Jun Zhang and his colleagues at the Courant Institute of New York University took our soap film and applied it to their studies of swimming things. The study of flags is related, but as you can see in the 'flag' section of this web site, a piece of fabric in the wind tunnel can behave in tremendously complex ways. Jun figured out that a thin silk filament flaps in our flowing soap films like a flag flaps in the wind. In this case, the behavior is much more controlled, as problems often are in lower dimensions. Here are some images we took of Jun's ideas.
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1D flag in a 2D flow. If you look closely, you can see that the filament ends a little over halfway above the middle of the picture |
The wake of the flag seen further downstream. The filament is still visible near the top. |
As with our vortex streets, we can use more sophisticated cameras and computers to measure the flow field behind the flags. We start by putting tiny particles into the flow, which you can see in the following images. The field of view is so you can just see the tail of the flag
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Particles in the flow behind the flag. |
The flag wagging the other way. |
We can compare these images with ones taken a fraction of a second later, and use computer algorithms to check how far things have moved during that time. This gives us a measure of the velocity of the fluid at many points. The data below were a crude first attempt, but get the idea across. The mean speed has been subtracted from these velocity fields.
