Low aspect ratio oscillating flexible wings at low Reynolds numbers

To identify the role of flexibility in lift generation, the force, deformation and flow fields of rigid and flexible low aspect ratio (sAR = 1.5) wings plunging with a fixed post-stall angle of attack of 15° and amplitude of 15% of chord were experimentally measured. It was shown that spanwise flexibility can significantly enhance lift. The peak increase in lift coefficient over the stationary case is more than three times larger for the flexible wing compared to the rigid wing. This increase is associated with significant deformation of the wing. The root is sinusoidally plunged with small amplitude but this motion is amplified along the span resulting in a larger tip motion. The amount it is amplified strongly depends on Strouhal number. A Strouhal number of Src = 1.5 was selected for detailed flow field measurements due to it being associated with high lift, near the natural frequency, and comparable with a previous study for high aspect ratio wings. For this Strouhal number the rigid wing exhibits a LEV dipole. This is where the clockwise upper-surface LEV pairs with the counterclockwise lower-surface LEV to form a vortex ring that self-advects upstream and away from the wing's upper surface. Conversely the flexible wing experiences deformation resulting in tip amplitude 1.53 times greater than the root amplitude and tip phase lag of 98°. This deformation inhibits the LEV dipole. Instead a strong upper-surface LEV forms during the downward motion and convects close to the airfoil upper surface thus explaining the significantly higher lift.