Computational modeling of unsteady aerodynamics in hummingbird flight
The flapping wings of flyers in nature such as birds, insects, and bats have recently attracted engineers' interest because of their applications in biomimetic and highly maneuverable micro air vehicles (MAVs). As unique animal flyers, hummingbirds are capable of sustained hovering, fast forward flight, and various rapid maneuvers. Therefore, they are great platforms for the study of unsteady aerodynamics that is generally involved in the biological wings. In this work, we collaborated with biologists to reconstruct the kinematics of real hummingbird wings and utilized a three-dimensional (3D) Computational Fluid Dynamics (CFD) approach to study the fluid dynamics and force production in different hummingbird flight modes, including both hovering and fast forward flight. First, a 3D CFD model was created to study the lift characteristics of a hovering rufous hummingbird. The results show that even though both downstroke and upstroke produce lift, significant force asymmetry exists within a stroke cycle, with downstroke providing much greater weight support than upstroke. Second, a quasi-steady model was developed to assess the performance of this reduced-order approach in representing the force production of hovering flight. The comparison with the full CFD model shows that the calibrated quasi-steady model is able to predict the overall trend of force production reasonably well but fails to capture detailed oscillations. Third, a biomechanical model was developed to study the rotational dynamics of the hummingbird wing in order to identify its pitching mechanism (i.e., active v.s. passive actuation). The results show that, similar with many insects, pitching reversal of the hummingbird wings is largely caused by the wing inertia; however, actuation power input at the root is also needed at some stages to assist with the pitch rotation. Finally, fast forward flight of a colliope hummingbird flying in a wind tunnel at 8.3 m/s was simulated. Similar with with other birds, the hummingbird generates all of lift and also significant thrust during downstroke. However, the hummingbird produces thrust during both downstroke and upstroke by properly setting the instantaneous angle of attack. These studies are useful for the development of hummingbird-like MAVs.