Schlieren photography on freely flying hawkmoth

Abstract

The aerodynamic force on flying insects results from the vortical flow structures that vary both spatially and temporally throughout flight. Due to these complexities and the inherent difficulties in studying flying insects in a natural setting, a complete picture of the vortical flow has been difficult to obtain experimentally. In this paper, Schlieren, a widely used technique for highspeed flow visualization, was adapted to capture the vortex structures around freely flying hawkmoth (Manduca). Flow features such as leading-edge vortex, trailing-edge vortex, as well as the full vortex system in the wake were visualized directly. Quantification of the flow from the Schlieren images was then obtained by applying a physics-based optical flow method, extending the potential applications of the method to further studies of flying insects.

Keywords: Schlieren; flow visualization; insect flight; vortex structure.

Figure 1.

Vortex structures captured on a near-hovering male hawkmoth (1.26 g) with an average flying speed of 0.3 m s⁻¹. (a) A combined structure of starting/stopping vortex after wing pronation. (b) Tip vortices created from tips of fore and hind-wings. (c) An attached cone-shaped leading edge vortex before wing supination. (d) An illustrative depiction of the vortex structure created in the down-stroke. (e) The vortex loop created in the down-stroke sheds into the wake. (f–g) Long, stretched tip vortices generated from the tips of fore- and hind-wings and connected to the shed vortex loops. (h) An illustrative depiction of the vortex structure created in the up-stroke.

Figure 2.

Velocity field quantification around wing tip. (a) Original Schlieren image with flow quantification region highlighted by red rectangle. The tip vortical flow image at a time instant is shown in the bottom left, while the tip vortical flow image in the next frame is shown in the bottom right. The tip vortex is highlighted by the red dashed loop. (b) Velocity vector field quantified using physics-based optical flow method. (c) Vorticity contour plot and streamline plot derived from the velocity quantification. Red dashed line from (a) matches well with the strong counter-clock vorticity.