turbulent mixing

Water in a tank spins at nearly four revolutions per second, and, as if by magic, a column the size of a dinner table emerges. The vortex lives for dozens of rotations, even though everything around it churns and trembles. Where does such stability come from? The secret is that the vortex is not isolated.

A hydrogen bubble on a platinum electrode behaves like a swimmer trying to surface, but every time it almost breaks free, something below grabs its legs. In the lab, high-speed cameras captured this behavior: a millimeter-sized bubble hangs near the surface, even though buoyancy pulls it upward with a force thousands of times stronger than surface tension can hold.

A weaver sits at her loom, her hands moving in a rhythm that cannot be predicted in advance. The threads converge and diverge, forming a pattern that seems random yet follows strict rules.

A crowd at the entrance to a narrow corridor behaves differently than in a spacious hall: some people get pressed against the walls, others are pushed forward. Roughly the same happens when an elongated particle finds itself in a fluid teeming with self-propelled dumbbells. Until now, it was unclear why such a particle moves faster along its long axis than across it, and how the crowd density affects its speed.

Water confined between two cylinders flows in smooth layers when the inner one rotates. But once the speed crosses a certain threshold, the layers crumple into toroidal rolls, like a stack of bagels. Physicists call these Taylor vortices: the flow loses stability and a regular pattern emerges.

Microwave plasma in a vortex is like a kitchen mixer that simultaneously heats and stirs the gas. In the reactor, the gas is swirled, and microwaves heat it to thousands of degrees, tearing carbon dioxide molecules apart. Scientists created a computer model of such a reactor to understand how pressure affects the process.

At night, when we sink into deep, dreamless sleep, a tide begins in the depths of the brain. Each heartbeat and each breath sends gentle pulses through the fluid-filled cavities (the ventricles). During the day, these pulses are timid and monotonous — the fluid sloshes back and forth almost in place. But with the onset of slow-wave sleep, the rhythm changes.

In the intricate dance of active turbulence, where invisible engines spawn chaos, a soft droplet takes on the role of a detective. Its shape is not merely an outline but a sensitive cipher: it reads the hidden signals of its environment.

Water boils in a pot: bubbles churn, heat quickly escapes into the air. In the center of the ARC fusion reactor, plasma behaves similarly — except instead of bubbles, there are turbulent eddies, and instead of air, there's a magnetic field that must hold the heat.

A stone falls into a pond, and ripples spread evenly in all directions. But some media behave as if the wave knows which way to turn, like water that only swirls clockwise. This asymmetry comes from odd viscosity and odd elasticity — properties of media with broken mirror symmetry.

A pot of water has just been stirred: the whirlpools and eddies gradually calm down, and the liquid becomes smooth. A similar process, but in the quantum world, occurs in superfluid helium or in a cloud of ultracold atoms.

A tiny oil drop squeezes through a water channel narrower than itself. In pure water, it crawls along calmly, like a round bead. But add a little soap to the water, and the drop begins to bend, like a caterpillar.

In a solid piece of tin-lead alloy, the layers of the two metals lie like pages in a book. When heated from one side, the layers don't melt uniformly but in whimsical tongues and fingers. Scientists studied this process using a transparent alloy that can be seen through under a microscope and compared it with a computer model.

At the heart of the Ophiuchus galaxy cluster floats a bubble of hot plasma the size of several galaxies. It rises like a giant hot-air balloon in the atmosphere, leaving a wake behind it — a region where gas is set in motion.

Turbulent flow near a wall resembles a crowd where everyone pushes in their own direction. Vortices form and break apart, creating chaotic jolts. If you embed a defective acoustic lining into the wall, like a tuning fork that rings only on one note, it starts responding only to a narrow range of frequencies.

In a cloud, thousands of tiny water droplets hang in the air. For rain to fall, they need to grow: stick together into larger drops and become heavy enough to fall to the ground. But how exactly do droplets find each other in a huge cloud? Scientists created a computer model that tracks each droplet individually, as if each one had its own little sensor.

In porous materials (soil, rocks, filters) fluids move through tiny, winding channels. When two immiscible fluids, such as water and air, are present simultaneously, their interface constantly shifts: air bubbles push water out of some pores and open paths through others.

When water boils in a kettle, waves of steam ripple through the kitchen. A similar process happens in space, in plasma — gas so hot that electrons are stripped from atoms. Around black holes and neutron stars, this plasma isn't just hot: its particles zip around at nearly the speed of light.

The young star HL Tau is surrounded by a disk of gas and dust — the cradle of future planets. Inside this disk, the gas is not static: it orbits the star, but it also trembles and churns, like water in a raging river. Astronomers have now directly measured the strength of this churning for the first time.

A swimmer cutting through water at the speed of the waves running on it finds themselves in a strange position: the waves they themselves generate can neither overtake them nor fall behind. They pile up at the swimmer's bow and freeze. A similar phenomenon occurs when a light source moves inside a medium at a speed matching the speed of energy transfer of the waves.

