Stuck here on Earth, it’s hard to know sometimes how greatly gravity affects the behavior of fluids. Fortunately, astronaut Don Pettit enjoys spending his free time on the International Space Station playing with physics. In his latest video, he shows some awesome examples of what is possible with a thin film of water–not a soap film like we make here on Earth–in microgravity. He demonstrates vibrational modes, droplet collision and coalescence, and some fascinating examples of Marangoni convection.
Tag: science
Soap Film Breakup
This high-speed video shows a soap film formed across two rings and its deformation and breakup as the two rings are pulled apart. As the rings get further apart, surface tension deforms the soap film until the distance is too great to continue sustaining that shape. The film breaks into two–a sheet of soap film in each ring–and a little satellite bubble. Note the similarities in breakup between this soap film and a thin liquid column or water from a faucet.
Colliding Jets
Two jets colliding can form a chain-like fluid structure. With increasing flow rate, the rim of the chains becomes wavy and unstable, forming a fishbone structure where droplets extend outward from the fluid sheet via tiny ligaments. Eventually, the droplets break off in a pattern as beautiful as it is consistent. (Photo credits: A. Hasha and J. Bush)
Examples of Flutter
Aeroelasticity is the study of the interaction of structural and aerodynamic forces on an object, and its most famous example is flutter, which occurs when the aerodynamic forces on an object couple with its natural structural frequencies in such a way that a violent self-excited oscillation builds. What does that mean? Take a look at the video above. This compilation shows examples of flutter on wind tunnel models, road signs, airplanes, and the Tacoma Narrows Bridge–one of the most famous examples of all time. When air moves over and around an object, like a stop sign, it exerts forces that cause the structure to twist or vibrate. Those vibrations then alter the airflow around the object, which changes the aerodynamic forces on the object. If the motion of the object increases the aerodynamic forces which then increase the oscillation, then a potentially destructive flutter cycle has been created. Flutter is very difficult to simulate computationally, so tests are usually performed experimentally to ensure that any vibrations in the system will damp out rather than grow to the point of structural failure like many of the examples in the film.
Vortices on an Airliner
Wingtip vortices form on airplanes due to the finite length of their wings. In general, lift on the wings results from low-pressure, high-velocity air moving over the top of the wing and high-pressure, low-velocity air moving below the wing. Near the wingtips, the high-pressure air is able to slip around the edge to the top of the wing, generating a vortex that then trails behind the airplane. The same thing is occurring in the video above, except the edges of the wing’s control surfaces are serving as the tip of the wing. Similar vortices also exist at the wingtips, but they are not made visible by condensation as the aileron vortices are.
Cloud Swirls
Two interesting sets of clouds are featured in this satellite photo of the Canary Islands and the coast of Africa. In the upper part of the picture, closed cell stratocumulus clouds cover the ocean. As the wind drives these clouds over the islands, their pattern is disturbed by mountains that force the lower layers of air up and around, forming von Karman vortices and wakes that mingle and twist the cloud patterns to the south of the islands. (Photo credit: European Space Agency; via Wired)
Ferrofluid
[original media no longer available]
The motion of ferrofluids in magnetic fields is always mesmerizing. Here a ferrofluid has been submerged in a clear alcohol-based solution in a shallow dish while a permanent magnet is used to perturb the liquid. Instead of forming its distinctive spikes due to the normal-field instability, the fluid forms ribbons and mazes due to the shifting magnetic field and the surrounding fluid.
Flow in Urban Areas
While we typically think about boundary layers as a small region near the surface of an object–be it airplane, golf ball, or engine wall–boundary layers can be enormous, like the planetary boundary layer, the part of the atmosphere directly affected by the earth’s surface. Shown above is a flow visualization of the boundary layer in an urban area; note the models of buildings. In these atmospheric boundary layers, buildings, trees, and even mountains act like a random rough surface over which the air moves. This roughness drives the fluid to turbulent motion, clear here from the unsteadiness and intermittency of the boundary layer as well as the large variation in scale between the largest and smallest eddies and whorls. In the atmosphere, the difference in scale between the largest and smallest eddies can vary more than five orders of magnitude.
Liquid Nitrogen and the Leidenfrost Effect
One of the tried and true cooking tips my mother gave me when I was younger was to test the temperature of my griddle before making pancakes by splashing a few drops of water on it. If it was hot enough that the water skittered across the surface before evaporating, then it was ready. Aside from being a way to make great pancakes, this tip demonstrates an everyday application of the Leidenfrost effect. When the surface of the pan is significantly higher than the boiling point of the water, the part of the water drop that hits the pan is vaporized, creating a thin layer of water vapor on which the rest of the droplet rests. The vapor serves as an insulator, protecting the rest of the water drop from the heat of the pan, as well as a lubricant, allowing the drop slip and slide easily across the surface. The same effect lets the brave plunge a hand into liquid nitrogen without damage, but they have to be quick, otherwise their hand will cool to the point that the liquid nitrogen contacts it without a protective layer of nitrogen. (In that case, a nasty case of frostbite may be the least of one’s worries. We do NOT recommend trying this one at home.)
Drops Through Drops
The splashes from droplets impacting jets create truly mesmerizing liquid sculptures. Corrie White is one of the masters of this type of high-speed macro photography. Her work captures the instantaneous battles between viscosity, surface tension, and inertia. The fantastic structure seen here through the falling droplets is created by a series of drops timed so that the later ones strike the Worthington jet produced by the initial drop’s impact. (Photo credit: Corrie White)
