Effect of flexion on the propulsive performance of a flexible flapping wing

A numerical study was carried out to investigate the effect of chordwise flexion on the propulsive performance of a two-dimensional flexible flapping wing. The wing undergoes a prescribed sinusoidal heaving motion with a local deflection. A deformable overset grid dynamic mesh method was employed to implement the motion of the grid instantaneously. The effect of flexural pattern, flexural amplitude and flapping frequency in terms of Strouhal number are evaluated. Unsteady flow around the wing is computed using an in-house developed Unsteady Reynolds-Averaged Navier-Stokes (URANS) solver. The results show that the different flexural patterns will create different flow fields, and thus the thrust generation will be significantly varied. The thrust and propulsive efficiency do not increase monotonically with the flexure amplitude while a peak value is revealed. It is found that the wake vortices after the flapping motion assembly behave as a reverse von-Karman vortex street, which can principally create thrust. The thrust is found to increase with increasing Strouhal number. Propulsive efficiency is beneficial from the chordwise flexibility and peaks within the range of 0.2 <St <0.4, which is evidenced by natural flyers.