Aerodynamic behavior of curved flexible wings

Abstract

Many creatures use flexible wings and membranes to increase propulsive efficiency when flying or swimming. This work explores the possibility of geometric tailoring in curved flexible wings, so that an asymmetric buckling phenomenon is elicited during wing heave. Several scaling arguments are presented and compared to static experiments and numerical simulations for wings undergoing deflection and buckling in concave-down and concave-up configurations. From this, predictions are made to identify the conditions in which wings may undergo asymmetric buckling during heaving, so that the wings remain relatively undeformed during the downstroke and buckle during the upstroke. A series of dynamic simulations and experiments are conducted to establish the accuracy of the scaling analysis and quantify the resulting performance gains in terms of average drag and lift. Good agreement between experiments, simulations and scaling analysis is observed. Observations suggest that the increase in average lift was most pronounced near the onset of concave-up buckling. A maximum lift coefficient increase of roughly 150% was recorded for the buckling wings over the most rigid wing tested herein. Additionally, the buckling wings were able to produce significant thrust output, leading to an increase of over 600% in combined resultant force coefficient over the shorter wings which did not appreciably deform. These experiments and simulations suggest that the geometry of flexible wings may be tuned, for example, by changing radius of curvature, to control lift and thrust.