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Search results for: “transition”

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    The Coexistence of Order and Chaos

    One of the great challenges in fluid dynamics is understanding how order gives way to chaos. Initially smooth and laminar flows often become disordered and turbulent. This video explores that transition in a new way using sound. Here’s what’s going on.

    The first segment of the video shows a flat surface covered in small particles that can be moved by the flow. Initially, that flow is moving in right to left, then it reverses directions. The main flow continues switching back and forth in direction. This reversal tends to provoke unstable behaviors, like the Tollmien-Schlichting waves called out at 0:53. Typically, these perturbations in the flow start out extremely small and are difficult or even impossible to see by eye. So researchers take photos of the particles you see here and analyze them digitally. In particular, they are looking for subtle patterns in the flow, like a tendency for particles to clump together with a consistent spacing, or wavelength, between them. Normally, researchers would study these patterns using graphs known as spectra, but that’s where this video does something different.

    Instead of representing these subtle patterns graphically, the researchers transformed those spectra into sound. They mapped the visual data to four octaves of C-major, which means that you can now hear the turbulence. When the audio track shifts from a pure note to an unsteady warble, you’re hearing the subtle disturbances in the flow, even when they’re too small for your eye to pick out.

    The last part of the video takes this technique and applies it to another flow. We again see a flat plate, but now it has a roughness element, like a tiny hockey puck, stuck to it. As the flow starts, we see and hear vortices form behind the roughness. Then a horseshoe-shaped vortex forms upstream of it. Aside from the area right around the roughness, this flow is still laminar. But then turbulence spreads from upstream, its fingers stretching left until it envelops the roughness element and its wake, making the music waver. (Video and image credit: P. Branson et al.)

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    Kilauea’s Rivers of Lava

    Kilauea continues to erupt without signs of abating. Aerial video, like this footage from Mick Kalber, shows the scope of the flow. Lava spurts like a hellish fountain from various fissures, then forms a gravity current that slowly flows downhill toward the ocean. Some of the angles give you an excellent view of the texture atop the flowing lava; it looks relatively rope-like now before solidification, indicating pahoehoe flow. Whether the flow will transition to the rougher appearance of a’a lava remains to be seen; as the lava cools and crystallizes, it may develop a yield strength. That would make it similar to fluids like your toothpaste, which only flow once a critical force is applied. Stay safe, Hawaiians! (Image and video credit: M. Kalber; via Colossal)

  • PyeongChang 2018: Ice-Making

    PyeongChang 2018: Ice-Making

    When it comes to winter sports, not all ice is created equal. Every discipline has its own standards for the ideal temperature and density of ice, which makes venue construction and maintenance a special challenge. Figure skating, for example, requires softer ice to cushion athletes’ landings, whereas short-track speed skating values dense, smooth ice for racing. The Gangneung Ice Arena hosts both and can transition between them in under 3 hours. Gangneung Oval hosts long-track speed skating and makes its ice layer by layer, spraying hot, purified water onto the rink. This builds up a particularly dense and therefore smooth ice. 

    The toughest sport in terms of ice conditions is curling, which requires a finely pebbled ice surface for the stones to slide on. Forming those tiny crystals on the ice sheet can only be done at precise temperature and humidity conditions. This is a particular challenge for Gangneung Curling Center due to its coastal location. To keep the temperature and humidity under control at full crowd capacity, officials even went so far as to replace all the lighting at the facility with LEDs! (Image credit: Pyeongchang 2018, 1, 2, 3)

  • Pilot-Wave Hydrodynamics: Walking Drops

    Pilot-Wave Hydrodynamics: Walking Drops

    This post is a collaborative series with FYP on pilot-wave hydrodynamics. Previous entries: 1) Introduction; 2) Chladni patterns; 3) Faraday instability

    If you place a small droplet atop a vibrating pool, it will happily bounce like a kid on a trampoline. On the surface, this seems quite counterintuitive: why doesn’t the droplet coalesce with the pool? The answer: there’s a thin layer of air trapped between the droplet and the pool. If that air were squeezed out, the droplet would coalesce. But it takes a finite amount of time to drain that air layer away, even with the weight of the droplet bearing down on it. Before that drainage can happen, the vibration of the pool sends the droplet aloft again, refreshing the air layer beneath it. The droplet falls, gets caught on its air cushion, and then sent bouncing again before the air can squeeze out. If nothing disturbs the droplet, it can bounce almost indefinitely.

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    Droplets don’t always bounce in place, though. When forced with the right frequency and acceleration, a bouncing droplet can transition to walking. In this state, the droplet falls and strikes the pool such that it interacts with the ripple from its previous bounce. That sends the droplet aloft again but with a horizontal velocity component in addition to its vertical one. In this state, the droplet can wander about its container in a way that depends on its history or “memory” in the form of waves from its previous bounces. And this is where things start to get a bit weird – as in quantum weirdness – because now our walker consists of both a particle (droplet) and wave (ripples). The similarities between quantum behaviors and the walking droplets, the collective behavior of which is commonly referred to as “pilot-wave hydrodynamics,” are rather remarkable. In the next couple posts, we’ll take a look at some important quantum mechanical experiments and their hydrodynamic counterparts.

