High speed Schlieren photography on flying insects

A comprehensive understanding of the complex flow topology generated by freely flying insects has eluded the scientific community due in part to the inability to adequately study the unsteady three-dimensional flow structure in a natural setting. In the absence, researchers have primarily relied upon either two-dimensional conventional flow visualizations/measurements on tethered insects or dynamically scaled experiments utilizing robotic flappers fitted with scaled insect wings undergoing simplified flapping motion. To overcome the limitations of these studies, high speed Schlieren photography is successfully implemented on freely flying hawkmoth Manduca sexta.

Flow features such as leading-edge vortex and tip vortex were directly visualized on the insect wings. A well linked vortex structure is captured under each wing, including a vortex loop produced in the down-stroke joint with tip and root vortices created in the upstroke. A physics-based optical flow method is then applied on the Schlieren images, deriving quantitative information about the flow around the flying insects.