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Fig. 9. Schematic reconstruction of vortices and local flow conditions at two different kinematic phase relationships between fore- and hindwing and at two different times within the stroke cycle. Hindwing lift depends on LEV strength and the velocity and angle of the oncoming fluid (local flow). Local flow conditions (black vector) on the wing segment (grey oval) are calculated from the velocity and angle of the combined fore-hindwing downwash determined in a region below the wing's surface in a single PIV image plane (green vector, see white box in Fig. 7), and the translational velocity of the hindwing section (grey vector). Blue arrows, lift attenuation; red arrow (F), lift enhancement of the hindwing compared to a wing flapping free of forewing downwash. Vortical circulation (cm² s^-1) in the hindwing's leading edge vortex is shown in square brackets close to the LEV icon. The different strengths of starting and leading edge vorticies are indicated approximately by the size of the plotted `vortex' icons. Effective angle of attack for the hindwing section (degrees, left value) and local flow velocity (m s^-1, right value) are shown respectively in parentheses under the vector diagram. (A-F) The flow characteristics for a flapping hindwing free from forewing downwash (single wing flapping) at (A) 15% and (B) 35% of stroke cycle; when the forewing (upper wing) leads wing motion by a quarter stroke cycle (C) at 15% and (D) 35% of stroke cycle; when the hindwing (lower wing) leads by a quarter stroke cycle (E) at 15% and (F) 35% of stroke cycle. A detailed description of vortex development and local flow is given in the text. In all diagrams, the motion of the hindwing (lower wing) is from left to right. White arrows indicate the direction of motion of the forewing. T, stroke cycle.

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