A rigid plate of aspect ratio S/C=0.64 (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=0.64 (this image, top and side views), the streamwise vorticity develops an instability which evolves into smaller-scale hairpin loops that convect vertically away from the symmetry plane.

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 = 0.43.