Abstract
There are two objectives in this thesis: investigate the asymmetrical hovering mechanism of Zosterops japonica; create biomimetic flapping machine to verify high-efficiency flapping mode. Using digital particle velocimetry, we demonstrated that Zosterops japonica was able to increase lift by using attached LEVs during hovering. Unsteady vortex structure- leading edge vortices (LEVs) and trailing edge vortices (TEVs) were created by flapping motion in the downstroke period during hovering. The use of unsteady aerodynamic mechanisms was limited to insects and bats in the past researches. It was the first time that LEVs appeared in bird’s asymmetrical hovering. Lift mechanism transition enhance lift production in the downstroke period during hovering. It transferred from “leading edge vortex lift mechanism” to “clap lift mechanism”. LEVs dominated lift production in the beginning of downstroke. When LEVs were absent in the end of downstroke, clap lift mechanism would then appear to enhance lift. LEV lift mechanism and clap lift mechanism alternately provided sufficient lift during dowstroke period. Flapping apparatus was created to imitate wing flexion of Zosterops japonica. The experimental results indicated that wing flexion would avoid excessive drag in hovering. Define span ratio R as one parameter that characterizes flexible geometry of bird wings and analyse the relationship between span ratio and cruising speed. From flow visualization data, increasing R would affect vortex jet inclined to horizontal axis. While jet inclined more horizontally, the resultant force was divided into greatly amount of thrust and limited amount of lift. While jet inclined more vertically, the resultant force was divided into greatly amount of lift and limited amount of thrust. With the experiment designed and analysis mentioned above, we illustrated that wing flexion could enhance thrust production during cruising flight. To conclude, LEVs lift mechanism and clap lift mechanism alternately provide sufficient lift in the downstroke period during hovering. Wing flexion would avoid excessive drag in hovering and enhance thrust production in cruising flight. Zosterops japonica can maneuver the flight highly-efficient.