Known by many names — including the Witch’s Broom Nebula — NGC 6960 is part of a supernova remnant visible in the constellation Cygnus. The wisp-like filaments of the nebula are shock waves moving through the cloud of dust and ionized gas. Based on observations using the Hubble Space Telescope, the nebula is expanding at around 1.5 million kilometers per hour. When the original supernova exploded thousands of years ago, astrophysicists estimate it would have been bright enough to see during daytime! (Image credit: K. Crawford)
Month: October 2021
Solid, Liquid, Both?
Materials like oobleck — a suspension of cornstarch particles in water — are tough to classify. In some circumstances, they behave like a fluid, but in others, they act like a solid. Here researchers sandwiched a thin layer of oobleck between glass plates and injected air into the mixture. For a fluid, this setup creates a classic Saffman-Taylor instability where rounded fingers of air push their way into the more viscous fluid. And, indeed, for low air pressures and low concentrations of cornstarch, the oobleck forms these viscous fingers. You can see examples in the top row’s first and third image, the second row’s middle image, and the bottom row’s third image.
Injecting air at high pressures and high cornstarch concentrations fractures the oobleck like a solid (middle row, first and third images). At intermediate pressures and concentrations, the oobleck forms a pattern called dendritic fracturing, where new branches can grow perpendicularly to their parent branch. Examples of this pattern are in the top row’s second image and the bottom row’s first and second images. (Image and research credit: D. Ozturk et al.; via Physics Today)
Seeking Magma
In 2009, drillers seeking geothermal energy in Iceland accidentally pierced a hidden magma chamber. After a billowing pillar of steam and glass shards poured out from the hole, it created the hottest geothermal well ever, until the casing failed. Now drillers are preparing to return to the area, this time with the intention of reaching magma. Capturing a sample of magma before it rises to the surface (thereby losing its trapped gases) is something of a holy grail for geophysicists, who otherwise rely on seismic wave detections and observations of magma that’s reached the surface. Building a long-term magma observatory will be an enormous engineering challenge, but the technologies developed may help us explore other hellish environments like the surface of Venus. (Image credit: G. Fridleifsson/IDDP; via Science)
Superfluid Instabilities
Superfluids — like Bose-Einstein condensates — are bizarre compared to fluids from our everyday experience because they have no viscosity. Without viscosity, it’s no surprise that they behave in unusual ways. Here, researchers simulated superfluids moving past one another. In each of these images, the blue fluid is moving to the left, and the red fluid is moving to the right. In a typical fluid, such motion causes ocean-wave-like curls due to the Kelvin-Helmholtz instability.
Instead, with a low relative velocity and high repulsion between atoms of the two layers, the superfluids form a tilted, finger-like interface (Image 1) that the authors refer to as a flutter-finger pattern. (Repulsion essentially sets the miscibility between the superfluids. With a high repulsion, the superfluids resist mixing.)
With a higher relative velocity (Image 2), the wavelength of the ripples becomes comparable to the thickness of the interface, and the superfluids take on a very different, zipper-like pattern. Note how the tips detach and reconnect to the neighboring finger!
With lower repulsion, the interface between the two liquids is thicker and breaks down quickly (Image 3). The authors call this a sealskin pattern. (Image credits: water – M. Blažević, simulations – H. Kokubo et al.; research credit: H. Kokubo et al.; via APS Physics)
RC Ground Effect Plane
The ekranoplan was a massive, Soviet-era aircraft that relied on ground effect to stay aloft. In this video, RC pilots test out their own homemade version of the craft, including some neat flow visualization of the wingtip vortices. When an aircraft (or, for that matter, a bird) flies near the ground, it experiences less drag than at higher altitudes. This happens primarily because of the ground’s effect on wingtip vortices.
