FYFD

Tag: fluid dynamics

  • The Wanderings of Micro-Scallops

    The Wanderings of Micro-Scallops

    In the 19th century, botanist Robert Brown observed pollen granules beneath his microscope jittering randomly. Einstein showed that this motion resulted from the impacts of much-smaller atoms against the particles. For small enough objects, the random walk of Brownian motion dominates their dynamics. A new study explores how flexible objects move at this Brownian scale.

    The researchers used trios of colloids — microscopic particles — held together by a lipid fluid layer that allows the three particles to change shape without losing contact. Essentially, each trio forms a tiny hinge. As atoms strike the colloids, they both move and change shape.

    Compared to rigid shapes, the researchers found their flexible hinges moved around in space about 3-15% faster. They also found coupling between the shape changes and motion. When the colloids hinge closed, it propels them in the direction the hinge points. Because this resembles the propulsion of scallops, the researchers refer to this as the “Brownian quasi-scallop mode.” (Image and research credit: R. Verweij et al.; via phys.org)

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    The Magic* Cork

    *Spoiler alert: it’s not magic. It’s science!

    Just what makes this dropped cork float beneath the surface? Just like a normal cork, it’s buoyancy! But this seemingly straightforward video is hiding a few key elements. Firstly, the cork has been modified; it has a metal sphere inside it so that its effective density is higher than that of water.

    Secondly, that liquid is not pure water; notice the hazy swirls near the bottom of the flask when the cork drops in? This is tap water that’s had a layer of salt dissolving in the bottom of it for the last day. That creates a density gradient with denser, salty water at the bottom and lighter, fresh water at the top. In fluid dynamics, we’d say the fluid is stably stratified; “stratified” meaning that there are distinct layers (strata) of different density and “stably” because the heavier ones are at the bottom.

    When the cork is dropped in, it settles at the fluid layer that matches its density. Because the surrounding fluid is stably stratified, poking the cork makes it bounce slightly but return to its initial height. Our atmosphere behaves just like this when it’s stably stratified. If you displace a parcel of air, it will oscillate up and down before settling back to equilibrium. In fact, the cork and the air even bounce at the same frequency! (Video and submission credit: F. Croccolo)

  • As the Fog Rolls In

    As the Fog Rolls In

    Although we talk about fog rolling in, it’s rare for us to have a perspective where we can truly appreciate that flow. But this photograph from Tanmay Sapkal provides just that for the low summer fogs sweeping over Marin, CA. When hot summer temperatures make inland air rise, cold, moist air from the ocean sweeps in to replace it. Once the moisture condenses, it forms thick, low clouds of fog that surge past the Golden Gate Bridge and into San Francisco Bay. (Image credit: T. Sapkal; via NatGeo)

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    Why Watering Globes Are Hard to Fill

    If you’re leaving home for a few days and want to keep your houseplants happy, you may have tried using a watering globe – those glass bulbs with long stems that slowly release water for your plant. And if you have used one, you’ve probably noticed what a pain it can be to fill. Pour water down the neck too quickly and you’ll get splashed by a sheet of water blown back at you.

    That splashback happens for the same reason that blowing across the top of a bottle plays an audible note: you’re compressing the air inside the container. When water tries to pour continuously down the watering globe’s neck, it can block the escape path needed by the air already in the globe. The increasing weight of water atop that volume of air compresses it, raising its pressure until it’s eventually high enough that it blows all the water back out the neck and into your face.

    The best method to ensure that doesn’t happen is to fill the globe slowly. Try tilting it at an angle and letting only a small stream of water fall into it such that there’s always an escape route for the air. (Image and video credit: E. Challita et al.)

  • Oil Drops and Filter Feeders

    Oil Drops and Filter Feeders

    Natural oils provide critical nutrients to filter feeders like zooplankton and barnacles. These creatures capture oil droplets on bristle-like appendages such as cilia and setae. But this droplet-catching turns into a disadvantage during petroleum spills, when capturing and ingesting oil can be lethal. A recent study looks at the fluid dynamics of oil droplet capture for these tiny creatures.

