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Search results for: “jet”

  • Superhydrophobic Drag

    Superhydrophobic Drag

    Using air or bubbles to reduce drag on boats is a popular idea, whether using supercavitation, the Leidenfrost effect, or superhydrophobic coatings. But most of the experiments done thus far use spheres rather than realisitic boat shapes. In this study, the researchers used two model boats — one with a hydrofoil and the other in a conventional motorboat shape — and applied superhydrophobic coatings to different parts of the model to see how superhydrophobicity affected the overall drag.

    Perhaps surprisingly, they found that superhydrophobic coatings can actually increase the drag! The effect was particularly stark for the hydrofoil boat (Image 2), where the surface jets (lower half) caused by the superhydrophobic coating slowed the boat by 30% compared to its unmodified speed (upper half).

    For the speedboat, a superhydrophobic hull made no overall difference in its drag, though it changed how water splashed in its wake. And coating the boat’s propeller was particularly detrimental, resulting in a speed up to three times slower. Overall, the study suggests that superhydrophobic coatings may be useful in some circumstances, but they have to be applied carefully, as they can have negative impacts, too. (Image credits: top – S. Anghan, others and research credit: I. Vakarelski et al.)

  • Luminous Fruits

    Luminous Fruits

    Light shines through citrus and melon in this photographic photorealistic series of paintings from artist Dennis Wojtkiewicz. The strong illumination reveals the underlying structure of pith, pulp, and juice. The deformable pockets of fluid in the peel of citrus fruits are the source of some incredible microjets. When the peel bends, it compresses these tiny fluid-filled pockets, creating incredibly high pressures that eventually drive a burst of oil at g-forces comparable to those felt by a bullet fired from a gun. Learn more about citrus jets here and see more of Wojtkiewicz’s work and purchase prints here on his site. (Image credit: D. Wojtkiewicz; via Colossal)

    ETA: Thanks to A.J. for pointing out that Wojtkiewicz is, in fact, a painter (and not a photographer), making his work all the more astounding! We regret the error.

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    Opera Singer Air Flow

    What does the air flow from a trained opera singer look like? That’s the question behind this study, which combines music and fluid dynamics. Using an infrared camera tracking carbon dioxide (CO2) exhalations from a singer during a performance allowed researchers to identify several important flow features. When breathing, air flows out the singer’s nose in a tight, downward jet with an initial velocity around 1 m/s.

    While singing, air leaves the mouth at a much lower velocity, especially during vowels where the mouth is open. With less momentum behind these exhalations, they can drift upward on the buoyant warmth of the singer’s breath. During consonants — especially plosives like t, k, p, b, d, and g — a rapid burst of air leaves the mouth, traveling at nearly 10 m/s. From the perspective of COVID-19 safety, it’s these plosive jets that are likely to spread contaminated droplets. (Image and video credit: MET Orchestra; research credit: P. Bourrianne et al.; via Improbable Research; submitted by Kam-Yung Soh)

  • Liquid Umbrellas

    Liquid Umbrellas

    Two well-timed and properly aligned droplets combine to create these umbrella-like fluid sculptures. The initial drop creates a jet that shoots upward. When the second drop hits that jet, it forms an expanding sheet of liquid like a miniature parasol. The higher the viscosity of the drops, the less lacy and unstable the sheet’s rim will be.

    Although set-ups for these sorts of pictures can be finicky, they’re very doable, even for amateur photographers. In fact, the techniques used here have been around for about a century! (Image and research credit: A. Kiyama et al.)

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    Pressure At The Dam

    Hydrostatic pressure in a fluid is based on the fluid’s depth. You’ll rarely see a more dramatic example of that power than with a water release from a dam. Here we see the outlet of the Verbund Hydro Power dam in Austria. With 190 meters of water behind the dam, the outlet jet is massive. It moves 20,000 liters of water per second at a speed of 50 meters per second. Imagine what it would be like to stand next to that! (Image and video credit: Discovery UK; submitted by Olwyn B.)

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    Really, Really Slow Mo Fluids

    Fluid dynamics is a perfect subject for high-speed video. So much goes on at speeds that are far too quick for our eyes and brains to perceive. But there is such a thing as too slow – a concept explored in this Slow Mo Guys video, which takes everyday activities like turning on a faucet or splashing into a pool and slows them down a speed where one second lasts an hour. The video I’ve embedded here isn’t nearly that long; it speeds up and slows down. But if you really want to, you can watch Gav fall into a pool for a full hour. (Image and video credit: The Slow Mo Guys)

  • Breaking Up Is(n’t) Hard to Do

    Breaking Up Is(n’t) Hard to Do

    Engineers often need to break a liquid jet up into droplets. To do so quickly, they surround the jet with a ring of fast-moving air in a set-up known as a coaxial jet. Shear between the gas and liquid creates instabilities that quickly distort the jet’s initial cylinder into sheets and ligaments. Those formations then undergo their own instabilities to break up into drops. The method is, as you can see in the high-speed images above, quite effective, though the breakup mechanism itself is tough to quantify. (Image credit: G. Ricard et al.)

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    Fish Versus Bird

    You’ve seen birds catch fish, but have you ever seen a fish that catches birds? In this video, giant trevally fish hunt fledgling terns — including those in flight! To do so, the fish must correctly assess the bird’s speed and trajectory across the water interface, a feat reminiscent of the archer fish’s aim. They also need the power and control to leap from the water and catch the birds in their mouth without relying on the suction technique so many fish use underwater. (Image and video credit: BBC Earth, from “Blue Planet II”)

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    Mud Pots

    Mud pots, or mud volcanoes, form when volcanic gases escape underlying magma and rise through water and earth to form bubbling mud pits. I had the chance to watch some at Yellowstone National Park a few years ago and they are bizarrely fascinating. In this Physics Girl video, Dianna recounts her adventures in trying to locate some mud pots in southern California and explains the geology that enables them there. And if you haven’t seen it yet, check out her related video on the only known moving mud puddle! (Image and video credit: Physics Girl)

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    When Squids Fly

    Some species of squid fly at speeds comparable to a motorboat for distances of 50 meters. The cephalopods get into the air the same way they swim underwater: by expelling a jet of water through the center of their body. Once aloft, the squids spread their tentacles to form a semi-rigid wing-like surface for lift. They can also use fins on their mantle as a canard for additional lift or control of their altitude. Researchers suspect the squids use flight as an escape mechanism to put distance between themselves and predators, but it could also be a low-energy migration strategy since a single pulse carries a squid farther in air than in water. (Video and image credit: TED-Ed)

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