FYFD

Tag: physics

  • Meander from Above

    Meander from Above

    This photo of the Amazon River taken by Astronaut Tim Kopra reveals the many meandering changes of the river’s course. Left untouched by human intervention, rivers tend to get more curvy, or sinuous, over time, simply due to fluid dynamics. Imagine a single bend in a river. Due to conservation of angular momentum, water flows faster around the inside curve of the bend than the outside – just like an ice skater spins faster with her arms pulled in. From Bernoulli’s principle, we know there is an accompanying pressure gradient caused by this velocity difference – with higher pressure near the outer bank and lower pressure on the inner one. This pressure gradient is what guides the water around the bend, keeping the bulk of the fluid moving downstream rather than bending toward either bank.

    At the bottom of the river, though, viscosity slows the water down due to the influence of the ground. This slower water, still subject to the same pressure gradient as the rest of the river, cannot maintain its course going downstream. Instead, it gets pushed from the outer bank toward the inner bank in what’s known as a secondary flow. This secondary flow carries sediment away from the outer bank and deposits it on the inner bank, which, over time, makes the river bend more and more pronounced. (Image credit: T. Kopra/NASA; submitted by jshoer)

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  • Underwater Landslides

    Underwater Landslides

    Turbidity currents are a gravity-driven, sediment-laden flow, like a landslide or avalanche that occurs underwater. They are extremely turbulent flows with a well-defined leading edge, called a head. Turbidity currents are often triggered by earthquakes, which shake loose sediments previously deposited in underwater mountains and canyons. Once suspended, these sediments make the fluid denser than surrounding water, causing the turbidity current to flow downhill until its energy is expended and its sediment settles to form a turbidite deposit. By sampling cores from the seafloor, scientists studying turbidites can determine when and where magnitude 8+ earthquakes have occurred over the past 12,000+ years!  (Video credit: A. Teijen et al.; submitted by Simon H.)

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  • Dam Release

    Dam Release

    Here the U.S. Army Corps of Engineers release 13,000 cubic feet per second (~370 cubic meters per second) of water at a dam in Oklahoma. That’s the equivalent of nine-and-a-half shipping containers a second! Releasing that much water at once has created an enormous hydraulic jump, seen on the right side of the animation. Hydraulic jumps are kind of like the shock wave of open channel flow. On the left side of the image, water is moving smoothly and swiftly down the sluiceway. At the center, the incoming water encounters the large, slow-moving mass of water already in the lake. There’s no way for the incoming water to sustain its kinetic energy while discharging into the lake. Instead a hydraulic jump forms, converting the incoming flow’s kinetic energy into potential energy, as seen in the sudden height increase. Some of the energy is also converted to turbulence and dissipated as heat. (Image credit: U.S. Army Corps of Engineers/AP, source; via Gizmodo)

  • Paint Flying

    Paint Flying

    Paint getting flung from a spinning drill bit can create some incredible art. Here the Slow Mo Guys recreate the effect in high-speed video. What we’re seeing is tug of war between centrifugal force, which tries to fling the paint outward, and internal forces in the paint, which struggle to hold the the fluid together. Primarily, it’s surface tension keeping the fluid together, but, depending on what sort of non-Newtonian fluid the paint may be, there could be other internal forces helping keep the paint intact. In this case, centrifugal force is clearly winning out, though the paint stretches pretty far before it thins enough to break. It would be interesting to see how the balance plays out with the drill bit spinning at a lower RPM. (Image credit: Slow Mo Guys, source)

  • Fluids Round-Up

    Fluids Round-Up

    New year, new (or renewed) experiments. This is the fluids round-up, where I collect cool fluids-related links, articles, etc. that deserve a look. Without further ado:

    (Video credit and submission: Julia Set Collection/S. Bocci; image credit: IRPI LLC, source)

  • Boiling Water to Snow

    Boiling Water to Snow

    When it’s really cold outside–to the tune of -40 degrees (Fahrenheit or Celsius)–physics can get a little crazy. In this photo, boiling-hot water from a thermos turns into an instant snowstorm when tossed. How is this possible? It turns out there are a combination of factors that affect this. Firstly, the rate of heat transfer between two objects depends on the magnitude of the temperature difference between them. The bigger the difference in temperature, the faster the hot object cools. Of course, as the hot object cools down, the temperature difference between it and its surroundings is smaller and the rate of heat transfer decreases.

