In recent years, a variety of techniques have been developed that allow the air movements around the wings of flying animals to be measured. Such techniques have been primarily applied to birds, or scaled-up models of insects. A key challenge for animal flight aerodynamicists has been to measure the flows, or wake, around the wings of life-sized insects. Whilst smoke can be used to visualise the form of the wake generated by a flying insect, it doesn't really give much quantitative information that can be used to determine, for example, the mechanical costs of flight. Richard Bomphrey and co-workers, at Oxford and Cranfield Universities, sought to determine,quantitatively, the patterns of airflow around a flying hawkmoth.
The animal they studied had a wing length of a mere 52 mm, so the technical challenges in measuring wakes from such a small animal are obviously quite profound. They conducted experiments on the moth while tethered in a wind tunnel. The tether was instrumented, to allow six force components to be measured during flapping. The movement of air around the wings was determined by using a digital particle image velocimetry (DPIV) system. The system used a laser to generate a 0.4 mm-thick sheet of light, produced in 5 ns pulses. Small smoke particles (less than 10 μm in size) were introduced into the flight tunnel, and the DPIV system automatically tracked the movement of the smoke particles within the wake. Analysis of the wake was triggered when the force produced by the moth was highest during the downstroke of each wingbeat. The tether of the moth could be moved horizontally in the wind tunnel, so that DPIV measurements could be made at different points behind the insect. Experiments were conducted at two wind speeds (1.2 and 3.5 m s-1).
The experiments showed that hawkmoths generate a curved, elliptical, vortex loop, arising from the leading edge of the wing and having a width equal to the wingspan of the insect. Bomphrey and colleagues show that the velocity profiles they observed matched very closely those predicted by existing theories for the aerodynamics of flapping flight of insects. The team found that the forces produced by air movement within the vortex loops could account for 85% of the forces measured on the tether to which the moth was attached. During the experiments, the team found that when the wings `clapped' together,two and a half times as much aerodynamic force was produced than during downstrokes. Whilst not every wingbeat cycle contains a `clap', they occur sufficiently often to supplement the force generated in the downstroke.
Despite collecting impressive data on the airflows around the moth, the team are planning to continue their work and use a stereoscopic DIPV system to improve the accuracy of measurements by determining airflows in all three planes.