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  • Water on Mars

    Water on Mars

    Recurring slope lineae (RSL) are seasonal features on Mars that leave behind gullies similar to those left by running water on Earth. Their discovery a few years ago has prompted many experiments at Martian conditions to determine how these features form. At Martian surface pressures and temperatures, it’s not unusual for water to boil. And that boiling, as some experiments have shown, introduces opportunities for new transport mechanisms.

    Researchers found that water in “warm” (T = 288 K) sand boils vigorously, ejecting sand particles and creating larger pellets of saturated sand. Water continues boiling out of the pellets once they form, creating a layer of vapor that helps levitate them as they flow downslope. The effect is similar to the Leidenfrost effect with drops of water sliding on a hot skillet; there’s little friction between the pellet and the surface, allowing it to travel farther.

    The mechanism is quite efficient in experiments under Earth gravity and would be even more so under Mars’ lower gravity. It also requires less water than alternative explanations. The pellets that form are too small to be seen by the satellites we have imaging Mars, but the tracks they leave behind are similar to the RSL seen above. (Image credit: NASA; research credit: J. Raack et al., 1, 2; via R. Anderson; submitted by jpshoer)

  • Space Shuttle Sonic Booms

    Space Shuttle Sonic Booms

    The Space Shuttle had a famous double sonic boom when passing overhead during re-entry. This schlieren flow visualization of a model shuttle at Mach 3 reveals the source of the sound: the fore and aft shock waves on the vehicle. The nose of the shuttle generates the strongest shock wave since it is the first part of the vehicle the flow interacts with. This initial shock wave turns the flow outward and around the shuttle. The second boom comes from the back of the shuttle and serves to turn the flow back in to fill the wake behind the shuttle. (The actual shock wave would look a little different than this one because there’s no sting holding the shuttle like there is with the model.) The other major shock wave comes from the shuttle’s wings, but, at least for this Mach number, the wing shock wave merges with the bow shock, making the two indistinguishable. (Image credit: G. Settles, source)

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    “Dance Dance”

    Artist Thomas Blanchard is no stranger to fluid dynamics. His previous short films focused on mixtures of oil and paint, but in “Dance Dance,” flowers are front and center. There are obvious splashes of color and clouds of diffusion toward the end of the video, but fluid dynamics are there throughout. The oozing, inexorable march of ice crystallizing over petals and leaves has a fluidity that’s heightened by timelapse. It’s a reminder that this phase change is unsteady and full of shifts too subtle to notice in real-time. In the second act, we see flowers blossoming in timelapse, bursting open dramatically before settling in with a subtle shift of their stamens. Motions like these are driven by the flow of fluids inside the plant. By shifting small concentrations of chemicals, plants drive the water in their cells via osmosis. This pumps up cells that cause the petals to spread and unfurl. (Video and image credit: T. Blanchard; via Colossal)

  • Rain on Car Windows

    Rain on Car Windows

    As a child, I loved to ride in the car while it was raining. The raindrops on the window slid around in ways that fascinated and confused me. The idea that the raindrops ran up the window when the car moved made sense if the wind was pushing them, but why didn’t they just fly off instantly? I could not understand why they moved so slowly. I did not know it at the time, but this was my early introduction to boundary layers, the area of flow near a wall. Here, friction is a major force, causing the flow velocity to be zero at the wall and much faster – in this case roughly equal to the car’s speed – just a few millimeters away. This pushes different parts of large droplets unevenly. Notice how the thicker parts of the droplets move faster and more unsteadily than those right on the window. This is because the wind speed felt by the taller parts of the droplet is larger. Gravity and the water’s willingness to stick to the window surface help oppose the push of the wind, but at least with large drops at highway speeds, the wind’s force eventually wins out. (Image credit: A. Davidhazy, source; via Flow Viz)

  • Jovian Polar Vortices

    Jovian Polar Vortices

    Jupiter’s atmosphere is full of enduring mysteries, and its poles are no exception. Instruments aboard the Juno spacecraft have gotten a better look at Jupiter’s North and South poles than any previous mission, and what they’ve found raises even more questions. Both of Jupiter’s poles feature a central cyclone ringed by other, similarly-sized cyclones. The North pole has eight outer cyclones (top image), while the South pole has five (bottom image), shown above in infrared. Despite being close enough that their spiral arms intersect, the cyclones don’t seem to be merging into something like Saturn’s polar hexagon. For now, scientists don’t know how this arrangement formed or why it persists, but the longer Juno can study the vortices up close, the more we’ll learn. (Image credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM; research credit: A. Adriani et al.; submitted by Kam-Yung Soh)

