Like breaking waves at the beach, these wavy clouds curl but only for a moment. The photo was captured near sunset on a late August evening in Arlington, MA. This short-lived cloud shape forms due to the Kelvin-Helmholtz instability, which is driven by shear forces between two layers of air moving at different speeds. The situation is a common one in the atmosphere, where air layers at altitude move in different directions and at different speeds. Most of the time we cannot see the curls that form between these air layers because of air’s transparency. But occasionally the mismatch happens right at a cloud layer and the condensation of the cloud gets pulled into these distinctive curls. (Image credit: B. Bray; submitted by Mark S.)
Search results for: “shear”
Ghostly Chandeliers
Under a black light, highlighter fluid creates ghostly trails as it drips through water. The vortices that form and break into this chandelier-like shape are the result of density differences between the ink and water. Since ink is heavier than water, it sinks, but as the two fluids flow past, they shear one another, forming elaborate shapes. Formally, this is known as the Rayleigh-Taylor instability. While you may be most familiar with it from pouring cream into coffee, it’s also a key to mixing in the ocean and the explosions of supernovas. (Image credit: S. Adams et al.; via Flow Vis)
The Vortex Beneath a Drop
While we’re most used to seeing levitating Leidenfrost droplets on a solid surface, such drops can also form above a liquid bath. In fact, the smoothness of the bath’s surface, combined with mechanisms discussed in a new study, means that drops will levitate at a cooler temperature over a liquid than they will over a solid surface.
Researchers found that a donut-shaped vortex forms in the bath beneath a levitating droplet, but the direction of the vortex’s circulation is not always the same. For some liquids, the flow moves radially outward from beneath the drop. In this case, researchers found that the dominant force was shear stress caused by the vapor escaping from under the droplet.
With other droplet liquids, the flow direction instead moved inward, forming a sinking plume beneath the center of the drop. In this situation, researchers found that evaporative cooling dominated. As the liquid beneath the droplet cooled, it became denser and sank. At the same time, the lower temperature changed the bath’s local surface tension, creating the inward surface flow through the Marangoni effect. (Image credit: F. Cavagnon; research credit: B. Sobac et al.)
A Lenticular Cloud With a Curl
Lens-shaped lenticular clouds are not terribly rare in mountainous areas, but observers at Mount Washington caught a very unusual cloud near sunrise in late February. This lenticular cloud had an added curl on top thanks to the Kelvin-Helmholtz instability!
Lenticular clouds form when air is forced to flow up over a mountain in such a way that its temperature and pressure drop and water vapor in the air condenses. The resulting water droplets form a cloud that appears stationary over the mountain, even though the air continues to flow.
To get that added wave-like curl, there needs to be another, faster-moving layer of air just above the cloud. As that air flows past, it shears the cloud layer, causing the interface to curl. Neither of these cloud types is long-lived — Kelvin-Helmholtz formations often last only a few minutes — so catching such a great dual example is lucky, indeed! (Image credit: Mount Washington Observatory; via Smithsonian Magazine; submitted by Kam-Yung Soh)
Studying Active Polymers Using Worms
I’ve covered some odd studies in my time, but this might be the strangest: to understand how active polymers affect viscosity, researchers loaded drunk worms into a rheometer. Active polymers are long-chain molecules that, like worms, can move on their own using stored energy or by extracting energy from their surroundings. Their dynamics are tough to study, though, because individual polymers are almost impossible to observe while a suspension of them is being deformed.
Enter the humble sludge worm. Often sold as fish food, these worms — like the polymers they’re meant to imitate — are individually quite wiggly but, given their size, are far easier to observe. Researchers placed them in a custom rheometer in a solution of water and observed how the worm mass responded when sheared by a spinning top plate (Image 3). Like active polymers, the worms exhibited shear-thinning; the faster the plate spun, the lower the worms’ viscosity, likely because the additional force helps align the worms.
But how do active worms compare with passive ones? The obvious solution would be to repeat their tests with dead worms, but the researchers found a more humane method: by adding some alcohol to the water, they temporarily reduced the worms’ activity, allowing them to compare active and passive worms (Image 2). Once rinsed in water, the worms sobered up and returned to their normal activity levels.
