A Study On Aerodynamic Performance Of Flapping Airfoils At Low Reynolds Numbers

Long time explorations have been carried out on the high-lift mechanism generated by insect wings, particularly when they stay in hovering states. This phenomenon cannot be explained by using the theory of quasi-steady flows. The flapping of insects is complicated, on which previous research is far from enough. In this thesis, the AcuSolve, a commercial code developed by Altair Company, has been used to model flapping wings numerically. The AcuSolve is a FEM-based CFD solver with high accuracy and simple modeling capability, while high-qualified grids are not strictly required. The purpose of this thesis is to investigate the high-lift mechanism of insect wings, by using a simplified geometrical model. Following previous researcher’s studies, the model can be simplified into a two-dimensional ellipse flapping foil in the stationary fluid without considering the incoming flow under the condition of low Reynolds numbers within the range of10～200. The complicated motion of a real insect wing can usually be divided into translation and rotation, which can be described by two sinusoidal functions, respectively.The results of this thesis include aerodynamic coefficients and flow structures surrounding a flapping ellipse airfoil, as well as the effects of kinematic parameters on these coefficients and flow structures. Furthermore, it is found that the phase difference between translation and rotation has a large influence on the forces and flow structures. Three common lift enhancing mechanisms have been confirmed and discussed in the thesis. They are delayed stall, rapid pitch-up and wake capture. In the delayed stall mechanism, the wing maintains a high level of lift throughout the whole flapping period. And rapid pitch-up and wake capture can further improve its instantaneous lift. In addition, parametric studies have been carried out, such as the flapping amplitude, the location of rotation center, and thickness of the airfoil. Their effects on the aerodynamic forces as well as the flow structures are discussed herein.