In the 1970s, the Voyager spacecraft discovered a hexagon near Saturn’s north pole that defied explanation for years. However, researchers have since simulated the shape in a laboratory by placing a fast-spinning ring on the top surface of a slowly spinning column of fluid. Fluorescent dye is used to visualize the flow pattern. #
Videos
Microfluidics
The field of microfluidics–where fluids are constrained to the sub-millimeter scale–is increasingly important in fields like chemistry, molecular biology, and microtechnology. At the microscale, surface tension often has greater effects than in our everyday world. This video shows how adding small amounts of a polymer drastically changes droplet breakup.
How Cats Drink
While humans use suction and dogs scoop water using their tongues*, cats use a dainty fluid mechanism to drink. Researchers used high-speed video to find that cats drink by touching the surface of their tongue to the water and drawing their tongue rapidly back into their mouth. Friction between their tongue and the water creates a fluid column about which the cat closes its jaw before gravity breaks off the column. They also built an artificial tongue to test different frequencies and found an optimal lapping frequency dependent upon the mass of the feline.
- Reis et al. in Science (11/11/10 edition)
- Wired article
- Scientific American article
*ETA: More recent research show that dogs actually use the same technique as cats, not a scooping method.
(Image credit: P. Reis et al.)
The Leidenfrost Effect
The Leidenfrost effect occurs when a liquid comes in contact with a mass significantly hotter than the liquid’s boiling point. Upon contact, a thin layer of the liquid will be vaporized, forming a lubricating gas layer that temporarily insulates the hot mass from the cold liquid. This effect is responsible for water skittering across a hot plate as well as protecting the hands of many a professor from a dunk in liquid nitrogen at the front of a classroom.
reblogged from fyeahchemistry:
(Thanks for the submission, singbird-sing!)
Flutter and the Tacoma Narrows Bridge
Sixty years ago yesterday the original Tacoma Narrows Bridge (a.k.a. Galloping Gertie) collapsed as a result of aeroelastic flutter during 42 mph winds. Flutter is a phenomenon in which the fluid dynamics and structural dynamics of a system are closely coupled, in this case resulting in a dramatic failure. The high sustained winds provided an energy source for self-excitation of one of the bridge’s torsional modes; as the bridge contorted, the motion caused additional vortices to be shed from the bridge deck, causing further vibrational forces on the bridge. For an analysis of the bridge’s collapse and its common misrepresentations, see Billah and Scanlan. The bridge’s spectacular collapse prompted reconsideration and redesign of the decks of modern suspension bridges.
Protecting an Egg with Oobleck
Using non-Newtonian fluids as “liquid armor” is an active area of research and development. Here students demonstrate the efficacy of shear-thickening as a defense against sudden impact by dropping a bag of oobleck containing a raw egg from different heights.
Morpho Towers
Artists Sachiko Kodama and Yasushi Miyajima use ferrofluids and magnetic fields to create their “Morpho Towers – Two Standing Spirals” artwork. Ferrofluids are formed from a suspension of ferrous material in a oil or water and are a popular subject for fluid dynamics as art. You can even make your own ferrofluid at home. (via ageekmom)
The No-Slip Condition
Viscosity plays an important role near surfaces in fluid mechanics. Friction between the fluid and the solid surface creates a “no slip” condition at the wall. In the video, dye injected near the wall remains there because there is little or no velocity of the fluid near the wall. As the dye filament is pulled away, the speed of the bulk flow–the freestream–is apparent. A strong velocity gradient exists between the wall and the freestream; this narrow region of changing velocity is called a boundary layer and is a major topic of research due to its importance in determining drag and thermal loads on vehicles.
Flow Visualization
This video, created by undergraduates as part of a fluid dynamics laboratory course, shows flow visualization of a von Karman vortex street in the wake of a cylinder in comparison to a computational fluid dynamics (CFD) simulation of the same phenomenon. If you’re wondering about the black-and-white segments and the peculiar speech patterns, look no further. The students are parodying a series of videos made by MIT in the 1960s that are still used in classrooms today.
Effects of Viscosity
[original media no longer available]
Today’s video demonstrates the effect of viscosity, which measures a fluid’s resistance to deformation. On the left is a column of highly viscous fluid; the fluids become less viscous as one moves right. When a jet of dye is released into the highly viscous fluid, the jet is very slow to penetrate, whereas, in the rightmost column, the dye expands quickly into a turbulent jet. Between these extremes, we see a laminar dye jet entering the liquid. The mushroom-like shape the laminar jet takes is the result of the Rayleigh-Taylor instability, which occurs when a denser fluid is on top of a lighter fluid in a gravitational field.
