FYFD

Tag: jets

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    The Kaye Effect

    Allow a stream of shampoo to fall into a pile and you’ll catch a glimpse of the bizarre Kaye effect. A jet of shampoo will briefly rise up before becoming chaotic and falling. The key to this behavior is the shear-thinning of the shampoo. When the shampoo is just sitting on a surface, it’s quite viscous, but slide your hand across it, and the shampoo will become much less resistant to flowing.

    When the jet of falling shampoo hits the pile, it creates a little dimple. Sometimes the incoming jet hits that dimple and slips along it, thanks to a sudden decrease in viscosity. That can send an outgoing jet of shampoo riding off the dimple like a ramp. As the dimple deepens, the outgoing streamer rises up until it hits the incoming jet and becomes unstable. The shampoo streamer collapses, only to be restarted when a new dimple forms. (Image and video credit: S. Mould; h/t to Guillaume D.)

  • Breaking With a Wave

    Breaking With a Wave

    For rocket combustion and other applications, like watering your lawn with a hose, a stream of fluid may need to be broken up into droplets. While simply spraying a liquid jet will make it break up, waving that jet back and forth will break it up faster. A recent study simulated this problem numerically to determine the exact mechanisms driving that break-up. The researchers found two major culprits.

    The first is a Kelvin-Helmholtz, or shear-based, instability. When a jet leaves the nozzle, there’s friction between it and the comparatively still air surrounding it. This creates tiny ripples in the surface that eventually grow into the distortions we can see, and it’s found in all jets, regardless of their side-to-side motion.

    The second culprit, which is only found in the oscillating jet, is a Rayleigh-Taylor instability. By moving the jet side-to-side, you’re driving the dense liquid into less dense air, which creates a different set of disturbances that also help break up the jet. The final result: swinging the jet side-to-side breaks it into smaller droplets faster. (Image and research credit: S. Schmidt et al.)

  • Sandy Splashes

    Sandy Splashes

    Sand and other granular materials can be strikingly fluid-like. Here the impact of a solid sphere on sand generates a splash remarkably similar to what’s seen with water. When the ball hits, it creates a crater in the surface and sends up a bowl-like spray of sand. As the ball continues falling through the sand, the grains try to fill the empty space left behind. The walls of sand collapsing around the void meet somewhere between the surface and the depth of the ball. This generates the tall jet we observe, as well as a second one under the surface that we can’t see. We know that collapse traps an air bubble under the surface because of the eruption that occurs as the jet falls. That’s the air bubble reaching the surface. (Image credit: T. Nguyen et al., source; see also R. Mikkelsen et al.)

  • Can Zooplankton Mix Oceans?

    Can Zooplankton Mix Oceans?

    Krill and other tiny marine zooplankton make daily migrations to and from the ocean surface. Previously, models of ocean mixing ignored these migrations; these animals are tiny, researchers argued, so any effects they could have would be too small to matter. But zooplankton make these migrations in huge swarms, and studies of a laboratory analog of their migrations (using brine shrimp rather than krill) reveal that, when moving en masse, these tiny swimmers create turbulent jets and eddies far larger than an individual. Their collective motion is enough to mix salty water layers 1000 times faster than molecular diffusion alone! Learn more in the latest FYFD video, embedded below. (Image and video credit: N. Sharp; research credit: I. Houghton et al.; h/t to Kam-Yung Soh)

  • Plasma From a Jet of Water

    Plasma From a Jet of Water

    There aren’t many naturally occurring plasmas in our daily lives; by far the most common one is lightning. So it’s something of a surprise that a stream of water hitting a material like glass is able to produce a toroid of plasma like the one above. The key here, though, is that the jet has to be fast – to the tune of 200 meters per second or faster. When a jet of deionized water strikes a surface at that speed, the water has to take a very sharp, 90-degree turn, and, thanks to the polar nature of water, this causes a (negative) charge to build up at that turn. It’s akin to rubbing a balloon to build up a static charge, and it’s known as a triboelectric effect. At rest (and without high shear rates), water and glass in contact tend to create in a positive charge in the water. The plasma is created when an arc forms through air between those two charged areas.

