Colloquia

Summary

The aerodynamics of vertebrates (bats and birds) are governed by all kinds of complex unsteady flow phenomena caused by the flapping motion of their wings, which create both lift and thrust. Due to this complexity, and the complexity of their wings there is still a relatively poor understanding of vertebrate aerodynamics. A better understanding of their aerodynamics could help in designing better drones, but also help humans understand birds’ behavior, which could help in protecting them.

The RoBird is a flapping wing drone that is developed by copying a peregrine falcon. By examining the aerodynamics of the relatively simple RoBird, more can be learned about how birds fly. This research focuses on the formation of Leading-Edge Vortices (LEVs) on the RoBird’s wing, which is an unsteady aerodynamic phenomenon that can create very high lift forces. From the literature, it is known that vertebrates are capable of developing LEVs. But even when a LEV is found on a vertebrate wing, it is often not known for which flight maneuvers they precisely use the LEV, and how different factors influence its formation.

The main goal of the research is to determine if LEVs form on the bottom side of the RoBird’s wing for a Reynolds number of 55.100 ≤ Re ≤ 106.400 and a Strouhal number of 0.20 ≤ St ≤ 0.41. This is investigated by placing a wing and flapping mechanism in the UTwente wind tunnel, and by measuring the flow with Particle Tracking Velocimetry (PTV). Three wings have been produced, of which two had a changed leading-edge radius relative to the original RoBird wing design, resulting in a blunt, original, and sharp wing. These wings were used to investigate the effect of leading-edge sharpness and the Reynolds- and Strouhal numbers on LEV formation if LEVs were found. Also, force measurements have been performed by mounting the flapping mechanism on a half-model balance in order to measure the high lift associated with LEVs.

The PTV data showed that a LEV is present for almost the entire measured regime. A crude vortex strength analysis showed that for increasing Strouhal numbers the vortex strength increases. For Reynolds numbers at St = 0.20, a peak in vortex strength was measured at approximately 70.900 ≤ Re ≤ 86.600, with no clear Reynolds dependency at higher Strouhal numbers. The effect of the leading-edge shape could not be determined accurately, possibly due to production errors of the wings. The force data has been filtered and recalculated to aerodynamic forces, but due to inconsistencies in the data, they were deemed too inaccurate to observe LEV formation.