Even something as simple as a falling sphere meeting a wall is composed of beautiful fluid motion. In Figure 1 above, we see side-view images of a sphere at low Reynolds number falling toward a wall over several time. Initially an axisymmetric vortex ring is visible in the sphere’s wake; when the sphere touches the wall, secondary vortices form and the wake vortex moves down and out along the wall in an axisymmetric fashion (Figure 2, top view). At higher Reynolds numbers, like those in Figure 3, this axisymmetric spreading of the vortex ring develops an instability and ultimately breaks down. (Photo credit: T. Leweke et al.)
Tag: vorticity
Supercell Thunderstorm
Photographer Mike Olbinski has captured a spectacular timelapse of a supercell thunderstorm over the plains of Texas. Supercells are characterized by a strong, rotating updraft known as a mesocyclone, seen clearly in the video. These storms are commonly isolated occurrences, forming when horizontal vorticity in the form of wind shear is redirected upwards by an updraft. Such a strong updraft is typically created by a capping inversion, a situation where a layer of warmer air traps the colder air beneath it. (This is why one sees a distinctive cut-off at the top of some clouds.) As warm air rises from the surface, either the air above the cap will cool or the air below the cap will warm. Either situation results in an instability with cooler air on top of warmer air, providing a catalyst for the kind of dramatic weather seen here. (Video credit: M. Olbinski; via io9)
Dye Droplet
A drop of fluorescent dye falling into quiescent water forms fantastical structures that are a mixture of vorticity, turbulence, and molecular diffusion. The horseshoe-like shape near the front of the drop is a typical shape for two fluids strained by moving past one another. The main section of the drop billows outward like a parachute, but the turbulence of its wake stretches the dye into fine threads that quickly disperse in the water. (Photo credit: D. Quinn et al.)
Mercedes-Benz Tornado
The world’s most powerful artificial tornado is part of the Mercedes-Benz Museum in Stuttgart, Germany. Though popular enough with visitors that the staff will bring out smoke generators to make it visible, the tornado was not built as an attraction – It’s actually part of the building’s fire protection system. The modern open design of the museum meant that conventional smoke removal systems were inadequate. Instead vorticity is generated in the central lobby with 144 wall-mounted jets. The angular velocity created by the jets is strongest at the middle, in the vortex core, due to conservation of angular momentum – exactly the way a spinning ice skater speeds up by pulling his arms in. The core of the vortex is a low pressure area, which draws outside air toward it – this is how the tornado pulls in smoke when there is a fire. The fan on the ceiling provides the pressure draw necessary for the smoke to be pulled up and out of the building at a supposed rate of 4 tons per minute. See the tornado in action here. (Photo credit: Mercedes-Benz Passion; submitted by Ivan)
Flapping Foil Wake
This gorgeous visualization shows the flow behind a flapping foil. Flow in the water tunnel is from right to left, with dye introduced to show streamlines. A flapping foil is a good base model for most flapping flight as well as finned swimming – anything that oscillates to create thrust. As the foil flaps, vorticity is generated and shed along the trailing edge, creating a regularly patterned wake of trailing vortices. (Video credit: R. Godoy-Diana)
(Source: https://player.vimeo.com/)
Real-Life Whirlpools
Literature is full of descriptions of monstrous whirlpools like Charybdis, which threatens Homer’s Odysseus. While it’s not unusual to see a small free vortex in bodies of water, most people would chalk boat-swallowing maelstroms up to literary device. But it turns out that, while there may not be permanent Hollywood-style whirlpools, there are several places in the world where the local tides, currents, and topology combine to produce turbulence, dangerously vortical waters, and even standing vortices on a regular basis.
One example is the Corryvreckan, between the islands of Jura and Scarba off Scotland. In this narrow strait, Atlantic currents are funneled down a deep hole and then thrust upward by a pinnacle of rock that rises some 170 m to only 30 m below the surface. The swift waters and unusual topology produce strong turbulence near the surface and whirlpools pop up throughout the strait. Other “permanent” maelstroms, such as those in Norway and Japan, arise from tidal interactions with similar structures rising from the sea floor.
For more, check out this Smithsonian article, Gjevik et al., Moe et al., and the videos linked above! (Photo credits: Manipula, Tokushima Gov’t, Wikimedia, and W. Baxter; requested by @kb8s)
Truck Vortices
The video above shows vortex rings of smoke ejected from the burning tire of a moving truck. Without seeing the damaged tire, it’s tough to pinpoint the cause with certainty, but here are a couple of ideas. Typically vortex rings are formed with a burst of air through a narrow orifice; this is, for example, how humans, dolphins, vortex cannons, and volcanoes all make smoke rings. If air is escaping the tire through small holes, this could cause rings. Unlike in those situations, though, the tire is spinning, which means its motion is already imparting vorticity to the flow, so that any air escaping the tire forms a vortex ring. (Video credit: The Armory; submitted by eruditebaboon)
ETA: Others are suggesting the vortex rings are due to a failure of the engine, with unsteady exhaust velocities resulting in the vortex structures. I think this might still depend on the exhaust pipe’s geometry. Regardless of the exact cause, the video remains an interesting bit of fluid dynamics.
Stirring Faces
This video features simulation of the laminar flow around a plate plunging sinusoidally in a quiescent flow. As the plate moves up and down, it mixes the fluid around it. This is visualized in several ways, beginning with the vorticity. Clockwise and anti-clockwise vortices are shed by the edges of the plate as it moves. The flow is also visualized using particle trajectories, which are classified by their tendency to accumulate (attract) or lose (repel) particles. These trajectories are particularly intriguing to watch develop as they appear to show ornate faces and designs. (Video credit: S. L. Brunton and C. W. Rowley)
Supersonic Bubble Shock Waves
Supercomputing has been an enormous boon to fluid dynamics over the past few decades. Many problems, like the interaction between a supersonic shock wave and a bubble, are too complicated for analytical solutions and difficult to measure experimentally. Numerical simulation of the problem, combined with visualization of key variables, adds invaluable understanding. Here a shock wave strikes a helium bubble at Mach 3, and the subsequent interactions in terms of density and vorticity are shown. This situation is relevant to a number of applications, such as supersonic combustion and shockwave lithotripsy–a medical technique in which kidney stones are broken up inside the body using shock waves. After impact, an air jet forms and penetrates the center of the structure while the outer regions mix and form a persistent vortex ring. (Video credit: B. Hejazialhosseini et al.; via Physics Buzz)
Fire Tornado
An artificial fire tornado makes for fascinating viewing. The box fans are positioned around a central firepit such that they impart the angular velocity needed to create a vortex. I’ve actually seen an even bigger live demonstration than this one at a fluid dynamics conference. Do not try this yourself. Fire tornadoes occur in nature, too: take a look at how they form. (submitted by acervant)