    (Image credit: D. Harris et al., source)

  • Breaking Up Turbulence

    Breaking Up Turbulence

    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.

  • Galapagos Week: Lava Flows

    Galapagos Week: Lava Flows

    The Galapagos islands are geologically similar to the Hawaiian islands; both are archipelagos that were born and continue to be formed by lava flows originating from a volcanic hot spot. Lava from this type of volcano is high in basalt content, which affects both its flow properties and the formations it creates. Geologists have actually borrowed words from the Hawaiian language to describe the two main kinds of lava formations seen in basaltic flows: pahoehoe and a’a.

    Pahoehoe formations tend to be relatively smooth and often leave behind a pattern of rope-like coils (below). In contrast, a’a lava features are sharp, rough, and challenging to traverse. Both flows are gravity-driven, and which features a given eruption forms depends on many factors. Many flows will even begin with a pahoehoe section that stretches for several kilometers before transitioning to an a’a structure. Researchers believe the transition occurs when the lava crystallizes enough to develop a yield-strength, meaning that it will behave like a solid until enough force is applied to make it flow again. Toothpaste, ointment, and mud are similar so-called yield stress fluids which will only flow after a critical force is applied.  (Image credits: lava flow – Epic Lava Tours, source; pahoehoe lava – J. Shoer)

    Galapagos Week continues tomorrow here on FYFD. Check out previous posts.

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    When Fire Ants are a Fluid

    Substances don’t have to be a liquid or a gas to behave like a fluid. Swarms of fire ants display viscoelastic properties, meaning they can act like both a liquid and a solid. Like a spring, a ball of fire ants is elastic, bouncing back after being squished (top image). But the group can also act like a viscous liquid. A ball of ants can flow and diffuse outward (middle image). The ants are excellent at linking with one another, which allows them to survive floods by forming rafts and to escape containers by building towers. 

    Researchers found the key characteristic is that ants will only maintain links with nearby ants as long as they themselves experience no more than 3 times their own weight in load. In practice, the ants can easily withstand 100 times that load without injury, but that lower threshold describes the transition point between ants as a solid and ants as a fluid. If an ant in a structure is loaded with more force, she’ll let go of her neighbors and start moving around.

    When they’re linked, the fire ants are close enough together to be water-repellent. Even if an ant raft gets submerged (bottom image), the space between ants is small enough that water can’t get in and the air around them can’t get out. This coats the submerged ants in their own little bubble, which the ants use to breathe while they float out a flood. For more, check out the video below and the full (fun and readable!) research paper linked in the credits. (Video and image credits: Vox/Georgia Tech; research credit: S. Phonekeo et al., pdf; submitted by Joyce S., Rebecca S., and possibly others)

    ETA: Updated after senoritafish rightfully pointed out that worker ants are females, not males. 

  • Spots of Turbulence

    Spots of Turbulence

    One of the enduring mysteries of fluid dynamics lies in the transition between smooth laminar flow and chaotic turbulent flow in the area near a wall. That region, known as the boundary layer, has a major impact on drag and other effects. The process begins with disturbances that are too tiny to see or measure, but eventually, those disturbances can grow large enough to generated an isolated turbulent spot, like the one imaged above. Flow in the photograph is from left to right. Turbulent spots have a distinctive wedge-like shape that expands as the spot grows and widens. These turbulent spots can merge together to create still larger spots, and when a surface eventually becomes completely covered in them, we call it fully-developed turbulent flow. (Image credit: M. Gad-El-Hak et al.)

  • Wrinkling Winds

    Wrinkling Winds

    If you’ve ever sat out on a lake and just watched the water’s surface, you’ve probably noticed how complex and variable it looks. There may be waves that rock your kayak but there are smaller variations, too, like little ripples or even tiny wrinkles that appear on the surface. Much of this activity comes from wind blowing across the water. When the wind exceeds a critical speed, waves form. They generally travel in lines that are aligned perpendicular to the wind (lower right). But what happens when the wind is below the critical speed?

    A recent study looked at just this question. By blowing air across the surface of different liquids and observing variations in the surface height as small as 2 micrometers, the researchers were able to measure tiny wrinkles on the water’s surface (lower left) when the wind speed was small. The size and shape of the wrinkles actually corresponds to structures in the turbulent air flow over the water! For fluids like water, there’s a smooth transition from wrinkles to waves as the wind speed increases, so both may be visible at the same time. For higher viscosity fluids, the switch from one to the other is more abrupt. (Image credits: water – M. Soveran; figure – A. Paquier et al. w/ annotations added in blue; research credit: A. Paquier et al.)

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    Supercritical

    Supercritical fluids are neither a gas nor a liquid. The video above shows a tube of pressurized xenon, initially below its boiling point of approximately ~16 deg C. As the temperature is raised, you see the meniscus that marks the liquid xenon disappear. At this point, the xenon has transitioned into the supercritical state. It takes up the entire tube – like a gas – but it is still capable of dissolving materials – like a liquid. At the same time, though, the xenon has no surface tension because there’s no liquid/vapor interface. Toward the end of the video, the temperature gets reduced and the xenon condenses back into a liquid state. Supercritical fluids can be used in a wide variety of industrial applications, including in decaffeination, dry cleaning, and refrigeration. (Video credit: wwwperiodictableru)

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