    The authors found that filter feeders capture a range of droplets regardless of size and oil viscosity. But not all droplets stay attached long enough to get consumed, and the larger a droplet is, the lower the flow velocity necessary to detach it from the animal. That suggests a method of limiting uptake of spilled petroleum into the marine food chain: use surfactants to break up the oil into droplets large enough that they’ll detach from filter feeders before getting eaten. (Image credit: D. Pelusi; research credit: F. Letendre et al.; submitted by Christopher C.)

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    Hot Ice, Buoyancy Tricks, and More DIY Fun

    Here’s a smorgasbord of DIY experiments from Dianna at Physics Girl. Some are fluidsy, some aren’t, but all of them give you a chance to stretch your science muscles at home. Personally, I think she saved the best for last with her laser-acoustics demo! (Video credit: Physics Girl)

  • Testing Granular Gas Theory

    Testing Granular Gas Theory

    When excited, a group of particles can behave much like a gas. These granular gases exhibit many similarities to molecular gases but contain one vital difference: without a constant input of energy, granular gases lose kinetic energy to collisions.

    Over the years, scientists have developed a special theory to describe the behaviors of granular gases, but most of its predictions could only be tested numerically. A new study used a microgravity experiment aboard a sounding rocket to physically test the theory.

    The experiment, shown above, consists of nearly 2800 magnetic particles, which the researchers could stir up using pairs of magnets. Once they shut off the magnets (which occurs at t=0 in the image above), the granular gas begins to “cool” as collisions sap away its energy. With this set-up, the researchers were able to confirm several key predictions of the granular gas theory. (Image and research credit: P. Yu et al.; via APS Physics)

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    Centrifugal Instability

    When it comes to geophysics, there are all kinds of phenomena that depend on rotation. In this short video, researchers demonstrate one such phenomena — the centrifugal instability — in a tank on a turn table. The experiment begins once the fluid in the tank is all rotating together, like a solid body would. Then, they reduce the rotation rate of the turn table. Almost immediately, we see rolls encircle the tank.

    The rolls form due to the difference in momentum between fluid in the interior and near the wall. Friction with the wall slows the fluid there down much faster than that in the middle of the tank. As the faster-moving fluid gets centrifuged outward, it forms rolls. As the video demonstrates, these rolls can be relatively uniform and laminar, or, with enough change in rotation rate, they can become turbulent. (Image and video credit: UCLA Spinlab)

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    Dendritic

    “What happens when two scientists, a composer, a cellist, and a planetarium animator make art?” The answer is “Dendritic,” a musical composition built directly on the tree-like branching patterns found when a less viscous fluid is injected into a more viscous one sandwiched between two plates.

    Normally this viscous fingering instability results in dense, branching fingers, but when there’s directional dependence in the fluid, the pattern transitions instead to one that’s dendritic. In this case, that directionality comes from liquid crystals, whose are rod-like shape makes it easier for liquid to flow in the direction aligned with the rods.

    For more on the science, math, and music behind the piece, check out this description from the scientists and composer. (Video, image, and submission credit: I. Bischofberger et al.)

  • Bright Volcanic Clouds

    Bright Volcanic Clouds

    Every day human activity pumps aerosol particles into the atmosphere, potentially altering our weather patterns. But tracking the effects of those emissions is difficult with so many variables changing at once. It’s easier to see how such particles affect weather patterns somewhere like the Sandwich Islands, where we can observe the effects of a single, known source like a volcano.

    That’s what we see in this false-color satellite image. Mount Michael has a permanent lava lake in its central crater, and so often releases sulfur dioxide and other gases. As those gases rise and mix with the passing atmosphere, they can create bright, persistent cloud trails like the one seen here. The brightening comes from the additional small cloud droplets that form around the extra particles emitted from the volcano.

    As a bonus, this image includes some extra fluid dynamical goodness. Check out the wave clouds and von Karman vortices in the wake of the neighboring islands! (Image credit: J. Stevens; via NASA Earth Observatory)