Not long ago, researchers showed that cats use friction to their advantage when drawing liquids into their mouths. New research shows that dogs rely on the same mechanism–they’re just far less efficient with it. The dog touches its backwards-curled tongue to the surface of the water; when it draws the tongue back, friction causes a column of fluid to follow. The dog then closes its jaws around the water. Some water also gets picked up by the back of the tongue, but since dogs have no cheeks, it spills out the sides, creating a mess familiar to dog owners. #
Videos
Volcanic Ash Plume
Video footage of Iceland’s Grimsvotn volcano erupting shows a massive turbulent plume of ash. The largest scales of the plume are of the order of hundreds, if not thousands of meters, and the eddies of the plume appear to move very slowly, especially far from the base. According to Kolmogorov, however, at the smallest scales of the flow (< 1 mm), the turbulent motions are isotropic. No one has been able to achieve Reynolds numbers high enough to fully prove or disprove Kolmogorov’s hypothesis, but natural events like volcanic eruptions produce some of the largest Reynolds numbers on earth. (See also: interview with videographer; via Gizmodo, jshoer)
Ferrofluid Self-Organization
The behavior of a ferrofluid subject to magnetic fields can be fascinating. Here a ferrofluid is subjected to a permanent magnet and thinner is added to the ferrofluid. As it spreads outward, the thinner carries ferrofluid with it. The thinner evaporates, increasing the concentration of ferrofluid in the outer ring and eventually forming peaks of ferrofluid that move inward toward the main body due to the attraction of the magnet. Near the main body, the peaks are repelled by the ferrofluid because they have the same magnetic orientation.
Feathering on SpaceShipTwo
Virgin Galactic and Scaled Composites recently performed their first feathered flight with SpaceShipTwo, which is on track to be the first commercial spaceship. Feathering is a re-entry technique devised by Scaled Composites founder Burt Rutan:
Once out of the atmosphere the entire tail structure of the spaceship can be rotated upwards to about 65º. The feathered configuration allows an automatic control of attitude with the fuselage parallel to the horizon. This creates very high drag as the spacecraft descends through the upper regions of the atmosphere. The feather configuration is also highly stable, effectively giving the pilot a hands-free re-entry capability, something that has not been possible on spacecraft before, without resorting to computer controlled fly-by-wire systems. The combination of high drag and low weight (due to the very light materials used to construct the vehicle) mean that the skin temperature during re-entry stays very low compared to previous manned spacecraft and thermal protection systems such as heat shields or tiles are not needed. During a full sub-orbital spaceflight, at around 70,000ft following re-entry, the feather lowers to its original configuration and the spaceship becomes a glider for the flight back to the spaceport runway. #
Though it works well for decelerating from sub-orbital speeds, feathering is sadly not useful for orbiting spacecraft due to the much higher kinetic energies that have to be dissipated.
Gelatin
Gelatins are actually colloidal gels, or a liquid dispersed inside a solid, cross-linked network. The crosslinks give the gelatin structure, but much of its dynamic behavior remains reminiscent of fluid motion.
Rayleigh-Taylor Art
The Rayleigh-Taylor instability occurs when a denser fluid lies atop a lighter fluid (relative to the gravitational field). The interface between the fluids deforms and the two fluids form finger-like protrusions that turn into mushroom caps and mix the dissimilar fluids together. This video, though based on a 2D Rayleigh-Taylor instability numerical simulation, was actually part of an art exhibit. (submitted by Mark S)
Personally, I recommend putting together a playlist of your favorite late 60s/early 70s rock (Pink Floyd, late Beatles, Jimi Hendrix, etc.) and sticking it on in the background while you watch the video in HD. It’s totally worth the 15 minutes. Especially in the later stages of each segment, the mixing between fluid layers really brings to mind cloud patterns on Jupiter or Saturn.
Supersonic Bullet
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This video shows a CFD simulation of a bullet passing through a parallel channel at Mach 2. The simulation captures 3 milliseconds of real-time and shows the Mach number in the top view and the temperature in the bottom view. Note how the bow shock near the front of the bullet and the trailing shock behind it reflect off the walls of the channel and interact. Even though the calculation is inviscid, the shock waves cause intense heating (white) in front of and behind the bullet.
Cloud Ocean
Time-lapse photography is great for capturing the fluid motion of clouds over the course of a day.
Giant Water Balloon Physics
Playing with a giant water balloon and high-speed cameras is like a giant experiment in surface tension, right up until the tensile strength of the balloon comes into play. The rippling in the balloon is reminiscent of the motion of droplet breakup or impact on superhydrophobic surfaces. (submitted by Daniel B)
DIY Non-Newtonian Fluids
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We’ve featured the non-Newtonian fluid oobleck here before, but it bears repeating as a fun and easy exercise for anyone to do at home or at school, especially with kids. For extra fun, try vibrating it, using it as liquid armor, or filling a pool and walking on it.