An astronaut captured this image of the Oyyl Dune Field in Kazakhstan from the International Space Station. To the south and east of the dune field (right and lower parts of image) there are fluvial floodplains, sources of sediment that feed the dunes. With sufficient wind and sand sources, the dune field has grown in a topographic low spot roughly 90 meters lower than the surrounding steppes. Dark specks scattered across the sands are clusters of vegetation, a sign that the dunes may get anchored rather than continue to shift in the wind. (Image credit: NASA; via NASA Earth Observatory)
Category: Phenomena
Backswimmers
Backswimmers rule the surface of ponds, streams, and other bodies of water. These insects spend much of their time clinging just beneath the air-water interface, where they hunt larvae and other insects. They use oversized, oar-shaped back legs to row, and they breathe using an air bubble that clings to their abdomen like a personal scuba tank. Oxygen from the water diffuses into the bubble, keeping the insect’s air supply fresh. When the time comes to move to greener pastures, they flip to the other side of the water’s surface, unfurl their wings, and take off. (Image and video credit: Deep Look)
Cloud Streets
Parallel lines of cumulus clouds stream over the Labrador Sea in this satellite image. These cloud streets are formed when cold, dry winds blow across comparatively warm waters. As the air warms and moistens over the open water, it rises until it hits a temperature inversion, which forces it to roll to the side, forming parallel cylinders of rotating air. On the rising side of the cylinder, clouds form while skies remain clear where the air is sinking. The result are these long, parallel cloud bands. (Image credit: J. Stevens; via NASA Earth Observatory)
Diving Together
Two spheres dropped into water next to one another form asymmetric cavities. A single ball’s cavity is perfectly symmetric, and so are two spheres’, provided they are far enough apart. But for close impacts, the spheres influence one another, creating a mirror image. The same asymmetric cavity also forms when a sphere is dropped near a wall. In fluid dynamics, this trick — using two mirrored objects in place of a wall — is used to make calculating certain flows easier! (Image credit: A. Kiyama et al.)
Blooms in the Black Sea
The Black Sea gains its name from its dark waters, but those waters don’t stay dark year-round. In this natural color satellite image, streaks of milky blue bloom through the summer waters, thanks to the presence of a species of phytoplankton armored with white calcium carbonate. Despite their microscopic size, the phytoplankton’s presence is visible from space. During other parts of the year, like the spring, another species of phytoplankton dominates the Black Sea, turning its waters darker. (Image credit: J. Stevens; via NASA Earth Observatory)
Yellowstone Flooding
In June of 2022, the area around Yellowstone National Park saw catastrophic flooding. The combined effects of rainfall and snowmelt overwhelmed waterways and washed out many roads and other structures in and around the park. In this video, Grady from Practical Engineering breaks down the floods and their aftermath, including how the area can be rebuilt. His depiction of the flood, from an engineering standpoint, is especially helpful, as he illustrates conditions across the park using flow sensor data. It helps explain the damage and gives viewers a sense for how engineers monitor and analyze these events. (Image and video credit: Practical Engineering)
Peering Into the Gap
This video offers a glimpse into turbulence developing in a classic flow set-up, a Taylor-Couette cylinder. The apparatus consists of two upright, concentric cylinders; the outer cylinder is fixed, and the inner one rotates. This video shows the gap between the cylinders, and it’s rotated so that the inner cylinder is at the bottom of the frame. Gravity points from left to right in the video. The fluid in the 8-cm gap between the cylinders is water, seeded with rheoscopic particles to visualize the flow.
The video begins as the inner cylinder has just begun to rotate, dragging nearby fluid with it. A thin, laminar boundary layer forms at the bottom of the frame, growing as time goes on. A few seconds in, the boundary layer transitions to turbulence; look closely and you’ll see hairpin-shaped vortices appear. Just after that, the boundary layer becomes entirely turbulent and continues to slowly move upward to take over the full gap. The video is available in a full 4K resolution if you really want to get lost in the flow. (Video credit: D. van Gils)
Aerated Faucets
So much goes on in our daily lives that we never see. But with the power of the smartphones in our pockets, we can catch more than ever before, as illustrated in this video. Here a researcher uses the standard “slo-mo” (240 fps) video mode on a smartphone to look at the flow from a typical kitchen faucet. Household faucets often have an aerator that adds air bubbles to the flow, something that’s particularly visible in slow motion at high flow rates. What you can see depends on more than just the frame rate, though. Without strong illumination — provided in this case by sunlight — you could easily miss the cloud of droplets ejected by the faucet. (Image and video credit: M. Mungal)
Fish-Scale Tides
On 31 July 2022, an unusual tidal phenomenon, a fish-scale tide, took place on the Qiantang River’s estuary in Zhejiang Province, China. Here are a couple videos. I’ve not found any explanations for it thus far, so I’m assembling my own. The Qiantang River and its estuary, Hangzhou Bay, are home to the world’s largest tidal bores, reaching 9 meters in places. That means the area regularly sees trains of large waves moving upstream against the normal current.
The area is also known to have rotating currents, meaning that the tide does not simply move inland and then smoothly reverse direction. Instead, a rotating current can change its direction of flow over the course of a tidal cycle without changing its speed. Taken together, this makes the Qiantang River region perfect for winding up with groups of waves colliding at oblique angles, similar to a cross sea. I believe that’s what’s going on here with the fish-scale tide. Two sets of tidal-bore-induced waves are colliding at an angle, creating some gnarly conditions and a very cool pattern. (Image credit: VCG; submitted by Antony B.)
Groundwater-Structure Interactions
Groundwater can sometimes wind up in unexpected places, given the way it interacts with subsurface structures. In this Practical Engineering video, Grady discusses the paths that groundwater takes around structures and how civil engineers account for groundwater-related forces on dams and other buildings. As always, he illustrates with excellent model demos, allowing viewers to see groundwater interactions for themselves. (Image and video credit: Practical Engineering)
