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"

Zesting the skin of a citrus fruit like oranges releases a spray of tiny oil droplets. Citrus oil has several volatile components, meaning that it evaporates quickly at room temperature. It is also a liquid with a relatively low flash point, meaning that only modest temperatures (~40-60 degrees Celsius) are needed to generate enough vapor to ignite a vapor/air mixture. With volatile and flammable liquid fuels, a spray of droplets is an ideal platform for combustion because the essentially spherical droplets have a high surface area from which they can evaporate and provide vaporous fuel.  (Video credit: ChefSteps)

Atomization is the process of breaking a liquid into a spray of fine droplets. There are many methods to accomplish this, including jet impingement, pressure-driven nozzles, and ultrasonic excitement. In the images above, a drop has been atomized through vibration of the surface on which it rests. Check out the full video. As the amplitude of the surface’s vibration increases, the droplet shifts from rippling capillary waves to ejecting tiny droplets. With the right vibrational forcing, the entire droplet bursts into a fine spray, as seen in the photo above. The process is extremely quick, taking less than 0.4 seconds to atomize a 0.1 ml drop of water. (Photo and video credit: B. Vukasinovic et al.; source video)

Rockets often utilize liquid propellants for their combustion. To maximize the efficiency during burning, the liquid fuel and oxidizer must mix quickly and break up into an easily vaporized spray. One method to achieve this is to inject the fuel and oxidizer as liquid jets that collide with one another. For high enough flow rates, this creates a highly unstable liquid sheet that quickly atomizes into a spray of droplets. The animation above shows an example of two impinging jets, but rockets using this method would typically have more than just two injection points. Other rockets use co-axial or centrifugal injectors to mix and atomize the fuel and oxidizer prior to combustion.  (Image credit: C. Inoue; full-scale GIF)

Rockets often utilize liquid propellants for their combustion. To maximize the efficiency during burning, the liquid fuel and oxidizer must mix quickly and break up into an easily vaporized spray. One method to achieve this is to inject the fuel and oxidizer as liquid jets that collide with one another. For high enough flow rates, this creates a highly unstable liquid sheet that quickly atomizes into a spray of droplets. The animation above shows an example of two impinging jets, but rockets using this method would typically have more than just two injection points. Other rockets use co-axial or centrifugal injectors to mix and atomize the fuel and oxidizer prior to combustion.  (Image credit: C. Inoue; full-scale GIF)

Photographers Cassandra Warner and Jeremy Floto produced the "Clourant" series of high-speed photographs of colorful liquid splashes. The artists took special care to disguise the origin of splashes, making them appear like frozen sculptures. The photos are beautiful examples of making fluid effects and instabilities. Many of them feature thin liquid sheets with thicker rims just developing ligaments. In other spots, surface tension has been wholly overcome by momentum’s effects and what was once ligaments has exploded into a spray of droplets. (Photo credit: C. Warner and J. Floto; submitted by jshoer; via Colossal)

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.)

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. To learn more, see the Bourouiba research group’s website.(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.)