Year: 2019

Astrophysical Turbulence
Subsonic turbulence – like the random and chaotic motions of air and water in our everyday lives – is something we have only a limited understanding of. Our knowledge of supersonic turbulence, where shock waves and compressibility rule, is even more tenuous. In part this is because, although we can observe snapshots of supersonic turbulence in astronomical settings like the Orion Nebula shown above, we cannot watch it evolve. On these scales, features simply don’t change appreciably on human timescales.
This has limited scientists to mostly numerical and theoretical studies of supersonic turbulence, but that is starting to change. Researchers are now building experimental set-ups that collide laser-driven plasma jets to generate boundary-free turbulence at Mach 6. Thus far, the observations are consistent with what’s been seen in nature: at low speeds, the turbulence is consistent with Kolmogorov’s theories, with energy cascading from large scales to smaller ones predictably. But as the Mach number increases, the nature of the turbulence shifts, moving toward the large density fluctuations seen in nebulae and other astrophysical realms. (Image credit: F. Battistella; research credit: T. White et al.; see also Nature Astronomy; submitted by Kam-Yung Soh)
April 25, 2019
Fluid at Work
For many engineering students, their first experience with flow visualization comes in undergraduate labs, where dye introduced into a flume demonstrates basic flow features around airfoils, cylinders, and spheres. This short video by undergraduate Nick Di Guigno and partners quietly illustrates that experience, from the introduction to the equipment to loading the dye and watching the flow develop under the commentary of one’s professor. For those of you who have done this, I suspect it may ignite a bit of nostalgia. For those who haven’t, I think it captures some of the magical feeling of stepping into the lab the first time, even when you’re just recreating a phenomenon others have seen a thousand times before. (Image and video credit: N. Di Guigno et al.)
April 24, 2019
As Ice Flows
The movement of glaciers is driven by gravity. The immense weight of the ice causes it to both slide downhill and deform – or creep. As glacier melting speeds up, scientists have debated how glacier flow will respond: will the loss of ice cause the glaciers to move more slowly since they have less mass, or will the increase in meltwater help lubricate the underside of glaciers and make them flow even faster?
By analyzing satellite image data of Asian glaciers collected between 1985 and 2017, researchers are finally answering that question. Their research shows that these glaciers are slowing down as they lose mass and speeding up as they gain mass. Nearly all – 94% – of the flow changes they observed can be accounted for solely from ice thickness and slope. This is valuable information as scientists continue to monitor and predict the changes we must expect as the world continues to warm. (Image credit: J. Stevens; research credit: A. Dehecq et al.; via NASA Earth Observatory)
April 23, 2019
Anak Krakatoa Tsunami
In late December 2018, a landslide on the island Anak Krakatoa triggered a deadly tsunami in Indonesia. The island (upper left, pre-landslide) lost an estimated 300 meters of height in the landslide, dramatically altering its appearance (upper right; post-landslide). Much of the slide occurred underwater, dumping material into a crater left by the famous 1883 eruption of Krakatoa.
The slide displaced a massive amount of water, creating a tsunami that spread, refracting around nearby islands and reflecting off shorelines in complicated patterns. A new numerical simulation, shown above, models the post-slide tsunami based on terrain data and fluid physics. Its wave predictions match well with the high-water readings from nearby islands. The scientists hope that such models, combined with monitoring, will help save lives should a future eruption trigger more tsunamis.
For a full picture of both the recent Anak Krakatoa eruption and its famous predecessor, check out this video. (Image credits: satellite views before and after landslide – Planet Labs; simulation – S. Ward, source; via BBC News; submitted by Kam-Yung Soh)
April 22, 2019
Fiery Backdraft
Combustion is ultimately a chemical reaction, and like any chemical reaction, it requires the right balance of ingredients. The only way to completely exhaust the reaction is to have the perfect amount of fuel (i.e. stuff to burn) and oxidizer (i.e. oxygen). When those ratios don’t match, the reaction can slow down or even appear to end, but that doesn’t mean a fire’s gone out.
