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)
Tag: science
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.)
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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)
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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)
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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)
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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)
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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)
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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)
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Thawing Permafrost Primes Slumps
As permafrost thaws on Arctic hillsides and shorelines, the land often deforms in a unique fashion, known as a slump. Formally known as mega retrogressive thaw slumps, these areas superficially resemble a landslide. They’re also prone to repeat performances: as many as 90% of Canada’s Arctic slumps recur in the same place as previous slumps. Researchers used ground-penetrating radar and other tools to study the underground structure at slumps and found that several factors contribute to this repetitive cycle.
Seawater soaking into the foot of a hilly shore can destabilize the permafrost, creating a slump. That changes the nearby ground cover, exposing more permafrost to warming; their measurements showed this warming could extend tens of meters underground, priming the area for future slumps. Similarly, the mudslides and narrow ravines that form on an active slump also shift away ground cover and warm the underlying permafrost. Together, these factors suggest that once a slump forms, more slumps will occur as the underlying permafrost warms. (Image credit: M. Krautblatter; research credit: M. Krautblatter et al.; via Eos)
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Twisting in the Flow
What happens to liquid crystals in a flow? In this video, researchers look at liquid crystals flowing through the narrow gap of a microfluidic device. Initially, all the crystals are oriented the same way, as if they are logs rolling down a river. But as the flow rate increases, narrow lines appear in the flow, followed by disordered regions, and, eventually, a new configuration: vertical bands streaking the left-to-right flow. The colors, in this case, indicate the orientation of the liquid crystals. As the researchers show, the crystals collectively twist to form the spontaneous bands. (Video and image credit: D. Jia and I. Bischofberger)
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