Vortices are a common feature of many flows. Here we see a helical vortex tube spinning in a swirling flow. The vortex itself is visible thanks to air trapped in its low-pressure core. As the vortex spins, two sections of it come together. This results in what’s known as vortex reconnection: the vortex lines break apart and rejoin in a new configuration – as a small independent vortex ring and a shorter section of helical vortex. Events like this are common but usually hard to observe directly. They’ve been previously visualized using vortex knots and have even been sighted in the quantum vortices of superfluid helium. (Image credit: S. Skripkin, source; research credit: S. Alekseenko et al., pdf)
Tag: flow visualization
Sorting by Bubble
Microfluidic devices, also known as labs-on-a-chip, require clever techniques for processes like sorting particles by size. One such technique uses an oscillating bubble to sort particles. When the bubble vibrates back and forth (left) it creates what’s known as a streaming flow – large regions of recirculation (shown as gray ellipses in the right image). If the bubble is placed inside a channel, we say that two flows have been superposed; the device combines both the left-to-right flow of the channel and the recirculating streaming flow.
Introduce a micron-sized particle into this combined flow, and it will get carried to the bubble and then bounced around by its effects (left). In fact, the larger the particle is, the more the bubble deflects it relative to the flow. You can see this in the image on the right as well. Here the frame rate has been matched to the bubble’s vibration, so the bubble appears stationary, and the particle paths look smooth. The gray lines show the fluid’s path, and individual solid particles are introduced at the left. The largest particle gets strongly deflected as it passes the bubble and exits at the top-right. A fainter, smaller particle follows after it. Being smaller, the bubble’s deflection on it is weaker, and this second particle exits along a path to the center-right. The result is a fast and simple method for particle sorting. (Image and research credit: R. Thameem et al., source)
Eroding Candy
When you pop a hard candy in your mouth, you probably don’t give much thought to the fluid dynamics involved in dissolving it. The series above shows a hard candy suspended in water being slowly eaten away. As sugars in the candy dissolve into the water, the fluid becomes denser and falls away. This creates the downward flow visible in the center of the image. As sugar-laden water sinks, fresher water is pulled in alongside the walls of the candy. That flow helps erode the candy, creating a rougher surface. Since rough surfaces have a greater surface area exposed (than a smooth surface), they prompt further and faster dissolution. That strengthens the downward flow, pulls in more ambient water, and keeps the whole process going. (Image credit: M. Wykes)
Quad Copter Schlieren
Schlieren photography is a classic method of flow visualization that utilizes small variations in density (or temperature) to make otherwise unseen air motion visible. Because changing air’s density or temperature changes its index of refraction, variations in either quantity show up as dark and light regions. Here researchers use it to reveal some of the airflow around a small quadcopter, including the vortices that spiral off each propeller and help generate the lift necessary for take-off. The full video includes a couple of neat demos, including what happens when the blades are wet (shown below). In that case, the wingtip vortices are somewhat disrupted by strings of water droplets being flung off the blades by centrifugal force. Beautiful! (Video and image credit: K. Nolan et al., source; submitted by J. Stafford)
A Drip’s Vortex
Drip food coloring into water and you can often see a torus-shaped vortex ring after the drop’s impact. That vortex rings form during droplet impact has been well known for over a century, but only recently have we begun to understand the process that leads to that vortex ring. Part of the challenge is that the vortex formation is very small and very fast, but recent work with x-ray imaging has allowed experimentalists to finally capture this event.
When a drop impacts a pool, surface tension draws some of the pool liquid up the sides of the drop. At the same time, the impact causes ripple-like capillary waves down the sides of the drop. This causes pool liquid to penetrate sharply into the drop, triggering the spirals that mark the forming vortex ring. When drops impact with even higher momentum, multiple vortex spirals can form, as seen on the lower right image. The authors observed as many as four rings during an impact. For more, check out the (open access) article. (Image and research credit: J. Lee et al., source)
Sky Glow
This short but spectacular timelapse video shows the Grand Canyon filled with fog. This phenomenon, known as a temperature inversion, occurs when a warm layer of air traps cold, moist air near the ground. As the inversion develops in the video, you can see wisps of clouds popping up in the canyon, seemingly out of nowhere, as moisture evaporated from the surface condenses in the cool air. Once fog fills the canyon, it flows and laps against the canyon’s sides, much like waves on the ocean. In fact, the physics here is quite similar, just at a much slower speed. (Video and image credit: H. Mehmedinovic / SKYGLOWPROJECT; via Gizmodo; submitted by Ian S.)
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.)
When Chaos is Not So Chaotic
In industry, tanks are often agitated or stirred to mix different elements. The goal is to create a laminar but chaotic flow field throughout the mixture. Introducing particles to such a system reveals that things are not quite as chaotic as they might seem. The photographs above show the pathlines of various large, glowing particles initially poured into the tank from above. Over time, the particles scatter off of structures in the mixed sections of the tank and end up trapped in vortex tubes that form above and below the agitator. Once trapped in the vortex tube, the particles follow helical paths inside the tube, creating patterns like those seen in the lower two photos. (Image and research credit: S. Wang et al., 1, 2, 3)
Flow Above the Treetops
As this smoke visualization shows, trees have a significant impact on airflow around them. Flow in the image is from left to right. On the left, the upstream air is traveling in smooth, laminar lines that are quickly disrupted as the flow moves into the trees. After the first shorter trees, flow inside the wooded area has been broken up and slowed. Above the canopy, the smoke streaklines have also slowed and become more turbulent. Understanding how wind and trees interact is important in a variety of applications, including when adding renewable energy options to buildings and when predicting the spread of forest fires. (Image credit: W. Frank et al.)
Fanning the Flame
A fan’s blade passes through the hot air rising above a flame in this iconic image by high-speed photography pioneer Harold Edgerton. This photo uses an optical technique known as schlieren photography that makes density differences in transparent media like air visible. Because of its lower density, the hot plume of air above the flame rises. When the fan blade swings past, it sheds a vortex off its tip and the rising air from the flame gets pulled into the vortex to make it visible. To the left, a ghostly counter-rotating vortex sits on the opposite side of the fan blade. (Photo credit: H. Edgerton and K. Vandiver)