  • Castle-like Clouds

    Castle-like Clouds

    An astronaut captured this towering cloud over Andros Island from orbit aboard the ISS. This is a cumulus castellanus cloud, named for the castle-like crenelations at its top. Castellanus clouds form in areas with strong vertical updrafts, often due to cloud-level atmospheric instabilities rather than heating at the Earth’s surface. These clouds frequently proceed rain or even thunderstorms. What distinguishes castellanus from other types of cumulonimbus clouds is their shape: castellanus clouds have protrusions that are taller than they are wide – like the castles for which they are named. (Image credit: NASA / Expedition 48; via NASA Earth Observatory)

  • Modons

    Modons

    The spin of the Earth creates myriad eddies in our oceans, most of which move slowly westward at a speed dependent on their latitude. You can see many in the animation above as green and red rings slowly marching to the left. According to theory, it’s possible for two of these eddies to combine to become more than the sum of their parts; under the right conditions, the two conjoined eddies could become a modon, which, like a vortex ring, is capable of traveling far faster than its parental eddies. Despite the theory, however, no one had ever observed a modon in nature.

    A new paper uses satellite imagery to identify nine modons in different locations around the world. One is shown above. Watch the eastern coast of Australia carefully, and you’ll see a modon form. It moves much faster than its surroundings, first southward toward Tasmania, then quickly eastward toward New Zealand. Thin black circles mark the two eddies that make up the modon. The strength and speed of these features makes them capable of pulling significant water mass with them. This suggests that they may play a role in ocean life, transporting water of different temperatures and nutrient content into regions it would not otherwise reach. (Image and research credit: C. Hughes and P. Miller; via Gizmodo)

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    Skiing, Avalanches, and Freezing Bubbles

    To wrap up our look at Olympic physics, we bring you a wintry mix of interviews with researchers, courtesy of JFM and FYFD. Learn about the research that helped French biathlete Martin Fourcade leave PyeongChang with 3 gold medals, the physics of avalanches, and how bubbles freeze. 

    If you missed any of our previous Olympic coverage, you can find our previous entries on the themed series page, and for more great interviews with fluids researchers, check out our previous collab video. (Video credit: T. Crawford and N. Sharp; image credits: GettyImages, T. Crawford and N. Sharp)

  • PyeongChang 2018: Curling

    PyeongChang 2018: Curling

    Curling is a deceptively engrossing sport with some unique physics among Winter Olympic events. Athletes slide 19kg granite stones at a target 28 meters away. Along the way, teammates sweep the pebbled ice with brooms, melting it with frictional heating to help the stone slide further. The underside of the stones is concave, so they only touch the ice along a narrow ring. Researchers think roughness in the leading edge of the sliding stone cuts into the ice, leaving scratches that the trailing edge tries to follow. This is what causes the stone’s trajectory to curl. By melting the ice, sweeping also prevents curling, so competitors must know exactly when and how much to sweep. Ice conditions shift throughout a match, and the best players can read the ice to keep their stones where they want them. (Image credit: AP; W. Zhao/GettyImages)

  • PyeongChang 2018: Cross-Country Skiing

    PyeongChang 2018: Cross-Country Skiing

    Cross-country skiing, also known as Nordic skiing, is a part of many longstanding disciplines in the Winter Games. Unlike downhill skiing, cross-country events typically involve mass starts, which allow athletes to interact, using one another for pacing and tactics. Drafting can be a valuable method to save energy and reduce drag. A following skier sees a 25% drag reduction while drafting; the lead skier gets about a 3% reduction in drag compared to skiing solo. Competitors usually wear tight-fitting suits to minimize drag, but unlike speedskating, for example, cross-country skiers don’t get much benefit from roughened surfaces and impermeable fabrics. Typical race speeds are 4 – 9 m/s, and most of these high-tech fabrics don’t provide tangible benefits until higher speeds. (Image credit: Reuters/S. Karpukhin, US Biathlon, GettyImages/Q. Rooney)

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