Research Record Insect Flight In Minute Detail

Credit: Oxford University

Previous experimental approaches had to rely on recording individual 2D slices through the wake and stacking them on top of each other to give a 3D picture. Scientists had to hope the wake didn’t change to much in the time between capturing each slice, and that they didn’t miss fine detail in between the slices. Such detail could be important in calculating the lift and thrust generated by the wings of the locust.

The new study, published today in the Journal of the Royal Society: Interface, shows the advantages of the new technique called ‘tomographic PIV’, and the circumstances under which the assumptions involved in previous methods fall down.

Credit: 
Oxford University

Oxford Science Blog asked Dr Per Henningsson of the Animal Flight Group in the Department of Zoology about the work.

OxSciBlog: Why is it important to understand more about insect flight?

Per Henningsson: There are several reasons. Understanding animal flight in general gives us knowledge about the cost and constraints flight imposes on animals, which directly links to their biology, ecology and behaviour. Furthermore, insects exhibit an extremely diverse collection of wing shapes – from long and slender to short and broad wings; there are two-winged and four-winged insects; and all sorts of wonderful shapes each with different aerodynamic characteristics. By studying insect flight, we learn not only about the life of the insects themselves, but also about how flight performance is determined by wing shape.

OSB: What have you demonstrated using this new equipment?

PH: This is the first time we have been able to capture three-dimensional wake volumes behind a flying animal. Not only that, but the measurements are also more detailed than ever before. This new approach has shown some of the limitations of more conventional methods.OSB: How does the experiment work?PH: The insect is attached to a small mount using a tiny drop of glue. It is placed in the wind tunnel and the wind speed is set to 3.3 m/s, which is the preferred flight speed of the desert locust. The wind tunnel is filled with a thin mist of small olive oil droplets that float with the air. This mist is used for visualising the flow patterns. Pulses of laser light are used to illuminate the oil mist over a small volume just behind the flying locust and the particles are then filmed using four high-speed cameras running at 2000 frames per second. The displacement of the particles between images is calculated and from this, flow fields are created for aerodynamic analysis.

OSB: What were the limitations to previous approaches?

PH: The biggest limitation of previous approaches was that no instantaneous three-dimensional data could be recorded. It was only possible to measure two-dimensional slices through the space behind the flying insect, with the third dimension artificially constructed afterwards by ‘stacking’ the slices together. This approach is very accurate under certain conditions and will continue to be of great use. Our new work, however, shows where certain assumptions made when using those methods can fail.

OSB: What does the 3D data reveal that’s new about locust flight?

PH: There are sections in the wake which simply could not have been captured by the 2D stacking technique. Vortex structures oriented close to the vertical and with a diameter less than the distance between the 2D slices would go unnoticed. We found several examples of this in the wake behind the locust, some of which correspond to considerable force generation. That means that they play an important role in the overall wake dynamics.

OSB: Do you expect that this new technology will lead to new advances in this field?
PH: Yes, tomographic PIV will allow us to make much more comprehensive and precise studies of animal aerodynamics. In many cases the emergent volumetric technique is necessary to really understand what is going on. The more refined our knowledge becomes, the more likely we will be able to transfer the intricacies to our own technologies.