Leading-Edge Vortex Characteristics of Surging-Translating Flexible Wings

Abstract

This study experimentally investigates the flow fields and properties of the leading-edge vortex (LEV) around rigid and chordwise-flexible wings performing rectilinear translational motion starting from rest. A rigid wing and two flexible wings with an intermediate degree of flexibility and a high degree of flexibility are considered in this study in order to explore the influence of wing deformation on the behaviour of the LEV. Phase-locked planar particle image velocimetry measurements are performed at the 75% span position of the wings at the Reynolds number of 7360. The results reveal the presence of similar flow structures for all wings in the acceleration phase: a coherent LEV and a series of vortices shedding from the trailing edge of the wing (i.e., trailing edge vortices). At the late stages of the constant-speed phase, the LEV moves away from the wing surface and bursts into smaller flow structures in the case of the rigid wing and the wing with an intermediate degree of flexibility. In the case of the wing with a high degree of flexibility, a coherent LEV which is positioned close to the wing surface is present throughout the motion. In this case, a smaller geometric angle of attack is attained due to a larger chordwise deformation of the wing. Accordingly, the flow reattaches downstream of the LEV and leaves the trailing edge smoothly. A stable LEV and the associated low-pressure region, and the increased size of the horizontal surface area may elevate the resultant lift force despite the fact that the relatively small LEV circulation level is achieved in the wing with a high degree of flexibility.