FYFD

Search results for: “jet”

  • Recreating Volcanic Lightning

    Recreating Volcanic Lightning

    Some natural phenomena, like volcanic eruptions or tornado formation, don’t lend themselves to fieldwork — at least not at the height of the action. The danger, unpredictability, and destructiveness of these environments is more than our equipment can survive. And so researchers find clever ways to recreate these phenomena in controllable ways. The latest example comes from a lab in Germany, where researchers are recreating volcanic lightning.

    To do so, they heat and pressurize actual volcanic ash in an argon environment and let the mixture decompress into a jet, like a miniature eruption. The lightning that accompanies the jet is thought to depend on friction between ash particles, which build up electric charges when rubbed, just like a balloon rubbed against one’s hair. When the charges get large enough, lightning discharges the build-up.

    Small-scale experiments like this one allow researchers to vary the temperature and water content of the ash and observe how this changes the lightning. Drier ash generates more lightning, but it’s hard to distinguish whether this is inherent to the ash or the result of the denser jets that form without the added eruptive force of steam. (Image credit: eruption – M. Szeglat, lab lightning – Sönke Stern/Ludwig-Maximilians-Universität München/Gizmodo; research credit: S. Stern et al.; via Gizmodo)

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    Waltzing Defects

    Liquid crystals are a peculiar state of matter with both liquid and crystalline properties. In this video, a microfluidic device breaks water into droplets surrounded by a shell of liquid crystal. Because the molecular structure of the liquid crystals is helical and cannot pack neatly in a spherical shell, there are visible defects in the liquid crystal shells. Given time, those defects can merge as the liquid crystal shell thickens. (Image and video credit: The Lutetium Project)

  • Events

    If you’re interested in scheduling an event, please contact [email protected].

    Upcoming Events

    Check the links below for specific event details.

    • 27 March 2020 – Online – Science Talk ’20

    Selected Past Events

    Slides from past events and talks are typically archived here. Past FYFD webcasts are available to FYFD patrons on Patreon.

    • Mar. 2016 – Online, The Internet – FYFD Webcast with Prof. Tadd Truscott and Ph.D. student Randy Hurd – Watch the webcast here
    • Apr. 2016 – Norton, MA – Wheaton College, APS New England Section Meeting, Banquet Keynote Address, “The Extremes of Fluid Dynamics”, slides here
    • Apr. 2016 – Online, The Internet – FYFD Webcast with Prof. Geoffrey Collins and NASA JPL mission operations engineer Keri Bean
    • Sep. 2016 – Cambridge, MA – 2016 Ig Nobel Prize Ceremony – Airplane Deluge Announcer and 24/7 Lecturer
    • Mar. 2017 – Pasadena, CA – “Celebrating the Beauty and Diversity of the Flow,” NASA’s Jet Propulsion Laboratory
    • Nov. 2017 – Denver, CO – APS DFD Talk, “Getting Into Science Communication,“ slides here
    • Oct. 2018 – Delft, The Netherlands – Workshop, “Communicating Your Science: Reaching Beyond Your Peers”
    • Nov. 2018 – Atlanta, GA – APS DFD Talk, “Tips for Connecting with Broader Audiences,” slides and video
    • Feb. 2019 – Denver, CO – Keynote speaker, ASME Students in Industry Day
    • Mar. 2019 – Washington, DC – “Communicating Science in the 21st Century,” United States National Committee on Theoretical and Applied Mechanics.
    • Nov. 2019 – Seattle, WA – APS DFD 2019; including a panel, a regular talk, and an invited talk
    • Jan. 2020 – Boston, MA – Northeastern
    • Jan. 2020 – Chicago, IL – ORDCamp
  • Bay of Fundy Tides

    Bay of Fundy Tides

    Canada’s Bay of Fundy has some of the wildest tidal flows in the world. Every six hours, the flow direction through the strait shifts and tidal currents rise to several meters per second. This creates distinct jets a couple kilometers long that pour from one side of the strait to the other. 

    What you see here is a numerical simulation of the flow using a technique called Large Eddy Simulation (or LES, for short). It’s one method used by fluid dynamicists to model turbulent flows without taking on the complexity of the full Navier-Stokes equations. At large lengthscales, like those of the jets and eddies we see above, LES uses the exact physics. But when it comes to the smaller scales – like the flow nearest the shores or the bottom of the strait – the simulation will approximate the physics in order to make calculations quicker and easier. Models like these make large-scale problems – including modeling our daily weather patterns – possible. (Image credit: A. Creech, source)

