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Investigation of Leading-Edge Vortex Formation on a Robotic Bird’s Wing.

Schalk, Laurens (2022) Investigation of Leading-Edge Vortex Formation on a Robotic Bird’s Wing.

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Abstract:The RoBird is a flapping wing drone that is developed by mimicking a peregrine falcon. It generates both lift and thrust by flapping its wings, similar to how real birds fly. The drone works, but there is still a poor understanding of how and why the aerodynamics work as they do. By investigating the RoBird’s aerodynamics, more can be learned about how birds fly. This could help humans for example in designing better drones, but could also help us understand birds, and therefore help us protect them. This research focuses on leading-edge vortex (LEV) formation on the bottom side of the RoBird’s wing. This LEV is an unsteady aerodynamic phenomenon that creates very high lift forces and is a key mechanism that enables insect flight. It’s however poorly understood in the context of vertebrate flight as the high Reynolds numbers associated with vertebrate flight and complicated wing designs change the flow and complicate research. It is known that vertebrates are capable of developing LEVs. But even when a LEV is found on a vertebrate’s wing, it is often not known for which flight maneuvers they precisely use the LEV, and how different factors influence its formation. The goal of the research is to find out if leading-edge vortices form on the bottom side of the RoBird’s wing during flapping flight. This has been investigated by performing measurements at 0.20 ≤ St ≤ 0.41, as most vertebrates fly in this region and cruise at 0.20 ≤ St ≤ 0.25 [Nudds et al., 2004] [Taylor et al., 2003]. The selected Reynolds regime is 55.100 ≤ Re ≤ 106.400, which has been chosen mainly due to limitations in the measurement set-up used. Three wings have been produced to investigate the effect of leading-edge sharpness on LEV formation. Of these wings, two had a changed leading-edge radius relative to the original RoBird wing design, resulting in a blunt, original, and sharp wing. Based on the performed literature study, the hypothesis has been formed that LEV formation conditions are the most optimal for wings with sharp leading-edges flapping at high Strouhal numbers, and at a Reynolds number of Re ≈ 100.000. The results of the measurements were used to investigate the effect of the Strouhal number, Reynolds number, and leading-edge shape on LEV formation. This knowledge can be used to test whether or not the mentioned hypothesis is true. A flapping mechanism that flaps a single RoBird wing up and down has been mounted in the open jet aero acoustic wind tunnel of the University of Twente, where the flow around the flapping wing was measured with particle tracking velocimetry (PTV). This technique works by injecting neutrally buoyant helium-filled soap bubbles into the airflow. By illuminating the bubbles with a powerful laser and recording the reflections with four high-speed cameras, the bubbles can then be converted into digital 3D particle tracks over time, which can be used to analyze the flow. 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. No information was available about the angle of the body relative to the wind during flight, so the body has been mounted under zero angle of attack, to serve as a stepping stone for more complicated research. Also, no phase change has been applied to the wing during the flapping motion, due to a non-functioning phase change mechanism. From the particle tracks, it was found that leading-edge vortices do form in the bottom side of the RoBird’s wing, with the only exception being the blunt and sharp wing not forming a LEV at Re = 55.100 and St = 0.20. The strengths of the LEVs that were found have been quantified using the maximum spanwise velocity inside the vortex Uy since this spanwise flow is crucial for LEV stability according to the literature. The noted values of Uy have been divided by the free stream velocity U∞, resulting in a relative, dimensionless spanwise velocity Uy/U∞. Despite being a crude quantification method, the measured vortex strengths could be used to compare the vortex strength for different Strouhal numbers, Reynolds numbers, and the three different leading-edge shapes. From this comparison, a clear Strouhal number effect could be seen, with increasing vortex strengths for increased Strouhal numbers. At constant St = 0.20 a clear Reynolds effect could be seen, with no or weak LEVs at Re = 55.100, an apparent peak in LEV strength at 70.900 ≤ Re ≤ 86.600 and a decrease in vortex strength for Reynolds numbers beyond the observed peak. This peak is thought to be close enough to the peak observed in literature to be in line with the hypothesis. At St = 0.26, no clear Reynolds number effect could be seen. From the comparison of the different leading-edge shapes, no definite statements could be made about its influence on LEV development, as no clear trends were visible and the data seemed to contradict the hypothesis. However, its believed to be likely that for example production inaccuracies have disrupted the measurements, making the comparison of the data of the different leading-edge shapes very hard. The force data has been filtered and recalculated into aerodynamic force data, showing highly detailed forces acting on the wings during their flapping cycles. However, the results showed large deviations in similar data sets, indicating that a large error has been introduced somewhere along the process, possibly by noise or the calculation of the aerodynamic forces. Therefore the data was deemed unreliable, and force enhancement by LEVs could not be accurately determined.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Programme:Mechanical Engineering MSc (60439)
Link to this item:https://purl.utwente.nl/essays/93868
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