FYFD

Tag: buoyancy

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    Antibubble Vortex Rings

    Bubbles are familiar, but antibubbles are a bit more unusual. An antibubble typically has a liquid-air-liquid interface, with a thin shell of air separating a liquid droplet from the surrounding fluid. Although they look rather like bubbles, antibubbles behave differently. Antibubbles are, for example, very sensitive to pressure changes. A sinking antibubble like the one in the video above, experiences a higher pressure on its lower face. This pressure compresses the gas shell and thins it on the bottom. The air shell bursts at the thin point and the antibubble collapses, generating two vortex rings and a small, buoyantly rising bubble. (Video credit: S. Dorbolo et al.)

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    The Physics of Sneezing

    Sneezing can be a major factor in the spread of some illnesses. Not only does sneezing spew out a cloud of tiny pathogen-bearing droplets, but it also releases a warm, moist jet of air. Flows like this that combine both liquid and gas phases are called multiphase flows, and they can be a challenge to study because of the interactions between the phases. For example, the buoyancy of the air jet helps keep smaller droplets aloft, allowing them to travel further or even get picked up and spread by environmental systems. Researchers hope that studying the fluid dynamics and mathematics of these turbulent multiphase clouds will help predict and control the spread of pathogens. Check out the Bourouiba research group for more. (Video credit: Science Friday)

  • “Smoke”

    “Smoke”

    Ethereal forms shift and swirl in photographer Thomas Herbich’s series “Smoke”. The cigarette smoke in the images is a buoyant plume. As it rises, the smoke is sheared and shaped by its passage through the ambient air. What begins as a laminar plume is quickly disturbed, rolling up into vortices shaped like the scroll on the end of a violin. The vortices are a precursor to the turbulence that follows, mixing the smoke and ambient air so effectively that the smoke diffuses into invisibility. To see the full series, see Herbich’s website.  (Image credits: T. Herbich; via Colossal; submitted by @jchawner, @__pj, and Larry B)

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  • Supernova Explosion

    Supernova Explosion

    Type 1a supernovae occur in binary star systems where a dense white dwarf star accretes matter from its companion star. As the dwarf star gains mass, it approaches the limit where electron degeneracy pressure can no longer oppose the gravitational force of its mass. Carbon fusion in the white dwarf ignites a flame front, creating isolated bubbles of burning fluid inside the star. As these bubbles burn, they rise due to buoyancy and are sheared and deformed by the neighboring matter. The animation above is a visualization of temperature from a simulation of one of these burning buoyant bubbles. After the initial ignition, instabilities form rapidly on the expanding flame front and it quickly becomes turbulent. (Image credit: A. Aspden and J. Bell; GIF credit: fruitsoftheweb, source video; via freshphotons)

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    Lighting a Match

    The schlieren optical technique is ideal for visualizing differences in fluid density and is an important tool for revealing flows humans cannot see with their naked eyes. In this high speed video, a professor lights a match. The initial strike generates friction and heat sufficient to convert some of the red phosphorus in the match head to its more volatile white phosphorus form. We see this in the schlieren as the cloud-like burst in the first several seconds. The heat from the phosphorus combustion ignites the sulfur fuel and potassium chlorate oxidizer in the match head to create a more sustained flame. During this period, wavy, smoke-like whorls of hot air rise from around the flame as buoyancy takes over. The upward movement of hot air draws in cooler air from the surroundings, providing the flame with an ongoing source of oxygen and allowing it to grow.  (Video credit: RMIT University)

  • Champagne Bubble Physics

    Champagne Bubble Physics

    Champagne is well-known for its effervescence, but its tiny bubbles do more than affect your sensation when sipping. Champagne bubbles form when carbon dioxide dissolved in the wine nucleates along imperfections in the glass. Buoyancy causes them to flow upwards, growing as they pull more carbon dioxide from the surrounding champagne. When the bubbles reach the surface, they pop, sending an almost imperceptible fountain of tiny droplets into the air, as seen in the photo above. You can sometimes feel the droplets if you hold a glass near your face. The droplets released from the bursting champagne bubbles spread the aroma of the wine, imparting additional flavor through our olfactory sense. (Photo credit: F. Beaumont et al.)

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    Double-Diffusive Convection

    Convection can be driven several mechanisms, including temperature and concentration differences. The video above shows convection between a a layer of sucrose solution and a layer of saline solution. Initially, the lighter sucrose layer sits over the denser salt water. After the interface is perturbed, the differences in concentration – and thus in density – between the fluids causes diffusion both upward and downward in the form of fingers. This instability behavior is analogous to salt-fingering, which occurs in the ocean when a layer of warm, salty water lies over a layer of cooler, less saline water. In the ocean, these temperature and salinity differences help drive ocean circulation as well as the mixing that occurs between different depths. (Video credit: William Jewell College)

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    Balloons in the Car

    Destin from Smarter Every Day has just made a video on one of my favorite fluids brain teasers: what happens to a helium balloon when you accelerate in a car? Take a moment to think about the answer before watching or reading further…

    Okay, so what happens? Contrary to what you may expect, hitting the accelerator with a balloon in the car will make it shift forward. This is a matter of buoyancy. As Destin demonstrates with the water bottle, when two fluids are accelerated forward, the denser one will shift backwards, which pushes the lighter one forward. Because the helium is lighter than the air filling the car, accelerating pushes the air backward (just as it does the pendulum and the car’s inhabitants) and that shifting of the air pushes the helium in the balloon forward. (Video credit: Smarter Every Day)

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    Convection Cells

    Human eyesight is not always the best for observing how nature behaves around us. Fortunately, we’ve developed cameras and sensors that allow us to effectively see in wavelengths beyond those of visible light. What’s shown here is a frying pan with a thin layer of cooking oil. To the human eye, this would be nothing special, but in the infrared, we can see Rayeigh-Benard convection cells as they form. This instability is a function of the temperature gradient across the oil layer, gravity, and surface tension. As the oil near the bottom of the pan heats up, its density decreases and buoyancy causes it to rise to the surface while cooler oil sinks to replace it. Here the center of the cells is the hot rising oil and the edges are the cooler sinking fluid. The convection cells are reasonably stable when the pan is moved, but, even if they are obscured, they will reform very quickly.  (Video credit: C. Xie)

  • Sneezes Vs. Coughs

    Sneezes Vs. Coughs

    Sneezing and coughing are major contributors to the spread of many pathogens. Both are multiphase flows, consisting of both liquid droplets and gaseous vapors that interact. The image on the left shows a sneeze cloud as a turbulent plume. The kink in the cloud shows that plume is buoyant, which helps it remain aloft. The right image shows trajectories for some of the larger droplets ejected in a sneeze. Like the sneeze cloud, these droplets persist for significant distances. The buoyancy of the cloud also helps keep aloft some of the smaller pathogen-bearing droplets. Researchers are building models for these multiphase flows and their interactions to better predict and counter the spread of such airborne pathogens. For similar examples of fluid dynamics in public health, see what coughing looks like, how hospital toilets may spread pathogens, and how adjusting viscoelastic properties may counter these effects. For more about this work, see the Bourouiba research group’s website. (Image credit: L. Bourouiba et al.)