When a flapping object moves through a fluid, it can leave a number of patterns of vortices in its wake. The von Karman vortex street, so often seen in clouds or behind cylinders, is only the beginning. In the above photo, a symmetric foil flaps in a vertical soap film; as the amplitude and frequency of the oscillation change, various wake patterns are formed. From left to right, a) a von Karman wake; b) an inverted von Karman wake; c) a 2P wake, in which two vortex pairs are shed with each cycle; d) a 2P+2S wake, in which two vortex pairs and two single vortices are shed per cycle; e) a 4P wake; and f) a 4P+2S wake. See some of these flows in action in these videos. (Photo credit: T. Schnipper et al.)
(via fuckyeahfluiddynamics)