  • Champagne’s Shock Wave

    Champagne’s Shock Wave

    The distinctive pop of opening a champagne bottle is more than the cork coming free. The sudden release of high-pressure gas creates a freezing jet that’s initially supersonic. It even creates a Mach disk, like those seen in rocket exhaust. That supersonic flow can only be maintained, though, with a large enough pressure difference between the gas in the bottle and the atmosphere outside. Once the pressure drops below that critical point, the jet slows down and becomes subsonic. For more on champagne popping and its colorful plume, check out this previous post. (Image and research credit: G. Liger-Belair et al.; via Nature; submitted by Kam-Yung Soh)

  • The Impressive Take-Off of Pigeons

    The Impressive Take-Off of Pigeons

    One reason that peregrine falcons are such amazing fliers is that their prey, pigeons, are no slouches in flight, either. Able to take off vertically and accelerate to 100 kph in two seconds, pigeons are pint-sized powerhouses. With this high-speed video, BBC Earth highlights the mechanics of this vertical take-off. Pigeons begin by bending their legs and jumping high enough that their first downstroke can extend fully and still clear the ground. That gives them a headstart on generating the force they need to propel themselves upward. 

    Note the way the pigeon’s wings move, sweeping from directly behind the bird’s back to a full extension in front of it. With the bird moving vertically, this motion tells us that it’s thrust – not aerodynamic lift – from the wingstroke that’s powering this take-off. In that sense, the pigeon is something like a Harrier jet, using the thrust of air downward to take off vertically. (Image and video credit: BBC Earth)

  • Cavitation Collapse

    Cavitation Collapse

    The collapse of a bubble underwater doesn’t seem like a very important matter, but when it happens near a solid surface, like part of a ship, it can be incredibly destructive. This video, featuring numerical simulations of the bubble’s collapse, shows why. 

    When near a surface, the bubble’s collapse is asymmetric, and this asymmetry creates a powerful jet that pushes through the bubble and impacts the opposite side. That impact generates a shock wave that travels out toward the wall. As the bubble hits its minimum volume, a second shock front is generated. Both shock waves travel toward the wall and reflect off it, generating high pressure all along the surface. (Image and video credit: S. Beig and E. Johnson)

  • Breaking Up

    Breaking Up

    The dripping of a faucet and the break-up of a jet into droplets is universal. That means that the forces – the inertia of the fluid, the capillary forces governed by surface tension, and the viscous dissipation – balance in such a way that the initial conditions of the jet – its size, speed, etc. – don’t matter to the process of break-up. 

    We’d expect that the inverse situation – the breakup of a gas into bubbles in a liquid – would be similarly universal, but it’s not. When unconfined bubbles pinch off, the way they do so is heavily influenced by initial conditions. But that changes, according to a new study, if you confine the gas to a liquid-filled tube before pinch-off. Confinement forces a different balance between viscous and capillary effects, one which effectively erases the initial conditions of the flow and restores universality to the pinch-off process. (Image and research credit: A. Pahlavan et al.; via phys.org)

  • Catching Fire

    Catching Fire

    Citrus fruits like oranges house tiny pockets of oil in their peels. When squeezed, the oils jet out in tiny micro-jets that are about the width of a human hair. Despite their small size, the jets reach speeds of about 30 m/s and quickly break into a stream of droplets. When exposed to the flame of a lighter, like in the animation above, those microdroplets combust easily, creating a momentary fireball used to augment some cocktails. For more on how the citrus peel generates these jets, check out this previous post. (Image credit: Warped Perception, source; research credit: N. Smith et al.)

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    Reducing the Force of Water Entry

    As anyone who’s jumped off the high board can tell you, hitting the water involves a lot of force. That’s because any solid object entering the water has to accelerate water out of its way. This is why gannets and other diving birds streamline themselves before entering the water. But even for non-streamlined objects, like a sphere, there are ways to reduce the force of impact.

    This video explores three such techniques, all of which involve disturbing the water before the sphere enters. In the first, the sphere is dropped inside a jet of fluid. Since the jet is already forcing water down and aside when the sphere enters, the acceleration provided by the sphere is less and so is the force it experiences.

    The second and third techniques both rely on dropping a solid object ahead of the one we care about. In the second case, a smaller sphere breaks the surface ahead of the larger one, allowing the big sphere to hit a cavity rather than an undisturbed surface. Like with the jet, the first sphere’s entry has already accelerated fluid downward, so there’s less mass that the bigger sphere has to accelerate, thereby reducing its impact force.

    In the third case, the first sphere is dropped well ahead of the second, creating an upward-moving Worthington jet that the second sphere hits. In this case, there’s water moving upward into the sphere, so how could this possibly reduce the force of entry? The key here is that the water of the jet wets the sphere before it enters the pool. Notice how very little air accompanies the second sphere compared to the first one. That’s because the second sphere is already wet. It’s also been slowed down by the jet so that it enters the water at a lower speed, all of which adds up to a lower force of entry. (Image and research credit: N. Speirs et al.)

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