Fig. 9. Vorticity plots at half-wing length at various times during one cycle. Solid and broken lines indicate positive and negative vorticity, respectively. The magnitude of the non-dimensional vorticity at the outer contour is 2 and the contour interval is 3. A-D, downstroke; E-H, upstroke. Reynolds number Re=20, stroke amplitude =150°, midstroke angle of attack _m=40° and non-dimensional duration of wing rotation _r=1.87 (mean angular velocity of rotation =0.93); symmetrical rotation.