Firefighters face one of the dangerous consequences of this situation in the form of backdrafts. When a fire has been burning in a sealed container and exhausted its oxygen supply, it can get extremely hot even if the flames seem to have died down. When oxygen is added back by opening a door or window, the fire can react explosively, as the Slow Mo Guys demonstrate above. The good news is that backdrafts are relatively rare and there are steps you can take to avoid them. (Image and video credit: The Slow Mo Guys)
April 19, 2019
Breaking
As waves fold over and break, they trap air, creating bubbles of many sizes. The smallest of these bubbles can be only a few microns across and persist for long times compared to larger bubbles. When they burst, they create tiny droplets that can carry sea salt up into the atmosphere to seed rain. Understanding how these bubbles form and how many there are of a given size is key to predicting both oceanic and atmospheric behaviors. Numerical simulations like the one featured in the video above reveal the dynamic collisions that create these tiny bubbles and help researchers learn how to model the tiniest bubbles so that future simulations can be faster. (Image and video credit: W. Chan et al.)
April 18, 2019
Resonating on a Bounce
When we think of resonance, we often think of it in simple terms: hit the one right note, and the wine glass will shatter. But resonance isn’t always about a one-to-one ratio between a driving frequency and the resonating system. Especially in fluid dynamics, we often see responses that occur at other, related frequencies.
One of the simplest places to see this is with a droplet bouncing on a bath of fluid. Above you see a liquid metal droplet bouncing on a bath of the same metal. At low amplitude, the pool surface moves at the driving frequency and a droplet bounces simply upon that surface, with one bounce per oscillation. Increase the amplitude, though, and the droplet’s bounce changes. It bounces twice – one large bounce and one small bounce – in the time it takes for the pool surface to go through one cycle. This is called period doubling because the bouncing occurs at twice the driving frequency.
Turn the amplitude up further, and the system undergoes another change. Faraday waves form on the surface. They resonate at half the driving frequency, and a droplet’s bouncing will sync up with the waves. That means the droplet returns to a one-to-one bounce with the waves, but the waves themselves are no longer reacting at the driving frequency. It’s this kind of complexity that makes fluid systems fertile grounds for studying paths toward chaos. (Image and research credit: X. Zhao et al.)
April 17, 2019
Recreating Pyroclastic Flow
One of the deadliest features of some volcanic eruptions is the pyroclastic flow, a current of hot gas and volcanic ash capable of moving hundreds of kilometers an hour and covering tens of kilometers. Since volcanic particles have a high static friction, it’s been something of a mystery how the flows can move so quickly. Using large-scale experiments (top), researchers are now digging into the details of these fast-moving flows.
What they found is that the two-phase flow results in a pressure gradient that tends to force gases downward. This creates a gas layer with very little friction near the bottom of the pyroclastic flow (bottom), essentially lubricating the entire flow with air. This helps explain why pyroclastic flows are so fast and long-lived despite their inherent friction and the roughness of the terrain over which they flow. (Image and research credit: G. Lube et al.; video credit: Nature; submitted by Kam-Yung Soh)
April 16, 2019
Coalescence at the Smallest Scales
The coalescence of two water droplets happens so quickly, it’s essentially impossible to see, even with high-speed cameras. For this reason, researchers have turned to simulating molecular dynamics – essentially building computer programs that model the actions of all the molecules contained in the water droplets. Viewed this way, the very first contact between drops comes from thermal fluctuations – the random jumping of molecules across the separating gap. Once the bridge starts to form, it continues to grow, driven by thermal forces and opposed by surface tension. Eventually, this thermal regime gives way to the more familiar hydrodynamic one, where the bridge is large enough for flow to drive its growth. (Image credits: experiment – S. Nagel et al.; simulation – S. Perumanath et al.; research credit: S. Perumanath et al.; submitted by Rohit P.)
April 15, 2019
Digging Sandpits
Antlion larvae dig sandpits to catch their prey, and, according to a new study, they rely on the physics of granular materials to do so. The antlion digs in a spiral pattern (bottom), beginning from the outside and working its way inward. As it digs, it ejects larger grains and triggers avalanches that cause large grains to fall inward. This leaves the walls of the final pit lined with small grains, which have a shallower angle of repose and will slip out from under any prey that wander in. The subsequent avalanche will carry the victim to the antlion lying in wait at the center of the pit. (Image credits: antlion larva – J. Numer; antlion digging – N. Franks et al.; research credit: N. Franks et al.; submitted by Kam-Yung Soh)
April 12, 2019