Fig. 9. Velocity vectors from a synchronous kick based on a lateral view (vertical light sheet; xy plane) plotted on a vorticity background. The reference vector equals 1 m s^–1. Red indicates counter-clockwise and blue indicates clockwise vorticity (s^–1). The three images are separated by 0.04 s, showing the buildup of the vortex ring shed by the foot. In the two upper images, the foot is still in the image, affecting the estimation of the vector fields. In the top image, the vorticity centers are upstream of the foot, while in the middle image the foot is the source of the vectors pointing upstream just upstream of the vortex ring. Free-stream flow velocity has been subtracted from each vector.