When a massive star dies, its core collapses into a neutron star, and the outer layers are blown off in an explosion. At that moment, an unimaginably dense cloud of neutrinos is born — particles that barely interact with matter but actively 'talk' to each other. There are so many of them that together they behave like a fluid, with waves rippling across it.

Planets are born from dust. In a disk around a young star, tiny dust grains collide, stick together, and gradually grow like a snowball rolling down a hill. But there's a catch: the gas in the disk moves faster than the dust and drags on it, preventing it from clumping into large chunks. For dust to stick together, there needs to be enough of it.

Ice on a warm frying pan doesn't just melt: it starts to move. A thin layer of water forms beneath it, and the piece glides as if on oil. The speed of this sliding depends on how quickly heat travels through the water film and how steep the incline is.

In turbulent air, droplets don't fly in straight lines: they get tossed around like dust motes in a beam of light. Physicists have built a setup that tracks each droplet, which is about a tenth of the thickness of a human hair. A camera captures their motion, and a computer reconstructs their trajectories.

Hot plasma in a tokamak is like a temperamental beast: you need to hold it in, but not let it overheat. In the ASDEX Upgrade reactor, researchers found a regime where plasma eddies solve this problem on their own. Stochastic modeling showed that oscillations at the plasma edge fling out blobs, like bubbles from a boiling pot.

A calm river suddenly starts churning in the middle of a flat stretch: that's the puzzle the authors of a new study set out to solve. They took a thin layer of charged gas with a very weak magnetic field and tracked on a computer what would happen to it. At first, everything flows smoothly, like water from a tap.

In a narrow vertical tube, alcohol flows upward, surrounded by water, and carbon dioxide bubbles are born inside it. As soon as they appear, they rush toward the axis of the flow, as if an invisible hand gathers them into a neat chain.

October 10, 2024. The Sun hurled a cloud of magnetized plasma toward Earth, and four spacecraft, stationed at a single point between the Sun and Earth, simultaneously recorded how that cloud churned. The distance between the outermost probes was just 80 Earth radii, about half a million kilometers.

In a plasma accelerator, particles are accelerated like surfers riding a wave created in the plasma. If the leading bunch and the trailing bunch are not aligned, an instability arises that shakes the trailing bunch. It's as if a surfer on a wave starts wobbling from side to side: their trajectory gets messed up, and they might fall off the wave.

A drop falls into a glass, and a tiny jet, a thin fountain, shoots up from the surface for an instant. Something similar happens with a bubble stuck to the wall of a tube when you yank the tube downward. Water around the bubble rushes in, and a jet fires from its top. The speed of this jet depends on the bubble's shape — how much it bulges, like an umbrella dome.

A blunt cylinder flying at six times the speed of sound isn't the most elegant object, but it's the star of a new simulation. The surface of such a vehicle is never perfectly smooth: during atmospheric entry, the heat shield ablates, leaving bumps and pits.

The diffusion equation, a mathematical tool describing how particles spread out in space, has long painted a paradoxical picture: a disturbance, like ripples on Water, instantly engulfs the entire universe. This challenged the theory of relativity, which holds that nothing can move faster than light.

Bacteria in a drop of water jostle and swim: each on its own, but together they create patterns. With weak motion, the fluid churns like soup on a high flame: vortices collide, and the flow direction changes every second. This is called bacterial turbulence.

In a glass of soda, bubbles merrily rise to the top. But if you freeze the water, some bubbles end up trapped in the ice. How do they get there? After all, ice grows, pushing gas aside, but it doesn't always manage to.

In a glass of water, ultrasound creates tiny bubbles. They grow and shrink, and sometimes they collapse sharply, like a popping soap bubble, but thousands of times faster. Near these bubbles float small particles — for example, nickel spheres the size of dust motes.

Water in a glass settles down, but before that, vortices go through a stage where energy losses spike. Physicists seek early signs of this stage to predict flow behavior — from the atmosphere to pipelines. In a new study, they test one such sign: the curvature spectrum of vortex lines, that is, how twisted small and large eddies are.

Toss a handful of peas into a pot of semolina porridge — the peas quickly sink, pushing the grains aside. But if you make the porridge very thick and sticky, the peas get stuck. That's roughly how light lithium atoms behaved in a cloud of heavy potassium atoms at temperatures just a few millionths of a degree above absolute zero.

Around young stars, flat clouds of gas and dust swirl: these are the birthplaces of planets. For matter in such a disk to clump together, it must be constantly stirred. But what makes the gas move chaotically when it would otherwise be calm? Part of the answer lies in the fact that the gas in the disk barely conducts electricity: it is weakly ionized.

The wind blows steadily, but suddenly a gust hits, and leaves swirl up in a vortex. Inside such a gust, the air speed changes not smoothly but in jumps — sometimes sharply forward, sometimes sideways. These rare and strong bursts are called intermittency, and they are the main mystery of turbulence.

Water swirls into a ring, and that ring flies through the bulk without breaking apart. Such vortex rings, from smoke rings to underwater currents, follow laws that are hard to capture on paper. Usually, the flow is divided into millions of cubic cells, and each one is tracked.