Ink released into water shows the swirling motion inside a vortex ring as well as its deformation and breakup upon stagnation against a wall. Although humans are known to make such vortex rings with smoke or bubbles, they are common in nature as well. Buoyant plumes often feature vortex rings at their head; dolphins and whales play with bubble rings; volcanoes blow smoke rings; and mosses use them to distribute spores.
Tag: vorticity
Antibubbles
Antibubbles–a liquid droplet surrounded by a thin film of gas and immersed in more liquid–are fragile things. This video explores how antibubbles behave when placed in proximity to a tornado-like whirl. When placed near the eye, where fluid motion is primarily vertical, the antibubble is stretched vertically. When placed in the rotating eyewall, the antibubble is distorted into a ring-like shape before it breaks down. (Video credit: D. Terwagne et al; APS Gallery of Fluid Motion 2009)
Tornadogenesis
Tornadogenesis–the formation of tornadoes–remains a topic of active research as there is relatively little direct experimental data, owing to the difficulty of prediction as well as measurement. Initially, a variation of wind speed at different altitudes in the atmosphere causes shearing, which can lead to the formation of a horizontal column of rotating air–a vortex line similar to a roll cloud. Beneath a developing storm, the updraft of warm local air can pull this vortex line upwards, creating vertical rotation in the cloud, thereby birthing a supercell. Supercells do not always spawn tornadoes, and the exact causes that result in tornadic or nontornadic supercells are not fully understood. However, the formation of tornadoes within the supercell seems dependent on the downdraft of cool air within the storm as well as stretching of the vortex line, which increases its rate of rotation. For more information, check out this explanatory video and some of the talks by Paul Markowski. (Thanks to mindscrib, aggieastronaut and others for their submissions related to this topic! Photo credits: P. Markowski and D. Zaras)
Vortex Cannon
Building a vortex cannon is a great way to demonstrate the power and longevity of vortex rings. As demonstrated here, it’s possible to create one with just a box with a round hole in it. Adding some smoke or stage fog helps visualize the rings. Vortex rings are found frequently in nature: volcanoes make them, some plants use them to distribute spores, and dolphins and whales use them to play. (submitted by @aggieastronaut)
Circulation Around an Airfoil
As a followup to yesterday’s question about ways to explain lift on an airfoil, here’s a video that explains where the circulation around the airfoil comes from and why the velocity over the top of the wing is greater than the velocity around the bottom. Kelvin’s theorem says that the circulation within a material contour remains constant for all time for an inviscid fluid. Before the airplane moves, the circulation around the wing is zero because nothing is moving. As shown in the video, as soon as the plane moves forward, a starting vortex is shed off the airfoil. As the plane flies, our material contour must still contain the starting position and thus the starting vortex. However, in order to keep the overall circulation in the contour zero, the airfoil carries a vortex that rotates counter to the starting vortex. This is the mechanism that accelerates the air over the top of the wing and slows the air around the bottom. Now we can apply Bernoulli’s principle and say that the faster moving air over the top of the airfoil has a lower pressure than the slower moving air along the bottom, thus generating an upward force on the airfoil. (submitted by jessecaps)
Vortex Ring Collision
Two vortex rings collide head-on in this video. If their vorticities and velocities are matched in magnitude and opposite in direction, their collision results in a stagnation plane–essentially a wall across which the fluid does not pass. In reality, there are slight variations that result in non-zero velocities where the vortices meet, so some mixing occurs, but the overall symmetry remains striking. The collision breaks up the vortex ring into filaments, some of which cross-link with the other vortex’s filaments, resulting in the little halo-like eddies around the perimeter. Videos of the same experiment at different Reynolds numbers can be found here. (Submitted by Charlie H; Video credit: T. Lim and T. Nickels)
Diesel Ignition
In a diesel engine, ignition of the injected fuel occurs due to the heat caused by the compression of the fuel/air mixture. (In petrol/gasoline engines, spark plugs are used for ignition.) The subsequent expansion of gases drives the pistons of the engine downward, creating mechanical energy. This high-speed video shows the in-cylinder combustion within a diesel engine. Note the symmetry and vorticity of the flow.
Smokestack Plumes
On a cold and windy day, the plume from a smokestack sometimes sinks downstream of the stack instead of immediately rising (Figure 1). This isn’t an effect of temperature–after all, the exhaust should be warm compared to the ambient, which would make it rise. It’s actually caused by vorticity.
Figure 2: Simple geometry (side view) In Figure 2, we see a simplified geometry. The wind is blowing from right to left, and its velocity varies with height due to the atmospheric boundary layer. Mathematically, vorticity is the curl of the velocity vector, and because we have a velocity gradient, there is positive (counterclockwise) vorticity generated.
Figure 3: Vortex lines (top view) According to Helmholtz, we can imagine this vorticity as a bunch of infinite vortex lines convecting toward the smokestack, shown in Figure 3. Those vortex lines pile up against the windward side of the smokestack–Helmholtz says that vortex lines can’t end in a fluid–and get stretched out in the wake of the stack. If we could stand upstream of the smokestack and look at the caught vortex line, we would see a downward velocity immediately behind the smokestack and an upward velocity to either side of the stack. It’s this downward velocity that pulls the smokestack’s plume downward.
Figure 4: Vortex wrapped around stack Now Helmholtz’s theories actually apply to inviscid flows and the real world has viscosity in it–slight though its effects might be–and that’s why this effect will fade. The vortex lines can’t sit against the smokestack forever; viscosity dissipates them.
The Silence of Owls
Owls are among the most silent hunters in nature, thanks to their feathers. The leading edge of the wing, shown in the bottom part of the photo, has a serrated comb-like edge, which breaks flow over the wing into small vortices, which are quieter than larger ones. The fringe-like trailing edge breaks the flow up further and helps absorb the sound produced by the turbulence. The fluffy feathers along the owl’s body can also help muffle noise. Researchers are investigating ways to use these techniques to quiet aircraft. # (via jshoer)
Steam Devils
The formation of the ethereal steam devil is quite similar to the formation of a fire tornado. In this case, the first frost of the season cooled air temperatures substantially below the temperature of the water of the lake, creating conditions for steam and for updrafts of rising, warmer air. A slight breeze across the lake is enough to create pockets of vorticity, which stretch due to the updrafts and intensify due to conservation of angular momentum. This creates the narrow spinning vortex filaments that pull steam up and dance across the lake’s surface. #
