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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Hydrophobic surfaces are great for creating some wild behaviors with water droplets, but they make neat effects with other liquids, too. The viscous honey in the first segment of this Chemical Bouillon video is a great example. Because the honey doesn’t adhere to the hydrophobic surface, the viscoelastic fluid does not maintain the form it had when drizzled on the surface. Instead, the honey contracts, with surface tension driving Plateau-Rayleigh-like instabilities that break the contracting ligaments apart to form nearly spherical droplets of honey on the surface.  (Video credit: Chemical Bouillon

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)

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)

Sharks have evolved some incredible fluid dynamical abilities. Instead of scales, their skin is covered in microscopic structures called denticles. To give you a sense of size, each denticle in the black and white image above is about 100 microns across. Denticles are asymmetric and overlap one another, creating a preferential flow direction along the shark. When water tries to move opposite the preferred direction, the denticles will bristle, like in the animation above. The bristled denticles form an obstacle for the reversed flow without any effort on the shark’s part. Since local flow reversal is an early sign of separation, researchers theorize that this bristling tendency prevents flow along the shark’s skin from separating. Keeping flow attached, especially along the shark’s tail, is vital not only to the shark’s agility but to keeping its drag low. Researchers have even begun 3D printing artificial shark skin to try and harness the animal’s hydrodynamic prowess. For much more shark-themed science, be sure to check out this week’s "Several Consecutive Calendar Days Dedicated to Predatory Cartilaginous Fishes" video series by SciShow, It’s Okay to be Smart, The Brain Scoop, Smarter Every Day, and Minute Physics. (Image credits: J. Oeffner and G. Lauder; A. Lang et al.; original video; jidanchaomian)

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)

Moving fluids around in microgravity can be a challenge. On Earth we experience buoyancy and other gravitational effects that dominate how fluids move. In space, on the other hand, the only options are to move fluids mechanically with pumps or fans or to use capillary action. Even on earth, adhesive forces between a liquid and its solid container can draw fluids in narrow tubes upward against the force of gravity. In microgravity, this capillary flow can be even more effective. But the best way to study and understand this flow regime is to do so in space. The Capillary Channel Flow experiment and similar studies have allowed astronauts on the space station and researchers back on Earth to explore the effects of capillary action on microgravity fluid transport. The results will be used to improve propulsion systems, heat exchangers, and life support systems used in space. (Photo credits: NASA, M. Dreyer et al., and A. Agrawala; submitted by jshoer)

Differences in viscosity or surface tension between two fluids can lead to finger-like instabilities. Here food dye placed on corn syrup forms narrow tendrils driven by the differing surface tensions of the two liquids. Similar dendritic shapes can be generated by injecting a low viscosity fluid into a high viscosity one (Saffmann-Taylor instability) or by pulling apart glass plates sandwiched around a high viscosity fluid. (Photo credit: T. Gaskill et al.)

A droplet falling onto a solid, dry surface seems like a simple situation, one that would be easy to understand. But splashes can be unpredictable.Velocity, viscosity, and surface tension all play clear roles, but the surrounding air also has an impact - drop the air pressure low enough and a droplet won’t splash. A new paper has tackled the problem, producing a mathematical model in agreement with experimental results. So why do some drops splash and others don’t? When a drop falls, its momentum flattens it into a pancake shape while surface tension struggles to hold it together. The spreading edge, called the lamella, can pull away from the surface. When it does, a pocket of high pressure forms beneath it due to lubrication effects, and the faster airflow over the top of the lamella creates a suction effect. This is analogous to a wing producing lift. Like the momentum that spread the droplet, the lift force pulls the lamella and ejecta sheet further up and outward, overcoming the restoring force of surface tension and tearing the droplet apart. For more on the effect, check out the research paper or this Inside Science article.  (Video credit: G. Riboux and J. Gordillo; via Inside Science)

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.)

Soap bubbles are ephemeral creations. The slightest prick will send them tearing apart in the blink of an eye. It may come as a surprise, therefore, that dropping a water droplet through a bubble will not break it. Instead, the bubble will heal itself using the Marangoni effect. In a soap bubble, the soap molecules act as a surfactant, lowering the surface tension of the water and allowing the fragile structure to hold together. When the water drop impacts the bubble, the local surface tension increases because of the relative lack of soap molecules. This increase in surface tension pulls at the rest of the bubble, drawing more soap molecules toward the point of contact. The effect evens out surface tension across the surface and stabilizes the bubble. You can test the effect at home, too. If you wet your finger, you can poke a soap bubble without popping it. (Video credit: G. Mitchell; via io9)

Pyrocumulus clouds tower tall above a wildfire in these photos taken last week from an Oregon National Guard F-15C. Most cumulus clouds form when the sun-warmed surface heats air, causing it to rise and carry moisture upward where it condenses to form clouds. In pyrocumulus clouds, the driving heat is supplied by a forest fire or volcanic eruption. The hot, rising air carries smoke and soot particles upward, where they become nucleation sites for condensation. Pyrocumulus clouds can be especially turbulent, and the gusting winds they produce can exacerbate wildfires. In some cases, the clouds can even develop into a pyrocumulonimbus thunderstorm with rain and lightning.  (Photo credit: J. Haseltine; via NASA Earth Observatory)