Imagine taking two panes of glass and setting them in a frame with a small gap between them. Then partially fill the gap with a mixture of dye, glycerol, water, and soap. After turning the frame over several times, the half of the frame will be filled with foamy bubbles. When you flip it again, the dyed glycerol-water will sink and penetrate the bubble layer, creating complex and beautiful patterns as it mixes. Some of the bubbles may get squeezed together until they coalesce into larger bubbles that shoot upward thanks to their increased buoyancy. Other smaller bubbles will wend their way upward as neighboring fluid shifts. If you examine the tracks left by individual bubbles, you can find patterns reminiscent of Impressionist paintings, as seen at the end of this Gallery of Fluid Motion video. (Image credit: A. Al Brahim et al., source)
Day and night mix in this flow visualization of watercolor pigments and ferrofluid. The former, as suggested by their name, are water-based, whereas ferrofluids typically contain an oil base. This means the two fluids are immiscible. Like oil and vinegar in salad dressing, the only way to mix them is to break one into tiny droplets floating in the other. This is what happens near their boundary, where brightly-colored paint droplets float in a network of dark channels. To the right, the paint and ferrofluid have been swirled around to create viscous mixing patterns among the paint colors with occasional intrusions of thin ferrofluid fingers. (Image credit: G. Elbert)
Cloud chambers were one of the first methods used to study radioactive decay and cosmic particles. Such chambers are filled with a cool, supersaturated cloud of alcohol vapor. When high-energy particles pass through, they collide with atoms in the chamber, ionizing them. Those ions then serve as nucleation sites for the alcohol vapor, creating a condensation streak that marks the particle’s passage. In some respects, they’re similar to the contrails that form behind airplanes. What you’re seeing is not the particle itself but evidence that it went by. YouTuber Nick Moore built his own cloud chamber. Learn more about it and see lots more great footage of it in action in the full video below. (Image and video credit: N. Moore)
The schlieren photographic technique is often used to visualize shock waves and other strong but invisible flows. But a sensitive set-up can show much weaker changes in density and pressure. Here, schlieren is used to show the standing sound wave used in ultrasonic levitation. By placing the glass plate at precisely the right distance relative to a speaker, you can reflect the sound wave back on itself in a standing wave, seen here as light and dark bands. The light bands mark the high-pressure nodes, where the pressure generated by the sound waves is large enough to counteract the force of gravity on small styrofoam balls. This allows them to levitate but only in the thin bands seen in the schlieren. Move the plate and the standing wave will be disrupted, causing the bands to fade out and the balls to fall. (Video and image credit: Harvard Natural Sciences Lecture Demonstrations)
The Space Shuttle had a famous double sonic boom when passing overhead during re-entry. This schlierenflow visualization of a model shuttle at Mach 3 reveals the source of the sound: the fore and aft shock waves on the vehicle. The nose of the shuttle generates the strongest shock wave since it is the first part of the vehicle the flow interacts with. This initial shock wave turns the flow outward and around the shuttle. The second boom comes from the back of the shuttle and serves to turn the flow back in to fill the wake behind the shuttle. (The actual shock wave would look a little different than this one because there’s no sting holding the shuttle like there is with the model.) The other major shock wave comes from the shuttle’s wings, but, at least for this Mach number, the wing shock wave merges with the bow shock, making the two indistinguishable. (Image credit: G. Settles, source)
Many chemical reactions, and the flows that accompany them, are invisible to the human eye. But in infrared wavelengths those same events are vibrant and energetic. In this video from the Beauty of Science group, various chemical reactions are shown in visible and IR wavelengths, revealing very different perspectives on the same thing. Many of the reactions are exothermic, meaning that they produce heat as they occur. Because of this the thermal imaging shows where the most intense reaction is occurring at a given time. Other areas gradually darken as diffusion and flow move and dilute the heat energy released. (Video and image credit: Beauty of Science, source)
A new interactive installation by the Japanese art collective teamLab uses the movement of visitors to drive vortex motion. Entitled “Moving Creates Vortices and Vortices Create Movement,” the installation uses projectors in a mirror room to create the sensation of an infinite, indoor ocean that’s constantly churned by the paths visitors take. In the absence of motion, the room slowly fades to darkness. The installation is currently in the National Gallery of Victoria in Melbourne, Australia, and will be open until April 15th, 2018. (Image credit: teamLab; via Colossal; submitted by jshoer)
P.S. – Winter Olympic coverage will start on Monday, February 12th! – Nicole
The Juno mission’s JunoCam has been producing stunning photos each time the spacecraft swoops past Jupiter. The instrument has a planning team, but its primary use is for citizen scientists, who have been suggesting images to take each orbit and have been processing those images. Most of the photos we see are like the one on the left above – photos that have been heavily color-enhanced to highlight details. The image on the right shows what Jupiter would look like to the human eye. Look closely, and you’ll catch many of the same colors and shapes in both photos.
Hot exhaust gases churn in the wake of this climbing B-1B Lancer. The high temperature of the exhaust changes the density and, thus, the refractive index of the gases relative to the atmosphere. Light traveling through the exhaust gets distorted, making the highly turbulent flow visible to the human eye. Note how the four individual engine exhaust plumes quickly combine into one indistinguishable wake. This is typical for turbulence; it’s hard to track where any given fluctuations originally came from. The airplane’s wingtip vortices are just visible as well, if you look closely. (Image credit: T. Rogoway; submitted by Mark S.)
Under most circumstances, we think about flows changing from ordered and laminar to random and turbulent. But it’s actually possible for disordered flows to become laminar again. This is what we see happening in the clip above. Upstream, the flow in this pipe is turbulent (left). Then four rotors are used to perturb the flow (center). This disrupts the turbulence and causes the flow to become laminar again downstream (right). To understand how this works, we have to talk about one of the fundamental concepts in turbulence: the energy cascade.
Turbulent flows are known for their large range of length scales. Think about a volcanic plume, for example. Some of the turbulent motions in the plume may be a hundred meters across, but there are a continuous range of smaller scales as well, all the way down to a centimeter or less in size. In a turbulent flow, energy starts at the largest scales and flows further and further down until it reaches scales small enough that viscosity can extinguish them.
That should offer a hint as to what’s happening here. The rotors are perturbing the flow, yes, but they’re also breaking the larger turbulent scales down into smaller ones. The smaller the largest lengthscales of the flow are, the more quickly their energy will decay to the smallest lengthscales where viscosity can damp them out. This is what we see here. Once the turbulent energy is concentrated at the smallest scales, viscosity damps them out and the flow returns to laminar. Check out the full video below for a cool sequence where the camera moves alongside the pipe so you can watch the turbulence fading as it moves downstream. (Image and video credit: J. Kühnen et al.)
ETA: As it turns out, there’s more going on here than I’d originally thought. Simulations show that breaking up length scales is not the primary cause of relaminarization in this case. Instead, the rotors are modifying the velocity profile across the pipe in such a way that it tends to cause the turbulence to die out. The full paper is now out in Nature Physics and on arXiv.
