We are used to associating convection with differences in temperature, but what’s actually necessary for a Rayleigh-Taylor-type instability is a density variation (and a gravitational field). The solutal convection seen above when mixing liqueur with cream is caused by the interaction of density and surface tension. When the alcohol of the liqueur mixes with the cream, it forms a less dense alcohol-cream that tries to rise to the surface. The alcohol also breaks the surface tension of the cream, causing it to contract and open cells where the alcohol surfaces. As the alcohol evaporates, the alcohol-cream mixture gets denser and sinks back down where it can pick up more alcohol and start the process again. (via jshoer and io9)
Search results for: “art”
The White Hole in Your Sink
Ever notice the distinctive ring that forms in your kitchen sink when you turn the water on? This phenomenon is known as a hydraulic jump; it occurs when a fast moving fluid (the water just discharged from the faucet) runs into a slow moving fluid (the water that’s been sitting in the sink) and transfers some of its kinetic energy into potential energy by increasing its elevation. Researchers have recently shown that this everyday occurrence is actually a physical analog to a white hole, the cosmological inverse of a black hole. (In theory, a white hole cannot be entered, but light and matter can escape it.) Check out Wired’s article for an explanation of the clever experiment that showed the equivalence of the two. #
Fluidized Sand
What’s shown in this video are some pretty spectacular demonstrations of fluidization, where a gas is introduced at the bottom of a bed of granular particles–like sand. At the critical gas velocity, the aerodynamic forces exerted on each particle by the gas will balance the gravitational force on the particle and it will become suspended. All of a sudden, the macroscopic behavior of the solid particles will be like that of a fluid; you can even make it “boil”!
The Silence of Owls
Owls are among the most silent hunters in nature, thanks to their feathers. The leading edge of the wing, shown in the bottom part of the photo, has a serrated comb-like edge, which breaks flow over the wing into small vortices, which are quieter than larger ones. The fringe-like trailing edge breaks the flow up further and helps absorb the sound produced by the turbulence. The fluffy feathers along the owl’s body can also help muffle noise. Researchers are investigating ways to use these techniques to quiet aircraft. # (via jshoer)
Canon Sound Sculptures
In a new series of ads for Canon, colorful paints are placed on a speaker cone and filmed at high speed to create beautiful “sound sculptures”. Paint, like oobleck, is a non-Newtonian fluid but does not react the same when excited by sound because it is shear-thinning. (When painting, you want the paint to run off the brush easily but not drip when it’s on the wall; hence, shear-thinning.) Both the photos and videos are lovely examples of fluid mechanics as art. Watch how they did it. # (Via jshoer, @ftematt, @JetForMe)
Jet-Based Control
Researchers have flown the first aircraft designed to maneuver without conventional control surfaces like ailerons and flaps. Instead of changing the wing geometry to alter the lift on different parts of the craft, the UAV uses strategically placed jets of air along the wing to control its flight. The plane can also alter the direction of its thrust, not by turning the nozzle as is conventionally done, but by modifying the thrust vector by directing and firing a secondary jet into the exhaust. #
Wind Turbines and Weather
A new study reports that wind turbine farms may be changing local surface temperatures, resulting in warmer temperatures at night and cooler temperatures during the day. The result is neither surprising nor new; the motion of the propellers increases the turbulence downstream of the turbines. Turbulent flow mixes much better than laminar flow, so air from above the ground is getting mixed into surface air in the wakes. At night, the air next to the ground cools more quickly than air higher up, so the mixing of higher, warmer air results in localized warmer air on the ground. Orange farmers use this effect when they put out fans at night to keep their crops from freezing. #
Wingtip Vortices in Ground Effect
In this flow visualization, wingtip vortices from an aircraft have been simulated using an apparatus with a couple of flaps that snap together like a book closing. Dye is pooled on the “ground” below the flaps and gets entrained by the vortices and lit up using laser light. The red vortices are the primary vortex generated by the aircraft wingtips and the green ones are secondary vortices generated by interaction with the ground. The lower half of the picture is a reflection off the ground. This photo was part of the 2009 Gallery of Fluid Motion. #
Wild Ferrofluid Shapes
In this fluid experiment, a ferrofluid–typically a liquid with a suspension of ferric particles–is exposed to rotating magnetic fields, which results in some wild shapes. #
Human-Powered Ornithopter
A team at the University of Toronto has flown the world’s first human-powered ornithopter, an aircraft that flies by flapping its wings like a bird. The concept dates back all the way to Da Vinci in the 15th century. Part of why it’s taken centuries to realize the dream is that bird flight is much more complicated than simply flapping up and down. Flapping a wing up and down will produce lift equally upward and downward. In order to create usable lift and thrust, it’s necessary to change the angle of attack during each stroke by twisting the wing while flapping. Watch the U of T craft carefully, and you can see this happening. #