And it is not easy to make reliable predictions of transient aerodynamic forces using unsteady aerodynamics theory. Based on the current measurement techniques, it is still challenging to obtain the details of the pressure distribution on the wing surface during the flight of hummingbirds. However, the experimental results were not complete though we have gained much understanding of the aerodynamics of hovering hummingbirds thanks to the efforts of previous researchers. So, the asymmetries in force generation were attributed to motion asymmetries. In addition, they found a longer wingspan and positive camber during the downstroke. ![]() proposed that the flapping speed and the angle of attack during the downstroke were higher than those during the upstroke. ![]() Some researchers tried to explain the mechanism of aerodynamic force generation using fluid dynamics theory by analyzing wing kinematics. They concluded that there existed a high degree of asymmetry in lift production based on the results of circulation. analyzed the strength of the shedding vortex of a hummingbird of the same species. In another study, they found two vortex rings in the region swept by a pair of hummingbird wings. measured the velocity field around a rufous hummingbird wing by conducting particle image velocimetry (PIV) experiments and discovered that a stable and conical leading edge vortex had a dominant role in the lift enhancement, especially during the downstroke. Previous studies primarily employed experimental measurements to examine the evolution of vortices in the flow field of hummingbird flight, which explained the unstable effect. Over the last decade, there have been many experimental studies on the underlying flight mechanism of hummingbirds. Compared to tiny insects, giant hummingbirds are one of the few extensively studied vertebrate species. Scientists and biologists have focused on the hovering flight of insects and hummingbirds for a long time in the past. Because hummingbird wings are anatomically and physiologically different from insect wings, hummingbirds generally do not fly in a similar way as insects do. And the dynamic wing morphing of the hummingbird wings is more remarkable than the passive deformations observed on smaller insect wings. Unlike insects, hummingbirds can reverse the angle of attack through active wing rotation at the wrist. Insects usually achieve the inversion of wings through muscle activation at the wing root and passive deformation of the wings. These behavioral characteristics are more similar to the flight characteristics of insects. The wings of hovering hummingbirds are usually rotated around their long axis during supination (transition from downstroke to upstroke) and kept outstretched during the upstroke. In particular, they can perform sustained hovering. Hummingbirds, the smallest bird species with excellent flight capability, can perform both regular cruise flights and aerobatic flights. The results of this study help to better understand the aerodynamics of the hovering hummingbird. It is worth noting that hummingbirds can maintain a similar wingtip speed by flapping their wings, but different strategies are adapted to hover efficiently due to the differences in size and body weight. Aerodynamic force coefficients and efficiency matched well with those of another hummingbird wing (the ruby-throated hummingbird). We found that the asymmetry in the time-averaged vertical force between the two half strokes was 3.5, and the value was higher than that reported earlier. A significant enhancement in aerodynamic forces was found during the downstroke, along with a large number of power consumptions. The difference is that the leading edge vortex induced a vortex ring near the root and a smaller and weaker vortex ring near the wingtip during the upstroke. In the results of the downward stroke, the different vortices separated from the surface and formed a vortex ring. This study showed that the leading edge vortex (LEV) attached to the wing was stable during the downstroke but extremely unstable and shed continuously during the upstroke. In numerical simulations, boundary-based smoothing and overset methodologies were used in combination to update the interior nodes of cells in the computational domain, allowing those nodes to accommodate the motion of the flexible wall. Navier-Stokes equations were solved on a dynamically deforming grid to study wing aerodynamics. To deepen our understanding of the aerodynamics by which hummingbirds use flexible wings to hover efficiently during flapping flight, a three-dimensional wing model with dynamic morphing was developed according to the morphological and kinematic data of a hovering hummingbird’s wing.
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