Effects of Wing Flexibility on Vortex Behaviors and Aerodynamic Performance in Various Flapping Flights

Abstract

<p>This dissertation presents various studies on vortex structures and aerodynamic characteristics in flapping flexible wings to offer design considerations for flapping micro aerial vehicle (FMAV) developments. A dynamically scaled-up flapping robot, mounted with a6-axis sensor, and a digital particle image velocimetry (DPIV) technique are used to measure aerodynamic force/moment and flow vector-fields during flapping wings in a water tank. Wing models with or without leading edge veins have different thicknesses, providing various wing flexibilities. Studies of flapping wings without veins in two different flapping kinematics are firstly proposed to demonstrate the wing flexibility effect on aerodynamic performance. The flexible wings generally lead to a leading-edge vortex (LEV) generation delay, causing a lift decline. However, the flexible wing in the specific range of flexibility (non-dimensional spanwise flexural stiffness of 35−80) enhances more lift than the rigid and highly flexible wings due to a wider LEV attachment area. The appropriate deformations lead to the attached LEV during the wing reversal. After the stroke reversal, the portion of the attached LEV flows over the leading-edge, causing a downward flux over a new LEV structure near the wingtip. In addition, flexible wings with leading-edge veins had a dynamic camber, causing a poor lift enhancement except for its initial augmentation. Nevertheless, the specific flexible wing (non-dimensional chordwise flexural stiffness of ~1) obtains a higher aerodynamic performance due to reducing the mechanical power requirement to fly. The above LEV attachment is influenced by the downwash from the wingtip vortex (TV). The TV is differed depending on the wingtip shape and the wing deformations, conducting a comparative study of hawkmoth-like and rectangular wings. In a rigid case, the rectangular wing has better aerodynamic performance than the hawkmoth-like wing, whereas in a flexible case, it has not. The flexible hawkmoth-like wing secures a wider LEV region with improved circulations, achieving about twice the increase rate from rigid to flexible wings on lift-drag ratio and a much more lift-power ratio. The results obtained will provide the specific range of wing flexibility and the importance of the wingtip design to allow better aerodynamic performance in FMAV developments with flexible wings.</p>