Effect of Wing Flexibility on Lift and Thrust Production in Flapping Flight

In bird and insect flight, wing deformation plays an important role in aerodynamic performance. The wing deformation is produced by neuromuscular control and/or by aeroelastic effects. The focus of the current study is to evaluate the effects of wing deformation by coupling a large-eddy simulation solver with a linear elastic membrane model. Different membrane prestresses are investigated to give a desired camber in response to the aerodynamic pressure. All simulations are carried out at Re = 10,000 for forward flight with an advance ratio of 0.5. The results show that the camber introduced by a flexible wing increases the thrust and lift production considerably. An analysis of flow structure reveals that, for flexible wings, the leading-edge vortex stays attached on the top surface of the wing and glides along the camber and covers a major part of the wing, which results in high force production. On the other hand, for rigid wings, the leading-edge vortex lifts off from the surface resulting in lower force production. Further, introduction of the camber also increases the force component contributing to thrust, leading to a high thrust-to-lift ratio. In comparison to a rigid wing, a 40% increase in thrust is observed for the low-prestress case, which results in a camber of 0.25 chord. Further, the results also show that the wing with high spanwise prestress and low chordwise prestress offers better performance both in terms of force production and uniformity in the force-induced cambering.