Some animals, like the common basilisk (a.k.a. the Jesus Christ lizard) are capable of running across water for short distances. The basilisk accomplishes this feat by slapping the water with sufficient force and speed to keep its body above the surface. This slap also creates a pocket of air around its foot. The lizard propels itself forward by kicking its leg back, then lifting its foot out of the water before the air bubble collapses. Water birds like the Western Grebe and tail-walking dolphins rely on similar physics to stay above the water line. # (submitted by Simon H)
Tag: fluid dynamics
Space Shuttle Flow Viz
When a space shuttle lands, a lucky few will hear twin sonic booms as it passes overhead. The double boom occurs due to the shock waves from in front of the shuttle and just behind it passing the observer on the ground. The colorized schlieren photograph above shows shock waves on a model of an early shuttle prototype. The fore and aft shocks that run from the craft to the ground are even clearer on this photo of a T-38 in flight. (Photo credit: Gary Settles)
Shock Waves from a Trombone
Shock waves emanating from a trombone have been captured on video for the first time using schlieren photography. With a harsh blast from the mouthpiece, it’s possible for pressure waves inside the trombone to build into a weak shock wave traveling about 1% faster than the speed of sound. It’s possible that musicians sitting in front of the trombones could receive hearing damage from these shock waves or similar ones from trumpets. # (submitted by jessecaps)
Microgravity Combustion
Combustion in microgravity is markedly different than that on earth, due to a lack of buoyant convection. The combustion of a droplet of heptane is shown here as a composite image. The bright yellow structure shows the path of the droplet, which gets smaller as it burns. The green structures show the initial development of soot, which eventually streams outward as long streaks. # (submitted by jshoer)
Viscous Fingers
The Saffman-Taylor instability occurs when a less viscous fluid is injected into a more viscous one, usually in a Hele-Shaw cell. Here oil paint and mineral spirits were painted onto flat surfaces that were pressed together before being pulled apart. The result is viscous fingering of the fluids. #
How Dogs Drink
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. #
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)
Flowing Up a Waterfall
Tea-drinking physicists found that it’s possible for particles to flow up a short (< 1 cm) waterfall to contaminate pure upstream sources. Their apparatus is shown above, along with an inset showing the velocity field on the surface of the channel. The blue arrows indicate flow downstream and the red arrows indicate counterflow that carries particles upstream. The researchers suspect that Marangoni effects may play a role in setting up the counterflow. The finding could have implications for pollution control and manufacturing. # (submitted by Gabe)
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.
