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=0.23 (this image, top and side views) the legs of each vortex loop are entrained by the tip of the subsequent loop forming a chain-like structure.

Studying such wakes is believed to be important for understanding the mechanisms of thrust production in fish-like swimming. See also St = 0.43 and St = 0.64.