Year: 2020

Capsule Impact and Bursting
Nature and industry are full of elastic membranes filled with a fluid, from red blood cells to water balloons. A new study looks at how these capsules deform — and sometimes burst — on impact. The researchers created custom elastic shells that they filled with various fluids like water, glycerol, and honey, then used the impacts to build a model of capsule deformation.
They found that there’s significant overlap between droplet impacts and capsule impacts, with a few key differences; instead of surface tension, capsules resist deformation through their elastic shell’s surface modulus — a combination of its elasticity and thickness. Capsules, unlike droplets, can also burst. To study this, the researchers used water balloons, which they were able to pre-stretch more easily than their custom shells. They found that their model could accurately predict the conditions under which the balloons burst.
The authors hope the model will be helpful both in designing capsules intended to burst — like a fire-fighting projectile — and in creating safety measures to prevent capsule burst — like car-crash standards that protect from organ damage. (Image and research credit: E. Jambon-Puillet et al.; via Physics World; submitted by Kam-Yung Soh)
May 8, 2020
How COVID-19 Affects the Lungs
One of the best known COVID-19 symptoms of this pandemic is difficulty breathing, and while you’ve likely heard a lot about ventilators used to help patients get oxygen, you may not know much about the processes that cause the breathing problems. This video from Deep Look provides a solid overview of the infection route and how lung damage occurs during infection. Perhaps unsurprisingly — this is FYFD, after all — fluid dynamics plays a major role in this process, both under normal conditions and when air sacs in the lungs get damaged by the body’s immune system responding to the virus. (Image and video credit: Deep Look)
May 7, 2020
A Year From Geostationary Orbit
Our planet is a complex fluid dynamical system, and one of the best ways to watch nature at work is through timelapse. This short film takes us through an entire year, from December 2015 to December 2016, as viewed from a geostationary weather satellite centered over Oceania.
The imagery is rather hypnotic, with clouds swirling day and night across the full field of view. Watch closely, though, and you’ll see a lot of neat phenomena from typhoons forming in the Pacific to wave clouds streaming from the islands of Japan. You can also see clouds blossoming (especially during the day) over the humid rainforests of Oceania.
There are neat non-fluids phenomena, too, like a total solar eclipse and the permanent sunlight of Arctic and Antarctic summers. What do you notice? (Image and video credit: F. Dierich)
May 6, 2020
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)
May 5, 2020
Holding Pipes in Place
Newton’s 3rd law states that any action has an equal and opposite reaction. Often engineers use this to our advantage; the thrust from expelling propellants is what lifts our rockets to space. But sometimes those reactions are undesirable, as illustrated in this Practical Engineering video with underground pipes.
Anytime flow through the pipe is forced to change direction, the flow causes an equal and opposite force on the joint. Just as with rockets, engineers refer to this reaction force as thrust. And if the thrust goes unaccounted for, it will force pipe joints apart. Civil engineers use several methods to fix pipelines against these forces, including concrete blocks that distribute the force to the surrounding soil and flange fittings that resist pipe movement. (Video and image credit: Practical Engineering)
May 4, 2020
Singing in the MRI
We rarely consider just how complex the process is when we speak or sing. Sound waves produced in our larynx are shifted and amplified by the geometry of our throats, mouths, sinus cavities, tongues, and lips. This video provides a glimpse of that hidden complexity through a trained vocalist singing inside an MRI machine. He sings the same aria in four distinctly different vocal styles, and it’s incredible to watch all the changes his tongue, lips, and soft palette go through to produce those different sounds. (Image and video credit: T. Ross; via Flow Vis)
May 1, 2020
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.)
April 30, 2020
Bristling Sharkskin Fights Separation
The speedy shortfin mako shark has a secret weapon to fight drag: bristling denticles that line its fins and tail. Denticles are tiny, anvil-shaped enamel scales on the mako’s skin. In the photo above, each one is about 100 microns across. Under normal conditions, with flow moving over the shark from nose to tail, the denticles lie flat, providing no interference.
But when sudden changes in flow near the shark’s skin cause water to begin moving in the opposite direction, the denticles flare up. Their rise interferes with the reversed flow, trapping it in small eddies beneath each denticle. Since that flow reversal is a precursor to the flow separating from the shark’s body, the bristling effectively cuts off flow separation before it can begin. The result is much less separation and much lower drag. Once the flow stops trying to move upstream, the denticles settle back into their original place. (Image credit: mako shark – jidanchaomian, denticles – J. Oeffner and G. Lauder, illustration – A. Lang, bristling – A. Lang et al.; research credit: A. Lang and A. Lang et al.; submitted by Kam-Yung Soh)
April 29, 2020
Expanding Water Beads
In this timelapse, we see hydrogel beads expanding as they absorb water. There are some interesting subtleties to the physics here. Notice how, in the Petri dish segments, the beads shift from a single crystalline structure to several smaller structures. I suspect those shifts are driven by the dropping water level, which changes how surface tension interacts with the beads’ shape to create attractive forces between beads.
Another interesting point comes as the beads expand through and out of the glass of water. Initially, the water level doesn’t change in the glass. This is because the water beads are taking up the same volume as the water that they’ve absorbed. But once the beads emerge past the water’s initial height, the water level drops dramatically. That’s because the beads are still absorbing what little water is left and continuing to expand in volume. (Image and video credit: Temponaut)
April 28, 2020
Icy Swirls
Rafts of sea ice follow swirling eddies in this satellite image of the Gulf of St. Lawrence. Just as with phytoplankton blooms and sediment, this thin sea ice can be moved by wind and currents to reveal hidden flow patterns. Experimentalists use many similar diagnostics that introduce bubbles, particles, smoke, and other tracers into flows to visualize motion that’s otherwise invisible. (Image credit: J. Stevens/NOAA/NASA; via NASA Earth Observatory)
April 27, 2020