Fig. 6. Estimation of fluid velocity and acceleration in a local area near the trailing wing edge during one-wing and two-wing flapping conditions. (A) Grey areas indicate the regions of interest at eight different times during dorsal stroke reversal. (B) Mean flow velocity and (C) acceleration in the fluid when flapping one (red, lower pictogram) and two wings (image wing, blue, upper pictogram). Green data in B (right scale) show the difference in flow velocity between both flapping conditions. Acceleration fields were calculated from the difference of two subsequent DPIV images taken at 65% wing length (center of wing area). Stroke kinematics is the same as in Fig. 1.