The researchers found that both the active and passive worms exhibited shear-thinning as the force on them increased, but the shear-thinning in the active worms was not as pronounced, presumably because the movements of individual worms prevented them from aligning smoothly. (Image and research credit: A. Deblais et al.; via Gizmodo and APS Physics)
Bioluminescence at the Beach
A bioluminescent phytoplankton bloom is causing a stir among California beachgoers. During the daytime, aggregations of Lingulodinium polyedra appear reddish-brown in color (think the classic ‘red tide’). But at night the phytoplankton bioluminesce, specifically when they’re disturbed by a change in shear force. This is why the brightest glows are visible in crashing waves or around the boards of surfers.
Beautiful as it appears, blooms like these are deadly to marine life. The excess numbers of phytoplankton strip water of oxygen, causing mass die-offs among fish. Even residents several miles inland of the beaches are reporting the unpleasant smell that results. (Image credits: AP; video credit: Scripps Institute of Oceanography; via Gizmodo)
Unifying Sediment Transport Theory
On windy days, streaks of snowflakes snake in the air above a mountaintop snowfield. And when snorkeling in the surf, you can watch the inbound waves sculpt underwater ripples in the sand. Both are examples of sediment transport, and scientists have struggled to understand why the physics of these grains seems to differ between air and water. We observe certain behaviors, like saltation, in air and very different behaviors for grains underwater.
One of the key differences is how much erosion occurs for a given amount of shear. In air, the relationship is linear; double the shear stress and you double the sediment transport rate. But in water, the relationship is nonlinear, meaning a small change in the shear stress can have a much larger effect on the rate of transport.
A new study suggests that these differences are really only skin deep. Through detailed simulations, the researchers showed that what really matters is the energy dissipation caused by collisions between grains. Whether the medium is air or water, there are two important regions in the flow: the bed region where particles experience little movement, and the overlying region where grains are energized and lifted by the flow. In this framework, the researchers found no difference in how energy is dissipated, regardless of the medium.
So why do measured sediment transport rates vary between air and water? The authors concluded that the relationship between shear and transport rate is, indeed, nonlinear. It’s just that the wind here on Earth is too weak to reach that nonlinearity. (Image credit: snow – wisconsinpictures, sand – J. Chavez; research credit: T. Pähtz and O. Durán; via APS Physics; submitted by Kam-Yung Soh)
Sliding Foams
What happens when a foam interacts with a sliding surface? That’s the question at the heart of this study, which finds three major regimes of foam-surface interaction. On smooth surfaces (Image 1), foams will simply slide against the wall without sticking or deforming. When surface roughness is about as large as the foam’s wall thickness (Image 2), the foam will stick to individual asperities, then slip to the next rough spot as the wall moves. But when the surface roughness is large compared to the foam wall (Image 3), the foam will remain anchored to the surface and all the shear from the wall’s movement goes into deforming the bulk of the foam.
Researchers thus found they could change foam’s behavior by changing the surface roughness. They also looked at the reverse situation: a surface with fixed roughness — like, say, a human tongue — and how tuning the size of foam bubbles might alter perception and ease of swallowing. That’s what we’re looking at in the last image, where a spoon slides a foam along a surface with roughness similar to the human tongue. (Image and research credit: M. Marchand et al.)
“Stormscapes 4”
Frightening as they can be in the moment, storms have a power and majesty all their own. I’ve never seen a better way to capture that than through timelapse, and photographer Nicolaus Wegner offers a great one in “Stormscapes 4″. I particularly like how his frame captures the motion of storms and how they shear, rotate, and billow as they evolve. With a quick glance upward, it’s easy to miss that motion, even if it is fundamental to these storms. Sit back and enjoy. (Video and image credit: N. Wegner)
Wave Clouds in the Front Range
Last Sunday night metro Denver was treated to a rare sight: clouds resembling breaking waves formed near sunset. These are Kelvin-Helmholtz clouds, and the comparison to ocean waves is apt, since the same physics is behind both. Winds were unusually calm near the ground Sunday night, but strong winds blew at the altitude just above the lower cloud layer. That velocity difference created strong shear where the two air layers met. With the cloud layer in place to differentiate the slower-moving air from the faster, we can what’s normally invisible: how the two air layers mix.
The Denver Post has several more views of the wave clouds from around the area, and you can learn lots more about the Kelvin-Helmholtz instability here. (Image credit: R. Fields; via the Denver Post)