Any object in the wind will lave a wake behind. A cylinder typically leaves a neatly arranged row of vortices. If you ever hear a humming sound from a bicycle rack on you car's roof, you are hearing the sound of many vortical tubes of air separating in every second. These photos show you what the phenomenon looks like.
Notice how the vortices in the last image stay neatly in line. This happens in part due to an effective hyrdodynamic repulsion between oppositely rotating nearest neighbor vortices.
The next movie clip shows you how vortices of opposite sign repel. We did this by letting vortices of one rotation interact with vortices of the opposite rotation in a second vortex street.
Quicktime Movie (0.9 MB, be patient while loading) |
When the film speed goes up, the nice orderly vortex street can become unstable. You can see this in the first few images of this page. In the following movie you can see that the breakdown of the vortex street starts by vortices jumping over their neighbors so they can merge with next nearest neighbors, which rotate in the same sense. Like sign vortices experience a hydrodynamically induced attraction. Red dots highlight vortices of one sign, blue dots of the other. Notice that the paring does not always occur.
Quicktime Movie (1.7 MB, be patient while loading) |
The last movie was a bit grainy for various reasons. a) The video was made with a special camera operating at 3000 images per second. b) I reduced the image quality somewhat to make the file smaller.
All of the above images were of millimeter size objects inserted through few micrometer thick soap membrane which is flowing at about 2 meters per second. The black and white contrast you see sub micrometer thickness variations which are being carried with the flow. Using a more sophisticated method than plain photography, we can use computer and thousands of tiny particles to measure the true motion of the flow. In the next to images the object in the fluid is slightly upstream from the field of view.
In the figure above the arrows indicate the velocity. There appears to be not much to see. All the fluid appears to be happily moving downstream. If you look very carefully you see that near the center the vectors are either a bit shorter than the rest, or wiggle slightly up and down. In the next image, we subtracted the average velocity out, and you see vortical patterns that are similar to the photos at the beginning of this page. The dark and light regions are in fact the vorticity as calculated from the field of red velocity vectors. Dark mean strong clockwise flow, and light means strong counterclockwise flow.
Even though the vortices in the pictures above are only a few mill meters in diameter, they can happen at any scale. Check out this photo, taken by NASA.
The brown obstacle on the left is Guadalupe Island, June 11, 2000. The Island is about 20 miles long, and the vortices have a similar size. For more of these amazing photos, check nix.nasa.gov and search for Karman.
Images Copyright (2000) W. Brent Daniel and Maarten A. Rutgers.