Uranus is one of our solar system’s oddest planets. An ice giant, it spins on its side. We originally estimated its rate of rotation using measurements from Voyager 2, the only spacecraft to have visited the planet. But that measurement was so imprecise that within two years, astronomers could no longer use it to predict where the planet’s poles were. Now a new study, drawing on over a decade of Hubble observations of Uranus’s auroras, has pinned down the planet’s rotation rate far more precisely: 17 hours, 14 minutes, and 52 seconds. While that’s within the original measurement’s 36-second margin of error, the new measurement has a margin of error of only 0.036 seconds. In addition to helping plan a theoretical future Uranus mission, this more accurate rotation rate allows researchers to reexamine decades of data, now with certainty about the planet’s orientation at the time of the observation. (Image credit: ESA/Hubble, NASA, L. Lamy, L. Sromovsky; research credit: L. Lamy et al.; via Gizmodo)
Tag: fluid dynamics
Escape From Yavin 4
In an ongoing tradition, let’s take another look at some Star Wars-inspired aerodynamics. This year it’s the TIE fighter’s turn. Here, researchers simulate the spacecraft trying to escape Yavin 4’s atmosphere at Mach 1.15. The research poster’s blue contours show pressure contours, with darker colors connoting higher pressures. The bright low pressure region immediately behind the craft suggests a difficult, high-drag ascent and a turbulent, subsonic wake despite the craft’s supersonic velocity. (Image credit: A. Martinez-Sanchez et al.)
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“Legend”
Filmmaker Roman De Giuli returns to his roots with this short fluid-filled film inspired by the color gold. He combines paint, ink, powders, and particles in a mix of micro- and macroscale photography. As always, the results are a mesmerizing plethora of fluid phenomena: Marangoni flows, turbulence, vorticity, viscous fingering and so much more. (Video and image credit: R. De Giuli)
Martian Mud Volcanoes
Mars features mounds that resemble our terrestrial mud volcanoes, suggesting that a similar form of mudflow occurs on Mars. But Mars’ thin atmosphere and frigid temperatures mean that water — a prime ingredient of any mud — is almost always in either solid or gaseous form on the planet. So researchers explored whether salty muds could flow under Martian conditions. They tested a variety of salts, at different concentrations, in a low-pressure chamber calibrated to Mars-like temperatures and pressures. The salts lowered water’s freezing point, allowing the muds to remain fluid. Even a relatively small amount of sodium chloride — 2.5% by weight — allowed muds to flow far. The team also found that the salt content affected the shape the flowing mud took, with flows ranging from narrow, ropey patterns to broad, even sheets. (Image credit: P. BroΕΎ/Wikimedia Commons; research credit: O. KrΓ½za et al.; via Eos)
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Quietening Drones
A drone’s noisiness is one of its major downfalls. Standard drones are obnoxiously loud and disruptive for both humans and animals, one reason that they’re not allowed in many places. This flow visualization, courtesy of the Slow Mo Guys, helps show why. The image above shows a standard off-the-shelf drone rotor. As each blade passes through the smoke, it sheds a wingtip vortex. (Note that these vortices are constantly coming off the blade, but we only see them where they intersect with the smoke.) As the blades go by, a constant stream of regularly-spaced vortices marches downstream of the rotor. This regular spacing creates the dominant acoustic frequency that we hear from the drone.
Animation of wingtip vortices coming off a drone rotor with blades of different lengths. This causes interactions between the vortices, which helps disrupt the drone’s noise. To counter that, the company Wing uses a rotor with blades of different lengths (bottom image). This staggers the location of the shed vortices and causes some later vortices to spin up with their downstream neighbor. These interactions break up that regular spacing that generates the drone’s dominant acoustic frequency. Overall, that makes the drone sound quieter, likely without a large impact to the amount of lift it creates. (Image credit: The Slow Mo Guys)
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Filtering Like a Manta Ray
As manta rays swim, they’re constantly doing two important — but not necessarily compatible — things: getting oxygen to breathe and collecting plankton to eat. That requires some expert filtering to send food particles toward their stomach and oxygen-rich water to their gills. Manta rays do this with a built-in filter that resembles an industrial crossflow filter. Researchers built a filter inspired by a manta ray’s geometry, and found that it has three different flow states, based on the flow speed. At low speeds, flow moves freely down the filter’s channels; in a manta, this would carry both water and particles toward the gills. At medium speeds, vortices start to form at the entrance to the filter channels. This sends large particles downstream (toward a manta’s digestive system) while water passes down the channels. At even greater speeds, each channel entrance develops a vortex. That allows water to pass down the filter channels but keeps particles out. (Image credit: manta – N. Weldingh, filter – X. Mao et al.; research credit: X. Mao et al.; via Ars Technica)
Depending on the flow speed, a manta-inspired filter can allow both water and particles in or filter particles out of the water. Fediverse Reactions
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Climate Change and the Equatorial Cold Tongue
A cold region of Pacific waters stretches westward along the equator from the coast of Ecuador. Known as the equatorial cold tongue, this region exists because trade winds push surface waters away from the equator and allow colder, deeper waters to surface. Previous climate models have predicted warming for this region, but instead we’ve observed cooling — or at least a resistance to warming. Now researchers using decades of data and new simulations report that the observed cooling trend is, in fact, a result of human-caused climate changes. Like the cold tongue itself, this new cooling comes from wind patterns that change ocean mixing.
As pleasant as a cooling streak sounds, this trend has unfortunate consequences elsewhere. Scientists have found that this cooling has a direct effect on drought in East Africa and southwestern North America. (Image credit: J. Shoer; via APS News)
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Ghostly Waterfalls
Photographer Jonathan Knight likes capturing waterfalls about 45 minutes after sunset, creating ghostly images that emphasize the shape of the cascading water. The dim surroundings and misty shapes remind me of old daguerreotypes. See more of his images on his website and his Instagram. (Image credit: J. Knight; via Colossal)
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Hot Droplets Bounce
In the Leidenfrost effect, room-temperature droplets bounce and skitter off a surface much hotter than the drop’s boiling point. With those droplets, a layer of vapor cushions them and insulates them from the hot surface. In today’s study, researchers instead used hot or burning drops (above) and observed how they impact a room-temperature surface. While room-temperature droplets hit and stuck (below), hot and burning droplets bounced (above).
In this case, the cushioning air layer doesn’t come from vaporization. Instead, the bottom of the falling drop cools faster than the rest of it, increasing the local surface tension. That increase in surface tension creates a Marangoni flow that pulls fluid down along the edges of the drop. That flow drags nearby air with it, creating the cushioning layer that lets the drop bounce. In this case, the authors called the phenomenon “self-lubricating bouncing.” (Image and research credit: Y. Liu et al.; via Ars Technica)
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Drops on the Edge
Drops impacting a dry hydrophilic surface flatten into a film. Drops that impact a wet film throw up a crown-shaped splash. But what happens when a drop hits the edge of a wet surface? That’s the situation explored in this video, where blue-dyed drops interact with a red-dyed film. From every angle, the impact is complex — sending up partial crown splashes, generating capillary waves that shift the contact line, and chaotically mixing the drop and film’s liquids. (Video and image credit: A. Sauret et al.)
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