Tag: fluid dynamics

Understanding Stars’ Seismology
Our understanding of Earth’s interior is based mostly on observations of seismic waves, which travel differently through our rocky crust and the molten core. Scientists similarly use seismic waves in stars to determine their interiors. But the pressure and temperature conditions in stars are far beyond anything we have here on Earth, which makes predicting how waves will travel in such exotic material difficult.
To better understand these extreme temperatures and pressures, scientists are using Lawrence Livermore’s National Ignition Facility (NIF) to mimic conditions similar to the outer envelope of a white dwarf star, like the one shown in the center of the image above. NIF’s laser array – shown as the blue lines in the artist’s conception above – can generate spherical shock waves that, as they converge on a solid sample, create pressures as high as 450 Mbar, more than 400 million times sea level atmospheric pressure here on Earth. Although the shock wave takes only 9 ns to travel across the sample, it’s enough to give researchers a glimpse into star-like conditions. (Image credit: NASA/ESA/C. O’Dell/D. Thompson, Lawrence Livermore National Laboratory; via Physics Today)
November 10, 2020
Spinning Bubbles
Fluid dynamics is largely about figuring out the relationship between forces. For a soap bubble sitting still, that’s primarily the effect of gravity, which makes the fluid in the soap film drain downward, and surface tension, which tries to maintain a spherical shape for the bubble.
Once you start spinning the bubble, though, there are new forces that come into play. One is the centrifugal force caused by the rotation, and another is the drag force between the rotating soap bubble and the air inside and outside of it. The addition of these forces drastically changes the bubble’s shape. It becomes wobbly and flattens out. Watch the contact line where the bubble meets the surface and you’ll also see it creeping outward toward the edge of the platform. (Image credit: C. Kalelkar and S. Paul, source)
November 9, 2020
Storm Eyes and Mushrooms in a Drop
In industry, drying droplets often have many components: a liquid solvent, solid nanoparticles, and dissolved polymers. The concentration of that last component — the polymers — can have a big effect on the way the droplet dries, as seen in the video above.
Without polymers, the droplet dries similarly to a coffee ring stain. But at moderate concentration, we see something very different. The droplet forms an eye in the middle, similar to a hurricane’s, and the edges of the droplet sprout mushroom-shaped plumes that grow and merge with one another along the edge. With even larger polymer concentrations, the mushrooms sweep their way inward, leaving a feathery stain behind. (Video, image, and research credit: J. Zhao et al.)
November 6, 2020
Granular Fingers
Finger-like shapes often form on fluids injected between glass plates, but what happens when that injected fluid contains particles? That’s the situation in this recent study, where researchers sandwiched a fluid between two glass plates and then injected a second, similar fluid laced with particles.
Despite the differences from the traditional Saffman-Taylor set-up, the granular-filled fluid still forms fingers as long as there’s even a slight density difference between the original and injected fluids. It doesn’t even matter which of the two fluids has the greater density! (Image and research credit: A. Kudrolli et al.)
November 5, 2020
Freshwater Mussels
Freshwater bivalves like these California floater mussels are critical species for the health of our waters. And although we don’t think of mussels as being very mobile, they’re actually quite active. As larvae, the mussels get released from their parent bivalve and attach to the fins or gills of a fish. While they develop, they cling to the fish, hitching a ride until they’re ready to strike out on their own. Considering the fluid forces typical on those areas of a fish, that means the larvae must have some impressive strength!
Once grown, the mussels anchor themselves using their tongue-like foot and begin their filter-feeding. They draw water in through a cilia-lined inlet, filter out algae, oxygen, and other nutrients, and expel clean water. This constant cycling, though largely invisible to the naked eye, is how bivalves keep their native waterways clean. (Image and video credit: Deep Look)
November 4, 2020
Recreating Infinity
In the ocean, tiny organisms can migrate hundreds of meters through the water column. Recreating and tracking those journeys in a lab is quite a challenge, but it’s one the researchers behind the Gravity Machine have conquered. This apparatus uses a wheel to essentially give micro-organisms an infinite water column to traverse while keeping them fixed in the lab microscope’s field of view.
With the device, researchers can watch organisms switch naturally between rising, sinking, and feeding behaviors as they would in the wild. The group is working to make it so that anyone with a microscope can recreate their set-up for observations. (Image, video, and research credit: D. Krishnamurthy et al.; see also Gravity Machine; submitted by Kam-Yung Soh)

November 3, 2020
Leidenfrost on Water
When a skillet is hot enough, water droplets will skitter across the surface almost frictionlessly thanks to the Leidenfrost effect. The incredibly high temperature of the surface relative the the liquid’s boiling point causes part of the drop to vaporize, enveloping the remainder of the liquid in a protective vapor cocoon.
We see this effect for more than just solid surfaces, though. This video demonstrates how pouring liquid nitrogen on a pool of water creates plenty of Leidenfrost weirdness as well. It looks as though the initial pour freezes some condensation to dust or other particles, which then stream outwards on a cloud of vapor. Larger droplets of liquid nitrogen actually manage to hold together on the pool’s surface. Their vapor keeps them from touching the water, but that flow also jostles them, creating a ring of ripples around the jiggling drop. (Video and image credit: Science Marshal)
November 2, 2020
Green Swirls and Dark Streaks
Green phytoplankton blooms swirl through the currents of the Baltic Sea in this satellite image. Individual phytoplankton are microscopic, which makes them excellent tracer particles in the flow; together, they make the ocean’s motion visible. Look closely and you’ll see dark streaks across the images showing where ships’ wakes are disrupting the bloom. (Image credit: J. Stevens/USGS; via NASA Earth Observatory)
October 30, 2020
Landings Beyond Earth
With planning for manned and unmanned missions to the Moon, Mars, and many asteroids underway, engineers are using numerical simulations to understand how spacecraft thrusters interact with planetary surfaces. Most practical data for this problem comes from the Apollo program and is of limited use for current missions. Recreating a Martian landing on Earth isn’t straightforward, either, given our higher gravity. Thus, supercomputers and numerical simulation are the best available tool for understanding and predicting how the plumes from a spacecraft’s thrusters will interact with a surface and what kind of blowback the spacecraft will need to withstand. (Video credit: U. Michigan Engineering; research credit: Y. Yao et al.; submission by Jesse C.)
October 29, 2020
Droplets From Speaking
Illnesses like COVID-19 can spread through droplets and aerosols produced by coughing, sneezing, or even speaking. New research looks at how regular speech patterns produce a spray of droplets. Researchers found that pronouncing many consonants causes a sheet of saliva to form between the speaker’s lips. That sheet stretches into filaments that then break into a spray of droplets.
Strong, plosive consonants like /p/ and /b/ create the most droplets (Images 2 and 3), but even milder consonants like /m/ create some (Image 1). Interestingly, the researchers found that wearing lip balm drastically decreased droplet production by altering the saliva sheet formation. Even so, there’s no substitute for wearing a properly fitted mask! (Image credits: masks – K. Grabowska, droplets – M. Abkarian and H. Stone; research credit: M. Abkarian and H. Stone; via APS Physics)
October 28, 2020