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#fluiddynamics
Ghostly Waterfalls
Photographer Jonathan Knight likes capturing waterfalls about 45 minutes after sunset, creating ghostly images that emphasize the shape of the cascading water. The dim surroundings and misty shapes remind me of old daguerreotypes. See more of his images on his website and his Instagram. (Image credit: J. Knight; via Colossal)
Hot Droplets Bounce
In the Leidenfrost effect, room-temperature droplets bounce and skitter off a surface much hotter than the drop’s boiling point. With those droplets, a layer of vapor cushions them and insulates them from the hot surface. In today’s study, researchers instead used hot or burning drops (above) and observed how they impact a room-temperature surface. While room-temperature droplets hit and stuck (below), hot and burning droplets bounced (above).
In this case, the cushioning air layer doesn’t come from vaporization. Instead, the bottom of the falling drop cools faster than the rest of it, increasing the local surface tension. That increase in surface tension creates a Marangoni flow that pulls fluid down along the edges of the drop. That flow drags nearby air with it, creating the cushioning layer that lets the drop bounce. In this case, the authors called the phenomenon “self-lubricating bouncing.” (Image and research credit: Y. Liu et al.; via Ars Technica)
Drops on the Edge
Drops impacting a dry hydrophilic surface flatten into a film. Drops that impact a wet film throw up a crown-shaped splash. But what happens when a drop hits the edge of a wet surface? That’s the situation explored in this video, where blue-dyed drops interact with a red-dyed film. From every angle, the impact is complex — sending up partial crown splashes, generating capillary waves that shift the contact line, and chaotically mixing the drop and film’s liquids. (Video and image credit: A. Sauret et al.)
Bifurcating Waterways
Your typical river has a single water basin and drains along a river or two on its way to the sea. But there are a handful of rivers and lakes that don’t obey our usual expectations. Some rivers flow in two directions. Some lakes have multiple outlets, each to a separate water basin. That means that water from a single lake can wind up in two entirely different bodies of water.
The most famous example of these odd waterways is South America’s Casiquiare River, seen running north to south in the image above. This navigable river connects the Orinoco River (flowing east to west in this image) with the Rio Negro (not pictured). Since the Rio Negro eventually joins the Amazon, the Casiquiare River’s meandering, nearly-flat course connects the continent’s two largest basins: the Orinoco and the Amazon.
For more strange waterways across the Americas, check out this review paper, which describes a total of 9 such hydrological head-scratchers. (Image credit: Coordenação-Geral de Observação da Terra/INPE; research credit: R. Sowby and A. Siegel; via Eos)
Playful Martian Dust Devils
The Martian atmosphere lacks the density to support tornado storm systems, but vortices are nevertheless a frequent occurrence. As sun-warmed gases rise, neighboring air rushes in, bringing with it any twisted shred of vorticity it carries. Just as an ice skater pulling her arms in spins faster, the gases spin up, forming a dust devil.
In this recent footage from the Perseverance Rover, four dust devils move across the landscape. In the foreground, a tiny one meets up with a big 64-meter dust devil, getting swallowed up in the process. It’s hard to see the details of their crossing, but you can see other vortices meeting and reconnecting here. (Video and image credit: NASA/JPL-Caltech/LANL/CNES/CNRS/INTA-CSIC/Space Science Institute/ISAE-Supaero/University of Arizona; via Gizmodo)
“Dispersion”
In “Dispersion,” particles spread under the influence of an unseen fluid. Like Roman de Giuli’s work, filmmaker Susi Sie creates macro images that look like ice floes, deserts, and river deltas viewed from above. This similarity of patterns at both large and small scales is a specialty of fluid physics. Just as artists use it to mimic larger flows, scientists use it to study planet-scale problems in the lab. (Video and image credit: S. Sie et al.)
Inside an Alien Atmosphere
Studying the physics of planetary atmospheres is challenging, not least because we only have a handful of examples to work from in our own solar system. So it’s exciting that researchers have unveiled our first look at the 3D structure of an exoplanet‘s atmosphere.
Using ground-based observations, researchers studied WASP-121b, also known as Tylos, an ultra-hot Jupiter that circles its star in only 30 Earth hours. One face of the planet always faces its star while the other faces into space. The team found that the exoplanet has a flow deep in the atmosphere that carries iron from the hot daytime side to the colder night side. Higher up, the atmosphere boasts a super-fast jet-stream that doubles in speed (from an estimated 13 kilometers per second to 26 kilometers per second) as it crosses from the morning terminator to the evening. As one researcher observed, the planet’s everyday winds make Earth’s worst hurricanes look tame. (Image credit: ESO/M. Kornmesser; research credit: J. Seidel et al.; via Gizmodo)
Channeling Espresso
Coffee-making continues to be a rich source for physics insight. The roasting and brewing processes are fertile ground for chemistry, physics, and engineering. Recently, one research group has focused on the phenomenon of channeling, where water follows a preferred path through the coffee grounds rather than seeping uniformly through the grounds. Channeling reduces the amount of coffee extracted in the brew, which is both wasteful and results in a less flavorful cup. By uncovering what mechanics go into channeling, the group hopes to help baristas mitigate the undesirable process, creating a repeatable, efficient, and tasty espresso every time. (Image credit: E. Yavuz; via Ars Technica)
Advancing mathematical physics for fluid motion:
#fluiddynamics #motion #mathematics #physics #research
Researchers claim to have solved Hilbert’s sixth problem by unifying three theories of #FluidDynamics at different levels of granularity:
+ Newton’s laws of motion at the microscopic level where fluids are composed of particles - little billiard balls bopping around and occasionally colliding
+ The Boltzmann equation at the mesoscopic level where the equation considers the likely behavior of a typical particle
+ Euler and #NavierStokes equations at the macroscopic level where the fluids are a single continuous substance
Preprint https://arxiv.org/abs/2503.01800
Flying Without a Rudder
Aircraft typically use a vertical tail to keep the craft from rolling or yawing. Birds, on the other hand, maneuver their wings and tail feathers to counter unwanted motions. Researchers found that the list of necessary adjustments is quite small: just 4 for the tail and 2 for the wings. Implementing those 6 controllable degrees of freedom on their bird-inspired PigeonBot II allowed the biorobot to fly steadily, even in turbulent conditions, without a rudder. Adapting such flight control to the less flexible surfaces of a typical aircraft will take time and creativity, but the savings in mass and drag could be worth it. (Image credit: E. Chang/Lentink Lab; research credit: E. Chang et al.; via Physics Today)
"In the paper, the researchers suggest they have figured out how to unify three physical theories that explain the motion of fluids. [...] This breakthrough won’t change the theories themselves, but it mathematically justifies them and strengthens our confidence that the equations work in the way we think they do."
