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Title: Japanese fan flow

Description: The picture shows the complex and fascinating flow of a rigid foil that flaps in an otherwise quiescent fluid, and thus consitutes a two-dimensional counterpart of the flow of a hand-held paper fan [1]. The foil penetrates a horizontally suspended soap film from below, and it is driven in simple harmonic pitching oscillations in a plane that is parallel with the soap film. The soap film is initially quiescent, and due to the flapping foil two vortices of opposite sign shed from the foil tip each oscillation period. Notice how these vortices travel away from the foil, while undergoing severe deformations. Two beautiful "butterfly wings" are formed by the secondary flow, which also contain traces of the flow some 80 oscillation periods back.

Credits: Teis Schnipper

References: [1] Teis Schnipper, Laust Tophøj, Anders Andersen, and Tomas Bohr (2010) "Japanese fan flow," Phys. fluids, 22, 091102-1.

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Contributed By: Teis Schnipper

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