FYFD

Tag: jets

  • Chains of Salps

    Chains of Salps

    Salps are small, jellyfish-like marine invertebrates that swim by ejecting a pulsatile jet. They are unusual creatures whose lives have two major stages: one in which salps swim individually and one in which they link together and swim in large chains. In the chain, salps don’t synchronize their jetting; each salp jets with its own phase and frequency. A new study suggests that, in spite of this lack of synchronicity, the salp chain’s swimming reduces the animals’ drag. There are several  factors that contribute to this result. One is that drag is generally lower on a body moving at constant speed compared to one moving in bursts. When linked together and firing randomly, all the individual jets tend to average out into one continuous swimming speed. There’s even a benefit to being out of sync: previous work showed that synchronized jets lose some of their thrust when they are too close together. Salps avoid that loss by keeping to their own beat. (Image and research credit: K. Sutherland and D. Weihs, source; via Gizmodo)

  • Inside Ink Jet Printing

    Inside Ink Jet Printing

    Inkjet printers produce droplets at an incredible rate. A typical printhead generates droplets that are about 10 picoliters in volume – that is, ten trillionths of a liter – moving at about 4 meters per second. Resolving the formation of those droplets would require ultra-high speed imaging at millions of frames per second. Instead researchers devised an alternative method to capture droplet formation, based on stroboscopic techniques. In this case the strobe is a 7 nanosecond laser pulse (7 billionths of a second) that illuminates a given droplet twice. By doing this for many droplets, the researchers can create a highly detailed time series like the one above, which shows the inkjet breakup and droplet formation. Here each droplet is 23 micrometers wide – about one-third the width of a human hair. (Image credit: A. van der Bos et al., source)

  • Superhydrophobic Splashes

    Superhydrophobic Splashes

    Superhydrophobic surfaces have a complicated microscale structure that changes how water interacts with them, like the hairs on a lotus leaf or the scales of a butterfly’s wing. The photo above shows snapshots at each millisecond as a water drop hits a superhydrophobic surface covered in rows of 18 micron-tall posts. The drop hits with enough speed to drive some water into the space between posts, as shown by the dark area near the center of the splash. As the rest of the droplet spreads, four microjets form along the directions of the micropost array. Those jets remain apparent until the drop reaches its maximum radius and starts to recoil. The rectangular shape of the post array affects how the water pulls away from the surface, or depins, causing the round droplet to instead take on a square-like shape as it pulls back. (Image credit: M. Reyssat et al.)

  • Putting Out Fires

    Putting Out Fires

    Fires in large, open spaces like aircraft hangers can be difficult to fight with conventional methods, so many industrial spaces use foam-based fire suppression systems. These animations show such a system being tested at NASA Armstrong Research Center. When jet fuel ignites, foam and water are pumped in from above, quickly generating a spreading foam that floats on the liquid fuel and separates it from the flames. Since the foam-covered liquid fuel cannot evaporate to generate flammable vapors, this puts out the fire. 

    The shape of the falling foam is pretty fascinating, too. Notice the increasing waviness along the foam jet as it falls. Like water from your faucet, the foam jet is starting to break up as disturbances in its shape grow larger and larger. For the most part, though, the flow rate is high enough that the jet reaches the floor before it completely breaks up. (Image credit: NASA Armstrong, source)

  • Using Jets to Find Food

    Using Jets to Find Food

    Archer fish are well-known for their ability to hit aerial targets with perfectly aimed jets of water, as we’ve discussed previously. But a new study shows they use a similar technique to form underwater jets that help them uncover food. The researchers found that the fish altered the timing of their jet formation based on the type of substrate – fine sand, course sand, or mud – that the food pellet was hidden in. A great next step in this research would be using a technique like particle image velociometry (PIV) to measure the flow field directly and see to what extent the fish’s actions are altering the jet they produce. (Image and research credit: J. Dewenter et al.; GIF source: freshphotons)

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    The Archer Fish’s Arrow

    Archer fish have a remarkable superpower. When hunting, they target insects above the water and knock them down with a precision strike from a jet of water they spit out. As previous research has shown, the archer fish packs an impressive punch by carefully modulating the water jet so that its tail travels faster and catches up to the front of the jet just as it strikes its target. Even more impressively, the archer fish can make this perfect strike on targets at different distances, which requires the fish to make significant adjustments to each jet. As this video from Deep Look discusses, the archer fish’s impressive hunting hints that it may have greater intelligence than we thought possible, given a comparison of its brain to ours. (Video credit: Deep Look)

  • Crowns On Impact

    Crowns On Impact

    Dropping a partially-filled test tube of water against a table makes the meniscus at the air-water interface invert into a jet of liquid. In some cases, the impact is strong enough to generate splashing crowns of water around the base of the jet. These crowns come in two forms – one with many splashes layered upon one another and the other with only a few splashes and a faster jet. 

    The many-layered splash crowns come from the pressure wave that reflects back and forth from the bottom of the tube to the surface and back. This pressure wave moves at the speed of sound and vibrates the water surface, creating the many splashes. The same reflected pressure wave occurs in the second type of splash crown, but it gets disrupted by cavitation bubbles that form in the water (visible in the lower left image). Instead the splash crowns form from the shock waves generated when the cavitation bubbles collapse. (Image credits: A. Kiyama et al.)

  • Reflecting in a Stream

    Reflecting in a Stream

    Total internal reflection traps three lasers in a stream of falling water. When light tries to pass from the water – a material with a high refractive index – to the air – which has a lower index of fraction – it can only do so if its angle of incidence is smaller than the critical angle. Here, the light impacts the water-air boundary at a large angle and rather than refracting across the interface – like the distorted view of a straw in a glass of water – the laser light is completely reflected. Instead of escaping, the laser light is trapped, becoming a ribbon of light that swirls inside the water stream until the light is diffused. (Image credits: L. Yarnell et al.; F. Batrack et al.)

  • Spore Squirting

    Spore Squirting

    The fungus Pilobolus spreads its spores with a squirt cannon. Each spore sits on the end of a round fluid-filled pod. Like many plants, the fungus uses a process called osmosis to pump water into the pod. Through osmosis, the fungus increases the concentration of certain molecules inside the pod, which draws water into the pod and increases its pressure. Eventually, the pod ruptures, sending the spore aloft on a jet of fluid that accelerates it at 20,000+g! (Image credit: BBC Earth Unplugged, source; research credit: L. Yafetto et al.)

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    Starting a Lighter

    Lots of fluids are transparent, which makes it hard for us to appreciate their motion. One technique for making these invisible motions visible is schlieren photography, which makes differences in density visible. Here it’s combined with high-speed video to show what happens when you use a lighter (minus the spark!). When the fuel starts flowing, it’s unstable and turbulent, but after that initial start-up, you can see the jet settle into a smooth and laminar flow. Wisps of fuel diffuse away from the jet as the fluid disperses. As the valve shuts off, the flow becomes unstable again, and the remains of the lighter fluid diffuse away. (Video credit: The Missing Detail)