Enhancing Flapping Wing Propulsion in Forward Flight Through Dynamic Twisting: A Numerical Investigation

To better understand the function of natural vertebrates such as hummingbirds to twist their wings, we presented a numerical investigation on the role of dynamic twisting based on a hummingbird-like flapping wing model. Computational fluid dynamic (CFD) simulations were performed to examine the effects of dynamic torsion on the unsteady flow field, generation of instantaneous aerodynamic forces, and time-averaged aerodynamic performance. This research uncovers the details of wake structures in the flow and explores the underlying mechanisms behind the positive effects of wing torsion. The results demonstrate that wing torsion can effectively maintain the favorable effective angle of attack distribution of the wing cross-section along the spanwise direction, resulting in a higher time-averaged thrust and vertical force. Further, the proper design of dynamic torsion parameters can also improve the propulsive efficiency of the flapping wing in forward flight. Dynamic torsion also showed superior ability in controlling the airflow separation over the airfoil surface and maintaining the stability of the leading-edge vortex (LEV). Under the currently specified time-varying profile of effective angle of attack variations, maintaining a constant effective angle of 9 degrees during the downstroke and - 9 degrees upstroke achieved the optimal propulsion performance. The findings in this paper have promising implications for both bio-inspired and robotic flapping wing applications.