Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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Posts tagged "vortex ring"

Italy’s Mount Etna is erupting again, producing a series of beautiful vortex rings. Like a dolphin’s bubble ring or a vortex cannon, the volcano's rings are formed when gases are rapidly expelled through a narrow opening. Such formations are extremely common but are generally not visible to the eye. In this case, steam has gotten entrained into the rings to make them visible. Vortex rings can maintain their structure over substantial distances. The photographer of these rings noted that they lasted as many as ten minutes before dissipating. (Photo credit: T. Pfeiffer; via NatGeo)

We take for granted that drops which impact a solid surface will splash, but, in fact, drops only splash when the surrounding air pressure is high enough. When the air pressure is low enough, drops simply impact and spread, regardless of the fluid, drop height, or surface roughness. Why this is and what role the surrounding air plays remains unclear. Here researchers visualize the air flow around a droplet impact. In (a) we see the approaching drop and the air it pulls with it. Upon impact in (b) and (c) the drop spreads and flattens while a crown of air rises in its wake. The drop’s spread initiates a vortex ring that is pinned to the drop’s edge. In later times (d)-(f) the vortex ring detaches from the drop and rolls up. (Photo credit: I. Bischofberger et al.)

Droplet rebound is frequently associated with superhydrophobic surfaces but can also be generated by very large temperature differences. For very hot substrates, a thin layer of the drop vaporizes on contact via the Leidenfrost effect and helps a drop rebound by preventing it from wetting the surface. This video shows almost the opposite: a water droplet hitting solid carbon dioxide (-79 degrees C). Upon contact, the solid carbon dioxide sublimates, creating a thin layer of gas that separates the droplet from the surface. You can also see the vortex ring that accompanies the drop’s impact. Water vapor near the carbon dioxide surface has condensed into tiny airborne droplets that act as tracer particles that reveal the vortex’s formation and the rebounding droplet’s wake. (Video credit: C. Antonini et al.; Research paper)

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.)

When a drop falls from a moderate height into a shallow pool, its impact creates a complicated pattern. The photo above is a composite image showing a top-down view 100 ms after such an impact. On the left side, the flow is visualized using dye whereas the right shows a schlieren photograph, in which contrast indicates variations in density. Both methods show the same general structure - an inner vortex ring generated at the edge of the impact crater and formed mostly of drop fluid and an outer vortex ring, consisting primarily of pool fluid, formed by the spreading wave. Both regions show signs of instability and breakdown. (Photo credit: A. Wilkens et al.)

Artist Corrie White uses dyes and droplets to capture fantastical liquid sculptures at high-speed. The mushroom-like upper half of this photo is formed when the rebounding jet from one droplet’s impact on the water is hit by a well-timed second droplet, creating the splash’s umbrella. In the lower half of the picture, we see the remains of previous droplets, mixing and diffusing into the water via the Rayleigh-Taylor instability caused by their slight difference in density relative to the water. There’s also a hint of a vortex ring, likely from the droplet that caused the rebounding jet. (Photo credit: Corrie White)

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.

When a droplet falls through an air/water interface, a vortex ring can form and fall through the liquid. In this video, the researchers investigate the effects of a stratified fluid interface on this falling vortex ring. In this case, a less dense fluid sits atop a denser one. Depending on the density of the initial falling droplet and the distance it travels through the first fluid, the behavior and break-up of the vortex ring when it hits the denser fluid differs. Here four different behaviors are demonstrated, including bouncing and trapping of the vortex ring. (Video credit: R. Camassa et al.)

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)

A drop of silicone oil falling through a liquid with lower surface tension distorts into multiple vortex rings connected by thin films. This behavior is caused by the interaction between viscous and capillary forces and is observable for only a narrow range of oil viscosities. (Photo credit: A. Felce and T. Cubaud)