Strands of green and brown mix in Langebaan Lagoon on the South African coast in this astronaut photograph. The shallow tidal estuary has a sandy floor and, since no river flows into it, the deeper green sections seen here are channels carved solely by the back-and-forth flow of the tides. To the north of the lagoon, Saldanha Bay is a busy hub for fishing and industry. The long reddish line extending into the water is a railroad pier responsible for loading 96 percent of South Africa’s iron ore gets loaded onto ships. (Image credit: NASA; via NASA Earth Observatory)
Tag: flow visualization
Spreading the Word
Just as prairie dogs bark to warn the colony of danger, many plants can signal their neighbors when they’re under attack. This thale cress releases calcium when caterpillars eat it; neighboring plants pick up the chemical signal and pass it along. To better understand how the signal gets passed, researchers genetically modified this plant to fluoresce when extra calcium is on the move. It’s incredible to watch the flow from one side of a leaf to another. (Image and research credit: Y. Aratani et al.; via Colossal)
Convection in Action
We’re surrounded daily by convection — a buoyancy-driven flow — but most of the time it’s invisible to us. In this video, Steve Mould shows off what convection really looks like with some of his excellent tabletop demos. The first half of the video gives profile views of turbulent convection, with chaotic and unsteady patterns. When he switches to oil instead of water, the higher viscosity (and lower Reynolds number) offer a more structured, laminar look. And finally, he shows a little non-temperature-dependent convection with a mixture of Tia Maria and cream, which convects due to evaporation changing the density. (Image and video credit: S. Mould; submitted by Eric W.)
Serpents and Ouroboros
Beads of condensation on a cooling, oil-slicked surface have a dance all their own in this video. Large droplets gobble up their fellows as they follow serpentine paths; each new droplet donates its interfacial energy to feed the larger drop’s kinetic energy. Eventually, the big drops switch to a circular path, like an ouroboros, the tail-eating serpent of mythology. This transition happens due to the oil shifted by the dancing droplets. You can recreate the effect at home by rubbing a thin layer of oil over glass and setting it atop a hot mug of your favorite beverage. (Video and image credit: M. Lin et al.; research credit: M. Lin et al.)
Swirls Off South Australia
Summer winds along Australia’s Bonney Coast push coastal waters offshore, triggering the upwelling of colder waters from depths below 300 meters. These cold waters from the deep are nutrient-rich, thanks to all the decomposition that happens along the ocean floor. The infusion of nutrients triggers an explosion of life, visible here in the form of a green phytoplankton bloom along the shelf break. In turn, the phytoplankton attract fish and blue whales. Even great white sharks are drawn to the cornucopia. (Image credit: W. Liang; via NASA Earth Observatory)
Skittering Drops
Drip some ethanol on a hot surface, and you’d expect it to spread into a thin layer and evaporate. But that doesn’t always happen, and a recent study looks at why.
Ethanol is what’s known as a volatile liquid, meaning that it evaporates easily at room temperatures, well below its boiling point. When dropped on a uniformly heated surface above 45 degrees Celsius, the drop contracted into a hemisphere and then began to wander randomly across the surface. Researchers trained an infrared camera on the drop from below (above image), and found an unsteady, roiling motion inside the drop. These asymmetric flows, they concluded, drive the drop’s erratic self-propulsion. They suspect the mechanism may explain why some ink droplets wind up in the wrong place on a page during ink-jet printing. (Image and research credit: P. Kant et al.; via APS Physics)
Rough Surfaces
In fluid dynamics, we’re often concerned with flow moving past a solid surface — air past an airplane wing, water past fish scales, oil between moving parts — and those surfaces are rarely perfectly smooth. Rough surfaces affect the flow near them, sometimes in unexpected ways. Here, researchers show a rough surface’s effect on the eddies of the atmospheric boundary layer. Put differently, this poster shows how buildings, trees, and other features influence the lowest layer of the atmosphere. From the tiny gaps between buildings to the eddies towering many times higher, the turbulence reflects roughness’s effects. (Image credit: J. Kostelecky and C. Ansorge)
Tracking Break-Up
In fluid dynamics, researchers are often challenged with complicated, messy flows. With so much going on at once, it’s hard to work out a way to keep track of it all. Here, researchers are looking at the break-up of two colliding liquid jets. This setup is often used to break rocket fuel into droplets prior to combustion. This video shows off a new data analysis tool that lets researchers break the flow into different parts, track them in time, and extract data about the changes that happen along the way. (Video and image credit: E. Pruitt et al.)
Feynman’s Sprinkler Solved
In graduate school, my advisor introduced us to a particularly vexing fluid dynamical thought experiment known as the Feynman sprinkler. After observing an S-shaped sprinkler that rotated when water shot out its arms, physicist Richard Feynman wondered what would happen if the device were placed in a tank of water with the flow reversed. If the sprinkler was sucking in water, would it rotate and, if so, in what direction?
This seemingly simple question has confounded physicists ever since, in part because you can make believable arguments for multiple different results. Attempts to build the apparatus experimentally produced differing results, too — often due to variables that don’t appear in the thought experiment, like friction in the sprinkler’s bearing. But, at long last, a group posits they have the final answer to the problem.
They cleverly built their sprinkler so that it floats in its tank, with the addition or removal of water from the sprinkler controlled by a second siphon-connected tank. With no solid-solid contacts, the sprinkler can rotate with very little friction.
Flow visualization of the sprinkler in reverse (suction) mode. For image clarity, the device is held in place to prevent spinning. Notice how the jets coming into the hub glance off one another and form counter-rotating vortex pairs at an angle. This asymmetry is the source of the sprinkler’s rotation when allowed to move. The team found that sucking water into the sprinkler does, indeed, reverse the sprinkler’s rotation, but it’s not a simple reversal of the forward sprinkler’s flow. To see why, check out the video above, which visualizes flow inside the sprinkler during suction. For clarity, the device is held fixed in place during flow visualization. Notice that the two arms of the sprinkler sit directly opposite one another in the hub. Thus, you’d expect their two jets to collide and form counter-rotating vortices along a vertical axis. But the vortex pairs are offset from the centerline.
This asymmetry takes place because the velocity profiles of flow across the hub inlets are skewed. Instead of the largest velocity occurring on the centerline of the inlet, each occurs slightly to one side. So when the jets collide, they do so off-center and impart a torque to the sprinkler. The reason for the skewed profiles at the inlets lies further upstream in the curved arms of the sprinkler. Centrifugal force from turning the corner leaves a mark on the flow, leading, ultimately, to the skewed velocity profiles, offset jets, and spinning sprinkler. (Image and research credit: K. Wang et al.; via APS Physics)
Spreading Frost
Condensation forms beads of water on a surface. When suddenly cooled, those drops begin to freeze into frost. This video looks at the process in optical and in infrared, revealing the patterns of spreading frost and the tiny ice bridges that link one freezing drop to the next. (Video and image credit: D. Paulovics et al.)