    The second important factor here is that the water is being tossed. When you throw water, it breaks into droplets, and droplets have a large surface area compared to their volume. As it turns out, the rate of heat transfer also depends on surface area. By breaking the hot water into smaller droplets, you increase the surface area exposed to the cold air, allowing the hot water to freeze faster. (Image credit: M. Davies et al.; via Gizmodo)

    Also: Since there are a few events scheduled around the country over the next couple months, I’ve added an events page where you can find details for those appearances. And as always, if you’re interested in scheduling a talk or event, feel free to contact me directly.

  • Inside APS DFD 2015

    Inside APS DFD 2015

    What do shark scales, underwater robots, blood flow, and art have in common? They’re all a part of the latest FYFD video! Check out my behind-the-scenes look at the latest American Physical Society Division of Fluid Dynamics meeting. Meet the researchers and find out about the science everyone was talking about! (Image/video credit: N. Sharp)

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    Pluto: Subsurface Convection

    Pluto’s rich and unexpected surface features indicate the dwarf planet is still geologically active. This is one of the largest surprises of the New Horizons mission because it was assumed that Pluto was too small, too isolated, and too old for such activity. Instead, its cryovolcanoes and surface convection cells point to significant and vigorous convection in Pluto’s mantle, likely heated by the decay of radioactive elements in its core. The simulation above shows a representation of mantle convection on Earth, simulated over billions of years.

    Mantle convection is described by the dimensionless Rayleigh number, which compares the effects of thermal conduction to those of convection. Above a fluid’s critical Rayleigh number, convection is the driving process in heat transfer. In Pluto’s case, if one assumes a mantle of pure water ice, the Rayleigh number is about 1600, barely enough to surpass the critical point where convection dominates. If, instead, one assumes a mantle containing 5% ammonia, the resulting composition has a Rayleigh number of more than 10,000–well past the critical point and large enough to support the vigorous convection necessary to explain Pluto’s surface features.  (Video credit: W. Bangerth and T. Heister; Pluto research credit: A. Trowbridge et al.; via Purdue University)

    This concludes FYFD’s week of exploring Pluto’s fluid dynamics. You can see previous posts in the series here.

  • Pluto: Convection in Sputnik Planum

    Pluto: Convection in Sputnik Planum

    The icy plain of Sputnik Planum, located in Pluto’s heart-shaped Tombaugh Reggio, is criss-crossed with troughs that divide the plain into polygons.  The current interpretation of these features is that they are the result of thermal convection. As with Rayleigh-Benard convection cells on Earth, the interior of the polygons is formed by the upwelling of warmer, buoyant material, and the troughs between cells mark locations where cooled material convects back into the mantle. On Pluto, these cells consist of nitrogen ice (and occasional water ice like the dirty black chunk seen in the upper right photo) that slowly rises and sinks from the planet’s surface, constantly refreshing the surface features. This would explain why Sputnik Planum is missing evidence of typical older features like impact craters. (Image credits: NASA/JHU APL/SwRI)

    Join FYFD all this week for a look at fluid dynamics and planetary science on Pluto! Check out the previous posts here.

  • Pluto: Cryovolcanoes

    Pluto: Cryovolcanoes

    Since its flyby last summer, NASA’s New Horizons mission has had planetary scientists questioning all our assumptions about Pluto and its fellow cold, icy worlds on the outskirts of the solar system. The two mountainous features above, the 4-km tall Wright Mons and 5.6-km tall Piccard Mons, are part of the mystery. Both mountains have a large depression in the middle, and their appearance from orbit is consistent with volcanoes seen on Earth and other planets. But instead of rock, these mountains are formed from water ice, and rather than spewing hot magma, it’s believed that these mountains are cryovolcanoes that erupt with a slurry of water, nitrogen, ammonia, or methane. Since no active eruptions were recorded during the flyby, scientists cannot be certain of the hypothesis, but it does explain the observed features. Check out the video below for a terrestrial demonstration of a “cryovolcano”. (Photo credits: NASA/JHU APL/SwRI; video credit: A. Cheri/U. Wash)

    Join FYFD all this week for a look at the fluid dynamics and planetary science of Pluto!