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Title: Vortex ballet
Description: A flapping foil (top center) in a downwards flowing soap film sheds six vortices each oscillation period. The vortices comprise a wake where two vortex pairs are formed, spaced by vortex singlets (a 2P+2S wake). Downstream of the foil, the vortices are advected under their mutual influence which appears as a graceful "vortex ballet". See  for more details on the vortex wakes behind a flapping foil.
Credits: Teis Schnipper, Anders Andersen and Tomas Bohr, Technical University of Denmark.
References:  Schnipper, Andersen and Bohr (2009) "Vortex wakes of a flapping foil," Journal of Fluid Mechanics, 633, 411-423.
Contributed By: Teis Schnipper