FYFD

Tag: jets

  • Protostellar Jets

    Protostellar Jets

    As young stars form, they often produce narrow high-speed jets from their poles. By astronomical standards, these fountains are dense, narrowly collimated, and quickly changing. The jets have been measured at velocities greater than 200 km/s and Mach numbers as high as 20. The animation above (which you should watch in its full and glorious resolution here) is a numerical simulation of a protostellar jet. Every few decades the source star releases a new pulse, which expands, cools, and becomes unstable as it travels away from the star. Models like these, combined with observations from telescopes like Hubble, help astronomers unravel how and why these jets form. (Image credit: J. Stone and M. Norman)

    ETA: As it happens, the APOD today is also about protostellar jets, so check that out for an image of the real thing. Thanks, jshoer!

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    Shooting a Bullet Through a Water Balloon

    This high-speed video of a bullet fired into a water balloon shows how dramatically drag forces can affect an object. In general, drag is proportional to fluid density times an object’s velocity squared. This means that changes in velocity cause even larger changes in drag force. In this case, though, it’s not the bullet’s velocity that is its undoing. When the bullet penetrates the balloon, it transitions from moving through air to moving through water, which is 1000 times more dense. In an instant, the bullet’s drag increases by three orders of magnitude. The response is immediate: the bullet slows down so quickly that it lacks the energy to pierce the far side of the balloon. This is not the only neat fluid dynamics in the video, though. When the bullet enters the balloon, it drags air in its wake, creating an air-filled cavity in the balloon. The cavity seals near the entry point and quickly breaks up into smaller bubbles. Meanwhile, a unstable jet of water streams out of the balloon through the bullet hole, driven by hydrodynamic pressure and the constriction of the balloon. (Video credit: Keyence)

  • Impacting a Viscous Pool

    Impacting a Viscous Pool

    Whenever a hollow cavity forms at the surface of a liquid, the cavity’s collapse generates a jet–a rising, high-speed column of liquid. The composite images above show snapshots of the process, from the moment of the cavity’s greatest depth to the peak of the jet. The top row of images shows water, and the bottom row contains a fluid 800 times more viscous than water. The added viscosity both smooths the geometry of the process and slows the jet down, yet strong similarities clearly remain. Focusing on similarities in fluid flows across a range of variables, like viscosity, is key to building mathematical models of fluid behavior. Once developed, these models can help predict behaviors for a wide range of flows without requiring extensive calculation or experimentation. (Image credit: E. Ghabache et al.)

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    What Makes Squids Fast

    Cephalopods like the octopus or squid are some of the fastest marine creatures, able to accelerate to many body lengths per second by jetting water behind them. Part of what makes its high speed achievable, though, is the way the animal changes its shape. In general, drag forces are proportional to the square of velocity, meaning that doubling the velocity increases the drag by a factor of four. The energy necessary to overcome such large drag increases generally prevents marine animals from going very fast (compared to those of us used to moving through air!) But drag is also proportional to frontal area. Like the bio-inspired rocket in the video above, jetting cephalopods begin their acceleration from a bulbous shape and then shrink their exposed area as they accelerate. Not only does this shape change help mitigate increases in drag due to velocity, it prevents flow from separating around the animal, shielding it from more drag. The result is incredible acceleration using only a simple jet for thrust. For example, the octopus-like rocket in the video above reaches velocities of more than ten body lengths per second in less than a second. (Video credit: G. Weymouth et al.)

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    Impacts on Sand

    Granular materials like sand are sometimes very fluid-like in their behaviors. The high-speed video above shows a ball bearing being dropped into packed sand. Many features of the splash are fluid-like; the initial impact creates a spreading crownlike splash, followed by a strong upward jet that eventually collapses back into the medium. At the same time, many of the impact characteristics are decidedly non-fluidic. Sand has no surface tension, so both the crown and the jet readily break up into small particles. The granular jet is very narrow and energetic, reaching heights greater than the impacter’s drop height. Interestingly, the column begins collapsing on its lower end before the jet even reaches its highest peak. This may be due to the lower energy of the sand particles that were ejected later in the crater formation process. (Video credit: J. Verschuur, B. van Capelleveen, R. Lammerink and T. Nguyen)

