In his recent short film, artist Roman De Giuli explores turbulence using metallic paints and inks in a fishtank. The effects are beautiful: sparkling pigments dispersing in clouds, mushroom- and umbrella-shaped Rayleigh-Taylor instabilities, and lots of swirling eddies. It’s exactly the kind of eyecandy to kick off your weekend with! (Image and video credit: R. De Giuli)
Gravity currents carry denser fluids into lighter ones, like cold air drifting under your door in winter or dense fogs flowing downhill in San Francisco. Here, researchers visualize the situation using denser salt water flowing into fresh water. Once the gate separating the two fluids rises, the salt water slides down an artificial slope into the fresh water.
Very quickly the flow forms a Kelvin-Helmholtz instability due to the different flow speeds between the two fluids. Kelvin-Helmholtz waves form distinctive swirls and billows that are reminiscent of a cat’s eye. As the swirls rotate, they can flow over one another, and break up into turbulence. (Image and video credit: C. Troy and J. Koseff)
“The Dark Days” is the third film in artist Thomas Blanchard’s N-UPRISING series. Like its siblings, this film features plants and insects, along with creeping — and sometimes overwhelming — fluid flows. The vivid colors of the orchids here make an uncomfortable juxtaposition with the air raid horns, sirens, and sounds of war that make up the soundtrack. It works well as a metaphor for life these days, where some of us can enjoy the new and the beautiful while others are caught up in war and suffering. (Image and video credit: T. Blanchard)
Capturing what goes on inside a combustion engine is incredibly difficult. It’s a problem that depends on turbulent flow, chemistry, heat transfer, and more. To represent all of those aspects in a numerical simulation requires enormous computational resources. It’s not simply the realm of a supercomputer; it requires some of the fastest supercomputers in existence.
Exascale computing, like that used for the simulations in this video, is defined as at least 10^18 (floating-point) operations per second. For comparison, my PC has a recent, high-end graphics card, and it’s about a million times slower than that. These are absolutely gigantic simulations. (Image and video credit: N. Wimer et al.)
Surfing looks entirely different from below the wave. Photographer Ben Thouard captures his images by freediving and observing what goes on overhead. Whether the surfers nearby ride a barrel roll or bail into the churn, the results are incredible. You can find more of Thouard’s artwork on his website and Instagram. (Image credit: B. Thouard; via Oceanographic Magazine)
When vortex rings collide, they reconnect into smaller, rings that eventually break down into chaos. Here, researchers experiment with colliding multiple vortex rings — focusing on an eight-ring collision. When they collide rings over and over, it creates a zone of isolated turbulence at the heart of the collisions.
Many of the theories and predictions that exist around turbulence assume that the flow is homogeneous and isotropic; what this means is that the (statistical) characteristics of the flow are the same in every direction. In reality, this kind of flow isn’t always easily achieved, which makes testing theoretical predictions challenging.
What’s neat about this set-up is that you get this desired turbulence in a very controlled way. It’s easy to tune the size and energy of your vortex rings, and those tweaks allow you to observe what — if any — changes occur in the resulting turbulence. (Image and video credit: T. Matsuzawa et al.)
Fall and winter storms rip Lake Erie with violent waves. Photographer Trevor Pottelberg of Ontario captures the dramatic eruptions of mist and spray from these massive, turbulent waves. It’s amazing how many different characters a wave can take on. Just compare Pottelberg’s waves with those caught by Lloyd Meudell or Ray Collins. It’s almost hard to imagine all of these waves growing from the same wind-driven start. See more from Pottelberg on his website and Instagram. (Image credit: T. Pottelberg; via Colossal)
One enduring mystery of the solar wind — a stream of high-energy particles expelled from the sun — is how the particles get accelerated in the first place. The sun frequently belches out spurts of plasma, but without further momentum, that material simply falls back to the sun’s surface under the star’s gravity. Mechanisms like shock waves can further accelerate particles that are already moving quickly, but they cannot explain how the particles get going in the first place.
A recent study used supercomputers to tackle this challenging problem in turbulent plasma physics. Each simulation tracked nearly 200 billion particles, requiring tens of thousands of processors. The results showed that turbulence itself provides the necessary initial acceleration and serves as the first step to getting particles moving fast enough to escape the sun. (Image credit: NASA SDO; research credit: L. Comisso and L. Sironi; via Physics World)
Accretion disks form everywhere, from around young, planet-building stars to massive black holes. As matter circles in the disk, it slowly loses angular momentum and falls inward toward the central gravitational body. But the details of this process have long vexed astronomers. The low-viscosity environment of gas and dust in accretion disks simply is not sufficient to account for the level of angular momentum lost. Turbulence is expected to provide a boost to the effect, but neither astronomical observations or Taylor-Couette experiments have shown how.
A new study uses a liquid metal, confined in a disk using radial and vertical electrical fields. Unlike prior experiments, this set-up creates a more gravity-like force to rotate the liquid. With it, researchers can tune both the rotation speed and the level of turbulence. They found that turbulence is, indeed, responsible for the loss of angular momentum that transports mass inward — even at the limit of zero intrinsic viscosity.
Unfortunately, the apparatus isn’t a perfect analog for astrophysical disks; in the experiment, the turbulence originates from secondary flows that aren’t present in real systems. So while the team demonstrated that turbulence can drive the accretion disk’s behavior, it can’t pinpoint where that turbulence originates in real accretion disks. (Image credit: NASA; research credit: M. Vernet et al.; via Physics World)
Plankton — microscopic creatures with often limited swimming abilities — can face daily journeys of hundreds of vertical meters in the ocean. That’s a daunting prospect for any tiny swimmer. A new mathematical model suggests that plankton can have an easier time of it, though, by riding turbulent currents.
The researchers modeled an individual planktar (singular of plankton) capable of sensing nearby velocity gradients and rotating its body to control its swimming direction. With this simple set of controls, their simulated planktar was able to “surf” turbulent currents, covering vertical distances at twice its normal swimming speed despite its curvy path.
Currently, there’s no direct experimental evidence that plankton do this, but it does seem to make sense of experimenters’ observations. With the model’s results to guide them, experimentalists are looking for microswimmers actively orienting themselves based on turbulence. (Image credit: top – B. de Kort, illustration – R. Monthiller et al.; research credit: R. Monthiller et al.; via APS Physics)