174 pages, 84 illustrations
In the past decade, there has been an increased effort to design small aircraft for special, limited duration missions, which may include, e.g., search and rescue or surveillance tasks. The present research aims at the development of small, autonomous aircraft called miniature or micro air vehicles (MAVs) that are of the size of birds or even insects. These MAVs must meet numerous requirements such as a high level of maneuverability and stability concerning their flight behavior.
The development of MAVs is challenging due to the aerodynamic problems that are caused by the usually low freestream velocity and the small size of the vehicle. The flow in this low Reynolds number regime is characterized by its high susceptibility to separation on the lifting surfaces, leading to a low aerodynamic efficiency of the flight vehicle. Still, although the flow separation is accompanied by drag increase, lift decrease and pressure fluctuations, some animals successfully operate under these conditions.
The barn owl (Tyto alba) is one example of how nature has adapted a bird species to the requirements of its habitat, especially to flight at low Reynolds numbers. Due to adaptations on its wings and plumage, the barn owl is able to perform highly-maneuverable gliding flight at very low flight velocities and therefore may serve as a role model for the future development of MAVs. Additionally, the noise emitted by the owl is low compared to other bird species. Hunting at dawn or night when sight is limited, the owl has to rely on acoustic information to localize and hunt down its prey. Therefore, the owl has to be nearly inaudible. All bird wings are formed by feathers, which are attached to the skeletal elements and thereby possess a high level of flexibility. Additionally, the geometry of the owl wing is characterized by a large nose radius and a location of maximum camber close to the leading edge. Furthermore, the wing size is large compared to birds of similar weight and its planform is shaped almost elliptical. Moreover, the surface of the upper side of the wing is equipped with a velvet-like structure whereas the distal part of the leading edge possesses a comb-like structure and the trailing edge of each remex as well as of the entire wing is fringed. These three adaptations are exposed to the flow during flight and hence interact with the surrounding flow field.
To further understand the mechanisms that enable the barn owl to perform its characteristic flight, three-dimensional wing models, which were constructed based on the geometry of a natural wing, were separately equipped with several owl specific features. First, the serrated leading edge is imitated and the corresponding measurements are compared to the results found on a clean wing model, which only possesses the owl-like shape with no special features to distinguish the influence of the leading-edge structure. Second, two models are equipped with artificial textiles which are selected based on their ability to mimic the natural structure. Again, the results are compared to the clean reference model. Third, a first attempt to understand the highly complex flight system of the natural wing is made by the measurement of two natural owl wings which are prepared in a position which resembles gliding flight.
The flow fields of the various configurations are analyzed using different forms of particle image velocimetry (PIV), i.e., two- and three component as well as high spatial resolution and high-speed PIV are performed depending on the investigated flow case. Since the PIV measurements are limited to few spanwise planes, time-resolved force measurements are additionally performed to investigate the aerodynamic forces acting on the full wing.
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