Near the surface of the sun, the interplay of magnetic fields and plasma flow creates solar prominences that appear to dance. The prominence shown here was recorded in 2012 by the NASA Solar Dynamics Observatory, and its arc is large enough to easily surround the Earth. This is fluid dynamics – specifically magnetohydrodynamics – on a scale difficult for us earthbound humans to imagine. Scientists are still working to understand the complex processes that drive flows like this one. Fortunately, we can appreciate their beauty regardless. (Image credit: NASA SDO, source; via APOD; submitted by jpshoer)
Category: Phenomena
Swirls of Color
These beautiful swirls show the wake downstream of a thin plate. Here water is flowing from left to right and dye introduced on the plate (upstream and unseen in the photo) curls up into vortices. The vortices in the top row rotate clockwise, while the vortices along the bottom rotate anti-clockwise. This pattern of alternating vortices is extremely common in the wakes of objects and is known as a von Karman vortex street. Similar patterns are seen in soap films, behind cylinders, in the wakes of islands, and behind spaceships. (Image credit: ONERA, archived here)
Stall with Pitching Foils
For a fixed-wing aircraft, stall – the point where airflow around the wing separates and lift is lost – is an enemy. It’s the precursor to a stomach-turning freefall for the airplane and its contents. But the story is rather different when the wing is actively pitching through these high angles of attack. In this case, you get what’s known as dynamic stall, illustrated in three consecutive snapshots above.
In the top image, the flow has clearly separated from the upper surface of the wing, but this isn’t a cause for panic. As the middle image shows, there’s a vortex that’s formed in that separated region and it’s moving backward along the wing as the angle of attack continues to increase. That vortex causes a strong low-pressure region on the upper surface of the wing, allowing it to maintain lift.
In the final image, the vortex is leaving the wing, taking its low-pressure zone with it. This is the point where the pitching wing loses its lift, but if the vortex’s departure is immediately followed by a pitch down to lower angles of attack, the aircraft will recover lift and carry on. (Image credit: S. Schreck and M. Robinson, source)
Phytoplankton Swirl
During the warm summer months, phytoplankton blooms pop up in waters around the world. This natural-color satellite image shows a bloom in the Gulf of Finland. The tiny phytoplankton serve as tracker particles for the flow, revealing large-scale features like the spectacular vortex in the center of this image. The presence of the phytoplankton here suggests that this vortex could be pumping nutrients up from the deep.
Researchers also use particles for flow visualization. This can be as simple as adding small, neutrally buoyant particles, illuminating smoke, or even using natural snowfall to see what’s happening in the flow. (Image credit: NASA/USGS/J. Stevens/L. Dauphin)
Oobleck Under Impact
Fluids like air and water are Newtonian, which means that the way they deform does not depend on how the force on them gets applied. Many other fluids, however, are non-Newtonian. How they behave depends on how force is applied to them. The Internet’s favorite non-Newtonian fluid is probably oobleck, a mixture of cornstarch and water with some fairly extreme properties. When deformed quickly, like when struck with a bat, oobleck doesn’t flow; it shatters.
What’s happening at the microscopic level is that the cornstarch particles in the oobleck are jamming together. They simply cannot move quickly and avoid one another. When they jam together, the friction between them goes way up and so does the apparent viscosity of the oobleck. Because it doesn’t have time to flow, all that energy goes into breaking off “solid” chunks instead. Once they hit the ground, the pieces of oobleck will puddle, just like any other liquid. (Image and video credit: Beyond the Press; via Nerdist)
Making Champagne for Space
Humanity’s ongoing quest to enjoy beloved beverages in space has a new entry: champagne. French champagne maker Mumm has announced a new line with specially designed bottles to dispense champagne in microgravity. The bottles feature an internal piston that allows users to release the contents from the bottle in a controlled manner. Rather than pouring the champagne, one dispenses a blob which can then be caught in the special cups that go with it. They’re shaped somewhat like a miniature coupe.
It certainly looks like a fun way to celebrate in microgravity, although it’s unclear to me that they’ve tested the after effects of consumption. Historically, astronauts have avoided carbonated beverages in orbit because the lack of gravity can cause unpleasant side effects with all those bubbles. (Image credits: Mumm Champagne, source; via Wired)
Supernumerary Bows
After the rain of Hurricane Florence came the rainbow, or rainbows, in this case. Photographer John Entwistle captured this image of a rainbow with several additional supernumerary bows. The inner fringes seen here form when light passes through water droplets that are all close to the same size; given the spread seen here, the droplets are likely smaller than a millimeter in diameter. Supernumerary rainbows cannot be explained with a purely geometric theory of optics; instead, they require acknowledging the wave nature of light. (Image credit: J. Entwistle; via APOD; submitted by Kam-Yung Soh)
The Livers of Our Rivers
To the naked eye, mussels and other bivalves don’t appear to be doing much. But these filter feeders are hard at work. The mussel takes in water through its incurrent siphon (on the right side in this image), and tiny cilia move the water through its gills, which filter out plankton and other edibles. Wastewater flows out the exacurrent siphon, seen here as the plume coming out the top of the mussel.
Mussel species are important indicators of the health of both fresh and marine water bodies. Because they’re stationary and they are constantly processing the water, the health of these bivalves is indicative of the ecosystem’s overall health. (Image credit: S. Allen, source)
The Actual Shape of Raindrops
If you imagine the shape of a raindrop, you probably think of a tear drop shape, but the reality of rain is much more complicated. It’s Okay to Be Smart has a great primer on the subject that takes a look at the forces on a raindrop and shows you the actual shape they take, which depends largely on their size.
Small raindrops tend to coalesce together over time and get larger and progressively flatter. When the drop’s volume gets too large (below), it balloons up like a parachute. Researchers call this a bag. Stretched into a film, the drop’s surface tension is no longer able to win its fight against aerodynamic forces, and the drop shreds into smaller droplets. (Video and image credit: It’s Okay to Be Smart)
Kelly Slater’s Surf Ranch
Many of us who grew up visiting water parks instead of ocean beaches have spent time bobbing in a wave pool. They’ve been around for decades. But a new generation of wave pools are aiming for a different goal: the perfect surf wave. One of the foremost current facilities is Kelly Slater’s Surf Ranch, shown above. Here a hydrofoil (draped in blue tarps on the left) is pulled along an artificial lagoon to create dozens of wave profiles, all engineered to give surfers a long ride on the perfect solitary wave.
Other facilities, like the surf ranch used by USA Surfing in Waco, Texas, design their waves with different goals in mind. The Waco wave pool uses air pressure to drive their waves, and aims for a larger quantity of shorter waves. They’re designed to help young surfers practice skills they’re working on, and to give them a place where they can experience waves like those they’ll face in the upcoming 2020 Olympics in Tokyo. (Image credit: R. Young/WIRED; CNet, source; submitted by Lionel V.)