In normal flight, the vortices from an aircraft’s wingtips create a downwash that reduces the wing’s overall lift. But in ground effect, the vortices cannot drift downward as they normally do. Instead, they spread apart from one another, thereby reducing the drag caused by downwash from the aircraft. The end result is better performance, though it comes with added risk since there’s very little time to correct an error when flying at an altitude less than half the aircraft’s wingspan. (Video and image credit: rctestflight; submitted by Simplicator)
As Above, So Below
I love a good crossover between fluid dynamics and something unexpected. Fiber artist Megan Zaniewski uses thread-painting techniques to embroider ducks, frogs, otters, and other animals as they appear both above and below water. I am blown away by how she captures the movement and turbulence of water in these pieces! Just look at that spectacular frog splash. You can find lots more of her art on her Instagram. (Image credit: M. Zaniewski; via Colossal)
Better Inhalers Through CFD
As levels of air pollution rise, so does the incidence of pulmonary diseases like asthma. Treatments for these diseases largely rely on inhalers containing drug particles that need to be carried into the small bronchi of the lungs. To better understand how the process works, researchers used computational fluid dynamics to simulate how air and particles travel through the human respiratory tract.
The team found that larger particles tended to get stuck in the mouth instead of making it down into the lungs. This problem was made worse at high inhalation rates because the particles’ inertia was too large for them to make the sharp turn down into the trachea. In contrast, smaller particles could travel down into the lungs and into the smaller branches there before settling. The authors concluded that inhalers should use fine drug particles to maximize delivery into the lungs. They also note that adjusting inhalers to deliver more medication to the lungs may also lower the overall price because less of the dosage gets wasted in the patient’s mouth.
Of course, the study’s results also serve as a warning about the dangers of air pollution from fine particulates. Here in Colorado, our summers are punctuated with wildfire smoke, much of it in the form of tiny particles about the same size as the drug particles in this study. If fine drug particles are effective at making it into the smaller branches of our lungs, so are those pollutants. That’s a good reason to stay inside in smoky conditions or use a high-quality N-95 mask while out and about. (Image credit: coltsfan; research credit: A. Tiwari et al.; via Physics World; submitted by Kam-Yung Soh)
The Noisy Gluggle Jug
The fish-shaped Gluggle Jug makes an impressive set of sounds when tilted for pouring. Steve Mould explores their origin in this video. When liquid is poured from a container, air needs a path in to replace the poured liquid. You’re likely most familiar with this from long-necked bottles, where trying to pour the liquid too quickly results in a glug-glug noise as air bubbles periodically force their way through the bottle neck. The same thing happens in the Gluggle Jug, particularly at the joint between the tail and body of the pitcher. The volume and resonance of the jug’s sounds comes from the shape; the open mouth of the container amplifies the sound of bubbles popping back from the tail region. (Image and video credit: S. Mould)
Sliding Along
Robust, self-cleaning surfaces are a holy grail for many engineers, but they’re tough to achieve. One necessary ingredient for a self-cleaning surface is the ability to shed water, which is why superhydrophobic coatings and surface treatments are popular. Here, researchers prompt their droplets to move at speeds up to 16 cm/s by dropping them onto a thin layer of heated oil.
Longtime readers will no doubt be reminded of self-propelling Leidenfrost drops, but this situation is not quite the same. In general, the oil layer suppresses the Leidenfrost effect. Instead, the oil heats the drop, evaporating its vapor. A bubble of vapor will nucleate at a random location in the droplet and eject itself, pushing the drop in the opposite direction. Because of the disruption caused by that ejection, new bubbles will preferentially form at the same spot, providing an ongoing supply of vapor that keeps the drop sliding in the same direction. It’s like a miniature rocket zooming along the oil film! (Image and research credit: V. Leon and K. Varanasi; via APS Physics)
Pressure At The Dam
Hydrostatic pressure in a fluid is based on the fluid’s depth. You’ll rarely see a more dramatic example of that power than with a water release from a dam. Here we see the outlet of the Verbund Hydro Power dam in Austria. With 190 meters of water behind the dam, the outlet jet is massive. It moves 20,000 liters of water per second at a speed of 50 meters per second. Imagine what it would be like to stand next to that! (Image and video credit: Discovery UK; submitted by Olwyn B.)
