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.
Tag: fluid dynamics
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
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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)
Reversing a Flow
The reversibility of laminar mixing often comes as a surprise to observers accustomed to the experience of being unable to separate two fluids after they’ve been combined. As you can see above, however, inserting dye into a highly viscous liquid and then mixing it by turning the inner of two concentric cylinders can be undone simply by turning the cylinder backwards. This works because of the highly viscous nature of Stokes flow: the Reynolds number is much less than 1, meaning that viscosity’s effects dominate. In this situation, fluid motion is caused only by molecular diffusion and by momentum diffusion. The former is random but slow, and the latter is exactly reversible. Reversing the rotation of the fluid undoes the momentum diffusion and any distortion remaining is due to molecular diffusion of the dye.
The Floating Water Bridge
The interaction of electric fields and fluids can lead to some unexpected results. Here we see the formation of a water bridge formed between two beakers of demineralized water across which a large voltage difference (~15kV) is applied. The bridge is stable for separation distances up to about 2 cm. In order to achieve this feat, the water is overcoming two destabilizing forces: gravity, which bends the bridge, and capillary action, which makes the liquid bridge thin until it breaks into droplets. According to the authors, both forces are countered by induced polarization forces at interface; in short, the electrical field around the liquid causes the positive and negative charges in the liquid to separate, thereby polarizing the liquid. This separation of charges then creates normal stresses along the surface of the water that oppose the gravitational and capillary forces trying to break the bridge. (Video credit: A. Marin and D. Lohse)
Mackerel vs. Eel: Who Swam It Better?
Which matters more, form or function? This simulation sets out to answer that question by comparing the swimming motion of eels and mackerels. Eels have longer, more rounded body shapes and swim in an undulatory fashion with their whole body, whereas mackerels have shorter bodies with a more elliptical cross-section and primarily move their tails when swimming. The simulation separates body type from swimming motion by creating virtual races between fishes of the same body type using the two forms of swimming. Eels swim at moderate Reynolds numbers where viscous and inertial effects are reasonably balanced. Under those conditions, eel-like swimming was faster, even with a mackerel’s body type. At the higher Reynolds numbers where mackerels usually swim, inertial forces domination and the racing fish moved faster if they swam like a mackerel, even with the body of an eel. The results suggest that the swimming motion matters more in each Reynolds number range than the shape of the swimmer. This is a neat way that simulation can answer questions we cannot test with an experiment! (Video credit: I. Borazjani and F. Sotiropoulos)
Flying Squid
Ever seen a squid fly? Not many have, but the behavior may be more common than you think. Thanks to a set of photos from an amateur photographer, scientists have managed to estimate the velocity and acceleration of squid as they propel themselves out of the water by squirting a jet behind them. Researchers found that their speeds in air are roughly five times that in water, thanks to decreased drag. Previously it was thought that the flying behavior might be linked to escaping predators, but some now suggest that it enables migration over long distances by saving energy.
