Fluid Motion Gallery


Six of the eight following photos were presented at the 1995 Gallery of Fluid Motion at the Division of Fluid Dynamics Meeting of the American Physical Society Meeting. The photos were one of six finalist photographic entries and have appeared in the 1996 issue of the journal Physics of Fluids, and some of them were later requested by the journal Nature.

M. A. Rutgers, X-l. Wu, and W. I. Goldburg. "The Onset of 2-D Grid Generated Turbulence in Flowing Soap Films," Phys. Fluids 8, S7, (Sep. 1996).

Nature, 383, 671 (Oct. 24, 1996).

Maarten A. Rutgers, Xiao-lun Wu, and Walter I. Goldburg.


Setup

A free standing soap film, bounded by two vertical nylon wires, originates about one meter above the photographed area from a bottle of soap solution. In the photographs the film has reached a terminal velocity between 2 and 3 meters per second and has a nearly uniform thickness. At the top of the photographs a comb punctures the film. The vortices generated in the wake of the comb's teeth mingle to varying degrees depending on the Reynolds number of the flow. The comb tooth diameter is 0.12 cm. The center to center tooth spacing is about 0.3 cm. The channel is about 4 cm wide.

Visualization

Visualization of the flow patterns is spontaneous. The film is slightly compressed as it hits the comb teeth and perhaps slightly expanded behind the teeth. The resulting thickness variations in the film are visible due to constructive and destructive interference of light. Illumination with white light results in the beautiful colors familiar to all from playing with soap bubbles. To enhance contrast the films in the photos were illuminated with monochromatic sodium light. Once created, thickness variations are mostly convected by the flow as a passive scalar.

1) A singe vortex street behind a glass rod which punctures the film. The rod is a few mm in diameter.
2) The Reynolds number is near the critical value for the onset of von Karman vortex streets behind each comb tooth. One tooth has already formed a vortex street.

3) At slightly higher Reynolds number each tooth is trailed by a vortex street, all of which eventually mingle further down stream.
4) A closeup of the comb teeth at a similar flow rate.

5) Another increase in Reynolds number. Individual vortex streets do not survive long.
6) A weakly turbulent state at higher Reynolds number. Note the increased length scales downstream, due to vortex pairing.

7) Close-up near the comb at a Reynolds number higher than that of all the other figures.
8) A study of a flat object. A utility knife tip punctures the film. The sharp edges produce small vortices in a shear layer roll-up. This fine structure is convected into a much larger von Karman vortex wake.

Finally, a movie of what this really looks like.  This film clip was recorded at 1000 frames per second and played back at a speed reduction of over 30 times. These soap films move very very quickly.

Quicktime Movie (0.9 MB, be patient while loading)