For a higher resolution picture, please email Jun, zhang (at) physics.nyu.edu
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A thin silk thread flaps in a fast flowing soap film, serving as a model system to
study flapping flags in the wind. Flow visualization is realized by the interference
technique, as the minute film thinkness variation revealed by the lighting from a
low pressure sodium lamp. The image below shows a "1D flag" that is straigtened
out by the laminar flow and this non-flapping state is dynamically stable (survives
small amplitude pertubations).
For more information, see
Nature, 408, 835 (2000)

The photo below shows a typical flapping state of the flag.
This state is also stable to finite amplitude purtubations.
Together with the above state, the system is said to be
bistable.
For more information, see
Nature, 408, 835 (2000)

Two tandem flapping flags in a flowing soap film. The fluid drag force on each of them
are directly measured. Unlike solid objects placed in fluids, the leading flag experiences
a reduced drag compared to the follower. We call this new effect the "inverted fluid
drafting."
For more information, see
Physical Review Letters, 101, 194502 (2008)

This photo shows a local flow field after the free end of a
flapping flag in soap film.
For more information, see
Physical Review Letters, 101, 194502 (2008)

An elastic fiber, made of very thin glass filament, is bent due to the force from the
oncoming flowing soap film. The drag is directly measured and its shape and
flow field are also recorded simultaneously. Many plants and even trees survive
in strong winds by deforming passively their shapes that yield much reduced air
drag.
For more information, see
Nature, 420, 479-481 (2002).

In a classical Rayleigh–Bénard convection (RBC) system, a fluid is subject to
heating from under and cooling from the top. The fluid undergoes an instability
and starts to move. The fluid casts a shadow on a screen as a parallel light
beam is sent through its transparent walls that confine the moving fluid.
Thermal "plumes", the mushroom-like structures, transport heat far more
effectively compare to that by thermal difussion. This image has appeared in
a review article in Physics Today by Leo Kadanoff (August 2001, P34-39).
See for more information:
Physics of Fluids 9, 1034 (1997).

A flat, symmetric, flapping wing moves through a fluid and leaves a wake that is similar to
the ones trailing off swimming fish. Their propelling mechanisms are essentially the same.
In this photo, the tracing particles are microscopic air bubbles that follow the flow quite
closely.
For more information, please see
Journal of Fluid Mechanics, 506, 147-155 (2004).



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