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    Vibrating Paint

    Paint is probably the Internet’s second favorite non-Newtonian fluid to vibrate on a speaker–after oobleck, of course. And the Slow Mo Guys’ take on it does not disappoint: it’s bursting (literally?) with great fluid dynamics. It all starts at 1:53 when the less dense green paint starts dimpling due to the Faraday instability. Notice how the dimples and jets of fluid are all roughly equally spaced. When the vibration surpasses the green paint’s critical amplitude, jets sprout all over, ejecting droplets as they bounce. At 3:15, watch as a tiny yellow jet collapses into a cavity before the cavity’s collapse and the vibration combine to propel a jet much further outward. The macro shots are brilliant as well; watch for ligaments of paint breaking into droplets due to the surface-tension-driven Plateau-Rayleigh instability. (Video credit: The Slow Mo Guys)

  • Bouncing Off The Surface

    Bouncing Off The Surface

    For the right angles and flow rates, it’s possible to bounce a fluid jet off a pool of the same fluid. As the jet flows, it pulls a thin layer of air with it, entraining the air. This air film is what keeps the jet separate from the pool when it initially hits. In the photo above, the jet is flowing right to left; notice how it maintains its integrity within the dimple during the bounce. The pool’s surface tension acts almost like a trampoline, redirecting the jet’s momentum into the bounce. It’s even possible to get a double bounce. In this video, the mechanism is the same, although the apparatus is different. In the photo above, the jet is introduced with a horizontal velocity to induce air entrainment and bouncing. In the video, the pool is spinning, which provides the necessary horizontal velocity between the jet and the liquid pool. (Photo credit: J. Bomber and T. Lockhart)

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    Avoiding Splashback

    Here’s a likely Ig Nobel Prize candidate from the BYU SplashLab: a study of splashing caused by a stream of fluid entering a horizontal body of water or hitting a solid vertical surface. In other words, urinal dynamics. The researchers simulated this activity using a stream of water released from a given height and angle and observed the resulting splash with high-speed video. They found a stream falls only 15-20 centimeters before the Plateau-Rayleigh instability breaks it into a series of droplets, and that this is the worst-case scenario for splash-back. The video above shows how a stream of droplets hits the pool, creating a complex cavity driven deeper with each droplet impact. Not only does each impact create a splash, the cavity’s collapse does as well. Similarly, when it comes to solid surfaces, they found that a continuous stream splashes less. They’ve also put together a helpful primer on the best ways to avoid splash-back. (Video credit: R. Hurd and T. Truscott; submitted by Ian N., bewuethr, John C. and possibly others)

    For readers attending the APS DFD meeting, you can catch their talk, “Urinal Dynamics,” Sunday afternoon in Session E9 before you come to E18 for my FYFD talk.

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    Fluid Juggling

    It’s that time of the year – the 2013 APS Division of Fluid Dynamics meeting is not far off, and entries to this year’s Gallery of Fluid Motion are starting to appear. This week we’ll be taking a look at some of the early video submissions, beginning with one that you can recreate at home. This video demonstrates a neat interaction between a slightly-inclined liquid jet and a lightweight ball. The jet can stably support–or, as the authors suggest, juggle–the ball under many circumstances, as seen in the video. Initially, the jet impacts near the bottom of the ball and then spreads into a thin film over the surface. This decrease in thickness between the jet and the film is accompanied by an increase in speed due to conservation of mass. That velocity increase in the film corresponds to a pressure decrease because of Bernoulli’s principle. This means that there is a region of higher pressure where the jet impacts the ball and lower pressure where the film flows around the ball. Just as with airflow over an airfoil, this generates a lift force that holds the ball aloft. (Video credit: E. Soto and R. Zenit)

  • Hydraulic Bumps

    Hydraulic Bumps

    If you’ve ever noticed the circular jump in your kitchen sink when you turn on the faucet, you’re familiar with what a jet does when it plunges into a horizontal layer of liquid. If the liquid is deep enough, the jet will perturb the surface into a circular depression, as in Figure (a) above. As the flow rate increases, a recirculating vortex ring and hydraulic bump forms (Figure b photo and flow schematic). At a critical flow rate, the bump will become unstable and form polygons instead of circles. At even larger flow rates, the system will shift toward a hydraulic jump, with a larger change in fluid elevation. Like bumps, these jumps can also appear in a variety of shapes. (Image credit: M. Labousse and J. W. M. Bush)