A rigid plate of aspect ratio S/C=0.54 (S=span, C=chord) is pitched in a uniform flow about its leading edge. The double-amplitude to span ratio is A/S=0.31. The leading edge is hinged to the trailing edge of a stationary symmetric 2-D airfoil. Fluorescent dyes are introduced through a series of holes on each side of the airfoil support. The Reynolds number based on the chord of the plate and the freestream velocity U is 640. Different flow regimes can be identified based on the Strouhal number, St=fA/U (f=pitching frequency). Twice in each flapping cycle, a horseshoe vortex is created, consisting of vorticity shed by the top, bottom, and trailing edges. At St=0.43 (this image, top and side views), the shed vorticity splits and forms two distinct branches.

Studying such wakes is believed to be important for understanding the mechanisms of thrust production in fish-like swimming. See also St = 0.23 and St = 0.64.