    Experiments in helium environments create a different color of plasma, confirming that the arc definitely travels through the gas. Similarly, if you use regular water instead of deionized water, the conductivity of the dissolved salts in the water is enough to prevent the necessary build up of charge. (Image and research credit: M. Gharib et al.; video credit: Applied Science; submitted by Kam-Yung Soh)

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    Bouncing, Floating, and Jetting

    Get inside some of the latest fluid dynamics research with the newest FYFD/JFM video. Here researchers discuss oil jets from citrus fruits, balls that can bounce off water, and self-propelled levitating plates. This is our third entry in an ongoing series featuring interviews from researchers at the 2017 APS DFD conference. Missed one of the previous ones? Not to worry – we’ve got you covered. (Video and image credit: N. Sharp and T. Crawford)

  • Nautilus Swimming

    Nautilus Swimming

    The shellbound chambered nautilus is a champion of underwater jet propulsion. It can eke out efficiencies as high as 75%, far outclassing other jet-based swimmers like squid, salps, and jellyfish. That high efficiency is especially important for the nautilus, which spends a great deal of time at depths where the oxygen needed to fuel movement is in short supply. To get around, the nautilus draws water in through an enlarged orifice, then squirts it out little by little. Its this asymmetry between drawing in and expending that keeps efficiency high. By releasing a jet slower and at lower speeds, the nautilus is able to reduce wasteful losses to friction and thereby keep the efficiency high. The drawback is that the nautilus swims relatively slowly at an average of around 8 centimeters–less than one body length–per second. (Image credit: Simon and Simon Photography/University of Leeds; research credit: T. Neil and G. Askew; via NYTimes; submitted by Kam-Yung Soh)

  • The Fishbone

    The Fishbone

    The simple collision of two liquid jets can form striking and beautiful patterns. Here the two jets strike one another diagonally near the top of the animation. One is slanted into the screen; the other slants outward. At their point of contact, the liquid spreads into a sheet and forms what’s known as a fishbone pattern. The water forms a thicker rim at the edge of the sheet, and this rim destabilizes when surface tension can no longer balance the momentum of the fluid. Fingers of liquid form along the edge, stretching outward until they break apart into droplets. Ultimately, this instability tears the liquid sheet apart. Under the right conditions, all kinds of beautiful shapes form in a system like this. (Image credit: V. Sanjay et al., source)

  • Oil Splatters

    Oil Splatters

    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)

  • Rocket Launch Systems

    Rocket Launch Systems

    If you’ve ever watched a rocket launch, you’ve probably noticed the billowing clouds around the launch pad during lift-off. What you’re seeing is not actually the rocket’s exhaust but the result of a launch pad and vehicle protection system known in NASA parlance as the Sound Suppression Water System. Exhaust gases from a rocket typically exit at a pressure higher than the ambient atmosphere, which generates shock waves and lots of turbulent mixing between the exhaust and the air. Put differently, launch ignition is incredibly loud, loud enough to cause structural damage to the launchpad and, via reflection, the vehicle and its contents.

    To mitigate this problem, launch operators use a massive water injection system that pours about 3.5 times as much water as rocket propellant per second. This significantly reduces the noise levels on the launchpad and vehicle and also helps protect the infrastructure from heat damage. The exact physical processes involved – details of the interaction of acoustic noise and turbulence with water droplets – are still murky because this problem is incredibly difficult to study experimentally or in simulation. But, at these high water flow rates, there’s enough water to significantly affect the temperature and size of the rocket’s jet exhaust. Effectively, energy that would have gone into gas motion and acoustic vibration is instead expended on moving and heating water droplets. In the case of the Space Shuttle, this reduced noise levels in the payload bay to 142 dB – about as loud as standing on the deck of an aircraft carrier. (Image credits: NASA, 1, 2; research credit: M. Kandula; original question from Megan H.)