Fluorescent oil sprayed onto a model in the NASA Langley 14 by 22-Foot Subsonic Wind Tunnel glows under ultraviolet light. Airflow over the model pulls the initially even coat of oil into patterns dependent on the air’s path. The air accelerates around the curved leading edge of the model, curling up into a strong lifting vortex similar to that seen on a delta wing. At the joint where the wings separate from the body those lifting vortices appear to form strong recirculation zones, as evidenced by the spiral patterns in the oil. Dark patches, like those downstream of the engines could be caused by an uneven application of oil or by areas of turbulent flow, which has larger shear stress at the wall than laminar flow and thus applies more force to move the oil away. Be sure to check out NASA’s page for high-resolution versions of the photo. (Photo credit: NASA Langley/Preston Martin; via PopSci)
Tag: vortices
Ig Nobel Fluids: Shower Curtain Science
Nearly everyone has faced the frustration of a shower curtain billowing inwards to stick to one’s leg. Various explanations have been offered to explain the effect, but David Schmidt won the 2001 Ig Nobel Prize in Physics for a numerical simulation suggesting that the spray of droplets from the shower head drives a horizontal vortex whose axis of rotation is perpendicular to the shower curtain. Since vortices have a low-pressure region in their core, this weak shower vortex has the power to suck a light curtain inward, much to the chagrin of the shower’s occupant. Of course, a heavier or weighted shower curtain will help avoid the effect. This post is part of a series on fluids-related Ig Nobel Prizes. (Photo credit: W. Taylor; research credit: D. Schmidt)
Vortex Street in the Clouds
Most objects are not particularly aerodynamic or streamlined. When air flows over such bluff bodies, they can shed regular vortices from one side and then the other. This periodic shedding creates a von Karman vortex street, like this one stretching out from Isla Socorro off western Mexico. From the wind’s perspective, the volcanic island forms a blunt disruption to the otherwise smooth ocean. This vortex shedding is seen at smaller scales, as well, in the wind tunnel, in soap films, and in water tunnels. If you’ve ever been outside on a windy day and heard the electrical lines “singing” in the wind, that’s the same phenomena, too. With the right crosswind, radial bicycle spokes will buzz for the same reason as well! (Photo credit: MODIS/NASA Earth Observatory)
Navigating the Interface
Walking on water may be the stuff of legend at human scales, but it’s a fact of everyday life for many smaller species. Waxy, hydrophobic coatings typically make such insects’ points of contact (feet, legs, etc.) water-repellent, and their light weight can be supported by surface tension. Navigating the interface between air and water is more complicated, though, and these creatures have evolved several mechanisms to help. Some, like water striders, use appendages they insert below the surface for propulsion. At 0:49 in the montage above, you can see flow visualization of the vortices generated by a stroke. Other insects release a chemical in their wake that lowers the local surface tension and drives them away via the Marangoni effect. For more, see here and especially this Physics Today article. (Video credit: D. Hu and J. Bush)
Contaminants Flowing Uphill
Here’s an example of some baffling fluid dynamics. Researchers have found that, when pouring a fluid from one container into a lower one containing a fluid with floating particulates, it’s possible for the floating particles to travel upstream against gravity and the flow. The phenomenon is driven by surface tension. The particulates floating in the lower container decrease the fluid’s surface tension relative to the pure fluid pouring in from above. This creates a gradient in surface tension that, via the Marangoni effect, drives a small flow upstream, in the direction of the greater surface tension. In the video above, this flow takes the form of two recirculating vortices in the pouring channel, oriented such that their upstream velocities run along the outside of the channel. Occasionally this flow draws particulates up the waterfall and into the recirculating zones, creating upstream contamination. We reported this previously, but the researchers have now released videos demonstrating the effect, including in pipettes and a water flume. Usually it’s taken for granted that matter cannot move upstream, so this could be a game-changer, especially at small scales where surface tension already dominates. For more, see their paper. (Video credit: S. Bianchini et al.)
Soap Film Butterfly
Originally posted: 14 Jan 2011 This gorgeous butterfly-like double spiral roll takes place on a horizontal soap film. The foil (seen top center) inserted in the film flaps back and forth. Each time the foil changes direction a vortex forms at the tip and gets advected away. The vortices stretch and distort in the roll, but if you look at the photograph closely, you’ll see the tiny shed vortices persisting throughout the roll structure. The bright colors that make this flow visible are due to interference patterns related to the local thickness of the film. (Photo credit: T. Schnipper et al.)
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Pendulum Soap Flow Viz
Soap films are a handy way to create nearly two-dimensional flow fields. Previously we’ve seen them used to show wake structures of pitching foils, flapping flags, and multiple bodies. In this video, we see the dynamics of a pendulum in a soap film. Initially its length is quite long, and the ring end of the pendulum bobs side-to-side in a figure-8 motion. There are two rotational effects here: one is the standard oscillation of a pendulum about its pivot, the other is the rotation of the pendulum’s ring about its attachment point. Interestingly, they have the same frequency. The major destabilizing force for the pendulum is the periodic shedding of vortices we see off the ring. By shortening the pendulum length, the pendulum’s behavior shifts; first it loses the stationary node in its string. Eventually, the string becomes so short that the pendulum no longer oscillates. (Video credit: M. Bandi et al.)
Reader Question: Energy from Whirlpools?
shiftymctwizz asks:
So I just read your post about vortices, and now I’m wondering if we could build structures similar to the Corryvreckan and put turbines in them for energy production? Would it be any more efficient than hydroelectric dams? Are you the right person to ask?
I can’t give you numbers off the top of my head, but I suspect that your typical hydroelectric dam will be more reliable if not more efficient. The trouble with things like the Corryvreckan, aside from the randomness of where the vortices pop up, is that they aren’t there every single day the way, say, Niagara Falls is.
That said, there is on-going work to effectively harness ocean waves for power, with ideas like buoy generators or sea snake generators. As with most concepts one of the difficulties in implementation is determining a safe and efficient manner to transmit the electricity generated from these offshore sites (we’re generally talking miles from shore) to where it’s needed. This problem is often similarly faced by solar and wind energy producers. There are already wave farms in place around the world, though, and it’s a promising field of renewable energy. (Photo credit: Wikimedia)
Wake Vortices at Night
The ends of an airplane’s wings generate vortices that stretch back in the wake of the plane. Most of the time these vortices are invisible, even if their effects on lift are distinctive. Here an A-340 coming in for a foggy landing demonstrates the size and strength of these vortices. Notice how the fog gets swept up and away by the vortices. Pilots will sometimes use this effect to their advantage in clearing a runway of fog by making repeated low-passes to clear the fog before landing. (Video credit: A. Ruesch; submitted by Jens F.)
Imitating Flapping Flight
Flapping flight, despite being utilized by creatures of many sizes in nature, remains remarkably difficult to engineer. In this experiment, a simple rectangular wing is flapped up and down sinusoidally. Above a critical flapping frequency, the wing–which is free to rotate–accelerates from rest to a constant speed. This rotation is equivalent to forward flight. The upper image shows a photo and schematic of the setup, while the lower images shows flow visualization of the wing’s wake. The wing moves to the right, shedding thrust-providing periodic vortices in its wake. (Photo credits: N. Vandenberge et al.)
