Colorful streaks of dye wrap like ribbons along the leading edge of a delta wing. At an angle of attack, this triangular wing forms a set of vortices that run along its edge, providing much of the low pressure–and therefore lift–on the upper surface of the wing. In contrast, the red streaks of dye in the middle of the wing demonstrate clean, laminar flow. Highly swept delta wings are popular for aircraft traveling at supersonic speeds, but they can also work well subsonically, as shown here. For more incredible and beautiful examples of flow visualizations by Henri Werlé, check out his 1974 film Courants et couleurs. (Photo credit: H. Werlé; via eFluids)
Category: Phenomena
Leaping Mobulas
Mobula rays engage in some pretty incredible aerial acrobatics. This species of ray, second only to manta rays in size, can jump up to 2 meters into the air. Large groups of mobula rays will engage in this behavior, including both males and females, but it remains unclear to scientists exactly what purpose the jumping serves. It may be a form of communication, which might explain the rays’ apparent preference for belly flopping. By striking the water surface with as much of their body as possible simultaneously, the rays generate both a large splash and a concussive clap that carries through the water. (Video credit: BBC; via J. Hertzberg)
Tides
Most of us think we understand why Earth’s oceans have tides, but it turns out that there are some misconceptions in the common explanation. Yes, it’s true that the moon’s gravity pulls on water in the ocean, but it equally pulls on everything else, too, and we don’t levitate at high tide! In reality, it’s the distribution of tidal forces across the enormity of the ocean that causes the ocean to bulge along the Earth-moon line and create high and low tides. Lakes, puddles, and humans experience tides, too, but we’re so small that the tidal forces we experience are too tiny to be noticeable. For the full explanation, I encourage you to watch PBS Space Time’s video. Don’t let the 15 minute run-time deter you; the tidal explanation is contained within the first 9 minutes. (Video credit: PBS Space Time; via It’s Okay To Be Smart)
Weaponizing Water-Repellency
St. Pauli, a neighborhood in the German city of Hamburg, has demonstrated one of the most unusual applications of superhydrophobicity I’ve ever heard of. St. Pauli is known as a party district, and the residents of the area have grown understandably frustrated with inebriated visitors publicly urinating on their buildings and, yes, playgrounds. When fines failed to curb the issue, they took to treating walls chemically to make them superhydrophobic. As the targeted audience has discovered, water repellency tends to make liquid jets bounce off rather than run down a surface. Well played, St. Pauli. (Video credit: IG St. Pauli; submitted by entropy-perturbation)
Jovian Dynamics
Our solar system’s largest planet is a mysterious and majestic font of fluid dynamics. Unlike rocky Earth, Jupiter is made entirely of fluids. Beneath its massive gaseous atmosphere lies an ocean of liquid hydrogen. The lack of solid ground to weaken storms may explain some of the longevity of Jupiter’s Great Red Spot, a hurricane that’s been raging on the planet for more than a hundred and fifty years. Part of the challenge of understanding Jupiter’s dynamics is that most of our data consists of observations of the uppermost layer of the atmosphere. It’s kind of like trying to describe an entire ocean based on the surface alone; what we see is part of the story, but it’s only a small portion of a much greater whole. (Image credit: NASA; submitted by jshoer)
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Carbonation in Space
Astronauts don’t typically drink soda or other carbonated beverages while in space. The reason is probably apparent if you watch this new video of an effervescent tablet in water on the space station (or, you could watch the older classic one from Don Pettit). Unlike on Earth, where the carbon dioxide bubbles are buoyant and rise to the surface, the bubbles in a fluid in microgravity are randomly distributed. Those few bubbles that happen to be located along the edge of the water sphere will sometimes burst, creating the halo of tiny droplets you see in the video. In the case of sodas, though, the bubbles’ behavior creates a foamy mess, and, after ingestion, the bubbles are stuck travelling through the astronaut’s digestive system instead of getting burped out. Sounds rather unpleasant to me. (Video credit: NASA; submitted by entropy-perturbation and buckitdrop)
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LAST CALL: Help us do some science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of FYFD readers. By participating, you’ll be helping me improve FYFD and contributing to novel academic research on the readers of science blogs. It should only take 10-15 minutes to complete. You can find the survey here.
Wave Clouds Over the Galapagos
This dramatic example of Kelvin-Helmholtz clouds was taken near the Galapagos Islands last week. The shark-fin-like clouds are the result of two air layers moving past one another. The velocity difference at their interface creates an unstable shear layer that quickly breaks down. The resemblance of the clouds to breaking ocean waves is no coincidence – the wind moving over the ocean’s surface generates waves via the same Kelvin-Helmholtz instability. In the case of the clouds above, the lower layer of air was moist enough to condense, which is why the pattern is visible. Clouds like these don’t tend to last for long because the disturbances that drive the instability grow exponentially quickly, leading to turbulence. (Image credit: C. Miller; via Washington Post; submitted by @jmlinhart)
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Help us do some science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of FYFD readers. By participating, you’ll be helping me improve FYFD and contributing to novel academic research on the readers of science blogs. It should only take 10-15 minutes to complete. You can find the survey here.
Alligators Water Dancing
Amorous alligators call to mates with a behavior known as water dancing. Their audible bellows are accompanied by infrasonic sound–vibrations below the 20 Hz limit of human hearing. These vibrations from their lungs excite Faraday waves in the water near the alligator’s back and make the surface explode in a dance of jets and atomized droplets. I’ve seen similar results in other instances of vibration, but this may be the only example of this I’ve seen in the wild. Researchers studying the phenomenon noted that the frequency of sound the alligators emit corresponds to a wavelength equal to the spacing of the raised scales, or scutes, on the alligators’ backs. They hypothesize that the shape of the scutes helps males create the display. (Image credit: N. Marven, source; research credit: P. Moriarty and R. Holt; h/t to io9)
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Raindrops in Puddles
Watching rain drops hit a puddle or lake is remarkably fascinating. Each drop creates a little cavity in the water surface when it impacts. Large, energetic drops will create a crown-shaped splash, like the ones in the upper animation. When the cavity below the surface collapses, the water rebounds into a pillar known as a Worthington jet. Look carefully and you’ll see some of those jets are energetic enough to produce a little satellite droplet that falls back and coalesces. Altogether it’s a beautifully complex process to watch happen over and over again. (Image credit: K. Weiner, source)
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Help us do some science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of FYFD readers. By participating, you’ll be helping me improve FYFD and contributing to novel academic research on the readers of science blogs. It should only take 10-15 minutes to complete. You can find the survey here.
Magnus Effect
Putting a little bit of spin on an object can have a big aerodynamic effect, thanks to the Magnus effect. As demonstrated in the video above, backspin on a basketball dropped from a big height will send it flying out and away. The reason spinning objects generate these counterintuitive motions is because the air flow over them creates differential pressures. On the side of the ball spinning with the flow, air is accelerated, dropping the local pressure; whereas on the opposite side, the ball spinning against the direction of flow makes the flow separate and no longer flow smoothly along that side. This causes a high pressure on that side. Like the difference in pressure on either side of an airfoil, the pressure difference across the ball creates a force that pushes the ball toward the low pressure side. Check out some of the other places Magnus effect shows up! (Video credit: Veritasium; submitted by Andrew C.)
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Help us do some science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of FYFD readers. By participating, you’ll be helping me improve FYFD and contributing to novel academic research on the readers of science blogs. It should only take 10-15 minutes to complete. You can find the survey here.
