The meeting of land and sea often creates a rich and colorful environment. This satellite image shows Mexico’s Laguna de Términos, a coastal lagoon off the Gulf of Mexico. A skinny barrier island forms the lagoon’s two connections to the ocean; the eastern side is the usual inlet (right), while the western side forms an outlet. Rivers feed freshwater into the lagoon from the south and southwest. These introduce sediments that cause some of the lighter swirls in the image. Winds and tides also contribute to this turbidity. The sheltered nature of the lagoon allows fresh and salt water to mix gradually, providing harbor for many forms of life. Oyster beds thrive in the river mouths; seagrasses prefer the calmer, saltier waters, and mangrove trees line the shore, slowly desalinating water for themselves as their roots shelter young fish and shrimp. (Image credit: NASA Earth Observatory)
Whether you’re into stickers or t-shirts, experimental fluid dynamics or CFD, we’ve got you covered. I’m running a special introductory sale through December 15th – holiday shopping anyone? – so it’s a great time to grab some merch!
See a design you want available on more products? Got a concept for a new design? Let me know!
For those attending the APS DFD meeting here in Denver in a few days, I’ll be sporting some of the new t-shirts there, and I’ll be selling selected sticker designs in person (no shipping costs for you!). More DFD details to come.
Birds are well-known for their vocalizations, but this isn’t their only way to produce noise. A new study on crested pigeons finds that the birds’ wings produce distinctive high and low notes during take-off. A low note takes place during each upstroke, and a high note is heard during the downstroke. A major source of the noise is the highly modified P8 feather. When airflow over the feather is fast enough, it sets off twisting and torsion in the feather through aeroelastic flutter. It’s this vibration that causes the noise. By playing back the notes at different speeds, researchers found that the crested pigeons use the notes’ timing as an alarm. When the cycle of high and low repeats in quick succession, they respond by taking off to escape the perceived danger.
Other bird species are also known to use aeroelastic flutter to make noise. Check out thesehummingbirds, which use flutter in their mating displays. (Video credit: Science; research credit: T. Murray et al.)
One of the most amazing things about fluid dynamics, in my opinion, is that the same rules apply across an incredible array of situations. The equations of motion are the same whether your fluid is water, air, or honey. Your flier can be a Cessna airplane or a fruit fly; again, the equations are the same. This is part of the reason that patterns in flows are repeated whether in the laboratory or out in nature – and it’s the reason why a timelapse of fog clouds can look just like ocean waves. Ultimately, the physics is the same; clouds just move slower than ocean waves! (Image credit: L. Leber, source; via James H.)
Oil and water are hard to mix, as any salad dressing aficionado will attest. Technically, the two fluids are immiscible – they won’t mix with one another – but one way around this is to emulsify them by distributing droplets of one in the other. This is usually accomplished by shaking or using sound waves to vibrate the mixture, but the results are typically short-lived. The larger a droplet is, the more gravity affects it, causing the buoyant oil to rise and separate from the water. The key to making an emulsion last is creating tiny droplets, which a new study accomplishes energy efficiently through condensation. Instead of mixing the oil and water immediately, the researchers used a surface covered in a mixture of oil and surfactant and cooled it in a humid chamber. As the temperature dropped, water condensed onto the oil and became encapsulated, creating nanoscale emulsion droplets. At such a tiny scale, buoyant forces are unable to overcome surface tension, so the emulsion remains stable for months. (Image credit: MIT, source; research credit: I. Guha et al.; via MIT News)
Most cooks have experienced the unpleasantness of getting splattered with hot oil while cooking. Here’s a closer look at what’s actually going on. The pan is covered by a thin layer of hot olive oil. Whenever a water drop gets added – from, say, those freshly washed greens you’re trying to saute – it sinks through the oil due to its greater density. Surrounded by hot oil and/or pan, the water heats up and vaporizes with a sudden expansion. This throws the overlying oil upward, creating long jets of hot oil that break into flying droplets. These are what actually hit you. This is a small-scale demonstration, but it gets at the heart of why you don’t throw water on an oil fire. (Image credit: C. Kalelkar and S. Paul, source)
Nature is full of remarkable patterns and moments of symmetry. This image shows the wake behind two rotating cylinders. Half of the cylinders are visible at the far left. The flow moves left to right. The cylinders are rotating at the same rate but in opposite directions, clockwise for the cylinder on top and counter-clockwise for the bottom one. At this speed relative to the freestream, there is a beautiful symmetry to the vortices in the wake, but the researchers found that even a slight deviation from this condition quickly destroyed the pattern. The flow is visualized here by introducing tiny hydrogen bubbles via electrolysis. The bubbles are small enough that their buoyancy has no appreciable effect. (Image credit: S. Kumar and B. Gonzalez)
Since the 1970s, fluid dynamicists have chased the idea that fish swim in schools for hydrodynamic advantage. The original 2D conception of the idea placed fish in a diamond pattern so that their wakes would constructively interfere and improve swimming efficiency. In nature, that exact pattern is rarely seen, possibly due to 3D effects or the difficulty of maintaining the exact orientation. Fish do, however, show signs of grouping themselves for efficiency – especially when they’re forced to swim quickly.
A recent study found that tetras, a type of small fish often used as pets, prefer a staggered diamond configuration (left) when free-swimming at low speeds around one body length per second. At higher speeds, around four body lengths per second, groups of tetras preferred a side-by-side or “phalanx” configuration (right). Here the fish tended to synchronize their tail-beat frequency with their neighbors, essentially working together for a mutually beneficial wake structure. The researchers found that this configuration was much more efficient than a lone swimmer or uncoordinated group, implying that fish do school for energy-savings when they’re swimming fast. (Image and research credit: I. Ashraf et al., source; via Hakai; submitted by Kam-Yung Soh)
Glycerol is a sweet, highly viscous fluid that’s very good at absorbing moisture from the ambient air. That’s why a drop of pure glycerol in laboratory conditions quickly develops convection cells – even when upside-down, as shown above. This is not the picture of Bénard-Marangoni convection we’re used to. There’s no temperature or density change involved; in fact, there’s no buoyancy involved at all! This convection is driven entirely by surface tension. As glycerol at the surface absorbs moisture, its surface tension decreases. This generates flow from the center of a cell toward its exterior, where the surface tension is higher. Conservation of mass, also known as continuity, requires that fresh, undiluted glycerol get pulled up in the wake of this flow. It, too, absorbs moisture and the process continues. (Image credit: S. Shin et al., pdf)