Salt Fingers
Any time a fluid under gravity has areas of differing density, it convects. We’re used to thinking of this in terms of temperature — “hot air rises” — but temperature isn’t the only source of convection. Differences in concentration — like salinity in water — cause convection, too. This video shows a special, more complex case: what happens when there are two sources of density gradient, each of which diffuses at a different rate.
The classic example of this occurs in the ocean, where colder fresher water meets warmer, saltier water (and vice versa). Cold water tends to sink. So does saltier water. But since temperature and salinity move at different speeds, their competing convection takes on a shape that resembles dancing, finger-like plumes as seen here. (Video and image credit: M. Mohaghar et al.)
Arctic Melt
Temperatures in the Arctic are rising faster than elsewhere, triggering more and more melting. Photographer Scott Portelli captured a melting ice shelf protruding into the ocean in this aerial image. Across the top of the frozen landscape, streams and rivers cut through the ice, leading to waterfalls that flood the nearby ocean with freshwater. This meltwater will do more than raise ocean levels; it changes temperature and salinity in these regions, disrupting the convection that keeps our planet healthy. (Image credit: S. Portelli/OPOTY; via Colossal)
Atmospheric Rivers Raise Temperatures
Atmospheric rivers are narrow streams of moisture-rich air running from tropical regions to mid- or polar latitudes. Though relatively short-lived, they are capable of carrying — and depositing — more water than the largest rivers. But researchers have found that their impact is not measured in water content alone. Instead, a survey of 43 years’ worth of data shows that atmospheric rivers also bring unusually warm temperatures. In some cases, the authors found surface temperatures near an atmospheric river climbed to as high as 15 degrees Celsius above the typical. On average, temperatures were about 5 degrees Celsius higher than expected for the region’s climate.
Several factors raise those temperatures — like the heat released when rising vapor meets cooler air and condenses into liquid — but the biggest effect came from carrying warm tropical temperatures to (usually) cooler regions. (Image credit: L. Dauphin/NASA; research credit: S. Scholz and J. Lora; via Physics Today)
Winter in Chicago
Fresh winter snow blankets Chicago in this satellite image. Over on Lake Michigan, ice dots the coastline out to about 20 kilometers from shore. Darker regions near land mark thinner ice being pushed outward by the wind. Further out, the ice appears white and may be thicker thanks to wind-driven ice piling up. (Image credit: M. Garrison; via NASA Earth Observatory)
Measuring Mucus by Dragging Dead Fish
A fish‘s mucus layer is critical; it protects from pathogens, reduces drag in the water, and, in some cases, protects against predators. But little is known about how mucus could affect terrestrial locomotion in species like the northern snakehead, which can breathe out of the water and move across land. So researchers explored the snakehead’s mucus layer by measuring the force required to drag them (and two other non-terrestrial species) across different surfaces.
The team tested the same, freshly euthanized fish twice: once with its mucus layer intact and again once the mucus was washed off. Unsurprisingly, the fish’s friction was much lower with its mucus. But they also found that the snakehead was slipperier than either the scaled carp or the scale-free catfish. The biologists suggest that the snakehead could have evolved a slipperier mucus to help it move more easily on land, thereby extending the distance it can cover.
As a fluid dynamicist, I think fish mucus sounds like a great new playground for the rheologists among us. (Image and research credit: F. Lopez-Chilel and N. Bressman; via PopSci)
Chaotic Hose Instability
Steve Mould is back with another video looking at wild fluid behaviors. This time he’s considering hose instabilities like the one that makes a water-carrying hose beyond a certain length to whip wildly back and forth. He tries to track down the reasoning for these flexible hoses snapping and whipping. In truth, both the hoses and the wind dancers do their thing due to interactions between the elasticity of the hose and the fluid dynamics of the flows within. These applications are ripe for a few control volume thought experiments. (Video and image credit: S. Mould)
Reclaiming the Land
Lava floods human-made infrastructure on Iceland’s Reykjanes peninsula in this aerial image from photographer Ael Kermarec. Protecting roads and buildings from lava flows is a formidable challenge, but it’s one that researchers are tackling. But the larger and faster the lava flow, the harder infrastructure is to protect. Sometimes our best efforts are simply overwhelmed by nature’s power. (Image credit: A. Kermarec/WNPA; via Colossal)
