex rings, plumes, and instabilities all appearing.
In many applications, chemical reactions and fluid dynamics are tied inextricably to one another because the rate of chemical reaction depends on local concentrations driven by fluid dynamics, and the fluid motion is itself influenced by those concentration gradients. This is why reacting flows, like those found in combustion, are among the hardest topics in fluids. (Image and video credit: Beauty of Science)
The Undisturbed Waters of Lake Kivu
Deep in Africa lies one of the world’s strangest lakes. Lake Kivu, over 450 meters in depth, is so stratified that its layers never mix. The upper portion of Lake Kivu consists of less-dense fresh water, which sits upon deeper layers of saltier water full of dissolved carbon dioxide and methane pumped into the lake by volcanic activity.
The lake’s lack of convection means that this deep water simply stays put for thousands of years as it collects gases that remain dissolved only thanks to the immense pressure of the water above. Should that deep water be disturbed — by an earthquake, climate changes, or simply oversaturation — the resulting eruption of carbon dioxide could be deadly for the millions of people living nearby. A similar eruption at smaller Lake Nyos in 1986 asphyxiated about 1,800 people.
Fortunately, Lake Kivu is well-monitored, so such an upwelling should not catch observers off-guard. Learn more about Lake Kivu’s oddities over at Knowable. (Image and research credit: D. Bouffard and A. Wüest, via Knowable Magazine; submitted by Kam-Yung Soh)
