Branching cracks wend through the slopes of Utah in this photograph by Matt Payne. It may seem strange to feature something so dry on a blog about fluid dynamics, but everything seen here depends as much on air and water as on soil, rock, and sand. How water intrudes into the porous landscape and the way it evaporates back out is critical to crack formation. (Image credit: M. Payne; via ILPOTY)
Tag: physics
Gliding Like a Grasshopper
Many biorobots are built after flies and bees–insects that rely heavily on flapping flight. For small robots, this means carrying heavy batteries or remaining tethered in order to power their motors. Instead, researchers have turned to grasshoppers for a lesson in small-scale gliding.
Grasshoppers have two sets of wings. The forward set provide protection and camouflage, while the hindwings are used to fly. The team studied the corrugated, foldable hindwings of the American grasshopper, then 3D-printed model wing designs and attached them to gliders. They found that the corrugated wings performed well at low angles of attack, but that non-corrugated wings–which still shared the outline and camber of the insect’s wings–were more efficient gliders over a range of conditions.
The team hopes that their grasshopper-inspired gliders give insect-like biorobots more efficient flying options. (Image credit: Princeton/S. Khan/Fotobuddy; research credit: K. Lee et al.; via Physics World)
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Instabilities in a Particle Flow
![Composite image of bed layers for 4 different particle density ratios. Text reads, "The wave amplitude and growth rate increase with particle density ratio but only if [the density of large particles is greater than the smaller particle density]."](https://fyfluiddynamics.com/wp-content/uploads/KHbed3.png)
Even though particles are not (strictly speaking) a fluid, they often behave like one. Here, researchers investigate what happens when two layers of particles–with different size and density–slide down an incline together. The video is tilted so that the flow instead appears from left to right.
When the larger, denser particles sit atop a layer of smaller, lighter particles, shear between the two layers causes a Kelvin-Helmholtz instability that runs in the direction of the flow. This creates a wavy interface that lets some small particles work upward while large particles shift downward.
At the same time, a slice across the flow shows that plumes of small particles are pushing up toward the surface, driven by a Rayleigh-Taylor instability. The researchers also look at what happens when the particles are fluidized by injecting a gas able to lift the particles. (Video and image credit: M. Ibrahim et al.; via GFM)
Watching Waves on the Nanoscale
It’s tough to simulate nonlinear wave dynamics, so scientists often test theories in wave flumes, where they can create more controlled waves than what we see in the wild. But conventional wave flumes are big–meters-long, complicated equipment–and can only test a small range of conditions. To reach more extreme nonlinear dynamics, researchers have turned to a chip-based approach. These 100-micron-long wave flumes carry a film of superfluid helium less than 7 nanometers thick. But despite that tiny size, the system can reach levels of nonlinearity five orders of magnitude greater than their full-sized counterparts. (Image and research credit: M. Reeves et al.; via Physics Today)
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A Supernova in Motion
In 1604, astronomers first caught sight of Kepler’s Supernova Remnant, a massive explosion some 17,000 light-years away. Twenty-five years of observations from the Chandra X-ray Observatory went into making this timelapse, which shows the supernova remnant‘s material pushing into the surrounding gas and dust.
In its fastest regions, the supernova remnant is moving around 2% of the speed of light–some 22 million kilometers per hour. Slower parts of the remnant are moving at just 0.5% of light-speed. (Image credit: NASA/CXC/SAO/Pan-STARRS; via Gizmodo)
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Caught in a Spider’s Web
Grains of pollen are caught amid droplets on a spider’s web in this award-winning image by John-Oliver Dum. How droplets behave on fibers has been a popular topic in recent years with research on how droplets nestle into corners, how they slide on straight or twisted wires, the patterns formed by streams of falling drops, and what happens to a droplet on a plucked string. (Image credit: J. Dum; via Ars Technica)
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Bouncing Indefinitely
On the surface of a gently vibrating liquid, a droplet can bounce indefinitely without coalescing, kept aloft by an air film too small to see. As long as the droplet lifts off before the air layer drains out from under it, the droplet won’t contact the water below. Now scientists have shown that this is possible with a solid surface, too.
Using an atomically smooth mica plate, researchers were able to bounce a droplet indefinitely without wetting the surface. At higher vibration rates (below), the droplet essentially hovers in place, bouncing so quickly that we simply see its shape vibrating in response to the surface. (Image and research credit: L. Molefe et al.; via APS)
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Jupiter in a Lab
The vivid bands of a gas giant like Jupiter come from the planet’s combination of rotation and convection. It’s possible to create the same effect in a lab by rapidly spinning a tank of water around a central ice core. That’s the physical set-up behind this research poster–note the illustration in the lower right corner. The central snapshots show how temperature gradients on the water surface change the faster the tank rotates. At higher rotational speeds, the parabolic water surface gets ever steeper and Jupiter-like temperature bands form. (Image credit: C. David et al.)
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Making Bubbles in Magma
When bubbles form in magma deep below the earth, volcanic eruptions follow. Scientists believe this happens when decompression of the magma allows volatile compounds to come out of solution and form bubbles–just as opening a bottle of seltzer allows carbon dioxide to bubble out. But a new study indicates that decompression may not be the only source of bubbles.
The team found that supersaturated fluids can nucleate bubbles when they’re sheared–even without decompression. They demonstrated this in the lab, not with magma but with a low-temperature magma analog, seen above. The more saturated with volatiles the fluid is, the less shear is needed to trigger bubbles.
Viscous shear is everywhere for magma, so this bubble formation mechanism is likely common. Better understanding how and when bubbles form in magma directly affects predictions for eruptions–especially for determining whether they’re likely to be explosive or effusive. (Image credit: volcano – A. Bonnerdeaux, experiment – O. Roche et al.; research credit: O. Roche et al.; via Physics World)
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Flow Through Granular Beds
We often rely on water draining through beds of grains, whether it’s the soil foundation beneath a building or the sand-and-gravel-filter used in water treatment. But how does water move through these tortuous porous passages? That’s what we see in this video, which places grains in a jig resembling an ant farm and lets us watch as water–and air–drain through the grains. The result is more complicated than you might imagine, with dry pockets, weak spots, and developing sinkholes. (Video and image credit: J. Choi et al.)
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