Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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Posts tagged "atomization"

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. (Image credit: L. Bourouiba et al.)

Much attention ahead of the Sochi Winter Olympics has been dedicated to the question of how this subtropical resort town would provide and maintain adequate snow cover for the Games. Officials promised a combination of natural snow, snow transported from elsewhere, snow stored from the previous year, and, of course, artificial snow. These days many ski resorts rely heavily on snow guns producing artificial snow. There are two main types of snow gun—those which use compressed air and those which have an electrically-driven fan—but the principles behind each are the same. The snow guns provide a continuous spray of air and water, atomizing the water into tiny droplets which freeze rapidly. The effectiveness of snow guns depends on both the temperature and humidity of the surrounding air. With sufficiently dry air, artificial snow can be made even several degrees above freezing. Sochi itself is relatively humid (72% on average for February), but most of the outdoor events are held in Krasnaya Polyana, higher in the mountains where temperatures are typically much lower and artificial snow can be manufactured. That said, temperatures have reached as high as 15 degrees Celsius during the Games so far, and athletes have complained about the changing snow conditions in several events. (Video credit: On The Snow)

FYFD is celebrating #Sochi2014 with a look at the fluid dynamics of the Winter Games. Check out our previous posts, including how lugers slide fast, how wind affects ski jumpers, and why ice is slippery.

Champagne owes much of its allure to its tiny bubbles. Unlike other wines, champagne undergoes a secondary fermentation in the bottle, during which the yeasts in the wine consume sugars and produce carbon dioxide, which dissolves into the wine. When opened, the carbon dioxide can begin to escape. Bubbles form in the glass around imperfections, either due to intentional etching of the glass or impurities left behind by cleaning. Once formed, trails of bubbles rise to the surface, swelling as more dissolved carbon dioxide is absorbed into each bubble. The bubbles then cluster near the surface of the champagne, occasionally popping and creating a flower-like distortion of the surrounding bubbles. The gases within the bubbles contains higher concentrations of aromatic chemicals than the surrounding wine, and the bursting of each bubble propels tiny droplets of these aromatics upwards, carrying the scent of the champagne to the drinker. For more beautiful champagne photos, I recommend this LuxeryCulture article; for more on the science of champagne, see Chemistry World’s coverage. Happy 2014! (Image credits: G. Liger-Belair et al.)

Hospital-acquired infections are a serious health problem. One potential source of contamination is through the spread of pathogen-bearing droplets emanating from toilet flushes. The video above includes high-speed flow visualization of the large and small droplets that get atomized during the flush of a standard hospital toilet. Both are problematic for the spread of pathogens; the large droplets settle quickly and contaminate nearby surfaces, but the small droplets can remain suspended in the air for an hour or more. Even more distressing is the finding that conventional cleaning products lower surface tension within the toilet, aggravating the problem by allowing even more small droplets to escape. (Video credit: G. Traverso et al.)

Nearly everyone has faced the frustration of a shower curtain billowing inwards to stick to one’s leg. Various explanations have been offered to explain the effect, but David Schmidt won the 2001 Ig Nobel Prize in Physics for a numerical simulation suggesting that the spray of droplets from the shower head drives a horizontal vortex whose axis of rotation is perpendicular to the shower curtain. Since vortices have a low-pressure region in their core, this weak shower vortex has the power to suck a light curtain inward, much to the chagrin of the shower’s occupant. Of course, a heavier or weighted shower curtain will help avoid the effect. This post is part of a series on fluids-related Ig Nobel Prizes. (Photo credit: W. Taylor; research credit: D. Schmidt)

Instabilities in fluids are sometimes remarkable in their uniformity. Here we see a hollow spinning cup with a thin film of fluid flowing down the interior. The rim of fluid at the cup’s lip stretches into long, evenly spaced, spiraling threads. These filaments stretch until centrifugal forces overcome surface tension and viscous forces and break the liquid into a multitude of tiny droplets. This process is called atomization and is vital to everyday applications like internal combustion and inkjet printing. (Photo credit: R. P. Fraser et al.)

Originally posted: 24 Aug 2011 That soap bubbles burst in the blink of an eye is a pity considering how fascinating their disappearing act is. This photo set from photographer Richard Heeks captures the bubbles mid-burst. Once the bubble’s film is breached, surface tension rips the smooth film back like a broken balloon, causing the liquid that used to be part of the bubble to erupt into droplets. (Photo credit: Richard Heeks)

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One of the most dangerous stunts for any fire-eater is breathing fire. Dr. Tim Cockerill explains some of the science behind the feat in this video. Volatility—the tendency of the liquid fuel to vaporize—is actually the enemy of a fire-eater. Use a fuel that is too volatile and it will catch fire too easily when the vaporous fuel mixes with the air. Instead fire-eaters use less volatile fuels and spray a mist of fine droplets to mix the air and fuel. This atomization of the fuel creates a spectacular fireball without endangering the fire-eater (as much). To see a similar fireball in high-speed, check out this post. (Video credit: T. Cockerill/The Ri Channel; via io9)

Bioaerosols—tiny airborne fluid droplets generated by coughing or sneezing—are a major concern for the spread of contagions like influenza. It may be possible, however, to mitigate some of these effects by manipulating biological fluid properties. The video above shows an experimental model of a cough, complete with the generation of bioaerosols from some fake human lung mucus. Contrast this with a cough where the model’s mucus has been treated to increase its viscoelasticity. The treated mucus generates substantially fewer droplets during a cough. The results suggest that drugs that increase viscoselasticity of biofluids may help stem the spread of disease. (Video credit: K. Argue et al.; research credit: M. D. A. Hasan et al.)

When two liquid jets collide, they can form an array of shapes ranging from a chain-like stream or a liquid sheet to a fishbone-type structure of periodic droplets. This series of images show the collision of two viscoelastic jets—in which polymer additives give the fluids elasticity properties unlike those of familiar Newtonian fluids like water. The jet velocities increase with each image, changing the behavior from a fluid chain (a and b); to a fishbone structure (c and d); to a smooth liquid sheet (e); to a fluttering sheet (f and g); to a disintegrating ruffled sheet (h), and finally a violently flapping sheet (i and j). The behavior of such jets is of particular interest in problems of atomization, where it can be desirable to break an incoming stream of liquid up into droplets as quickly as possible. (Photo credit: S. Jung et al.)