Date of Award
Master of Science (MS)
The traditional flapping wing high lift mechanism research mainly focuses on the wing in unbounded flow. However, the real insect flight includes not only the unbounded flow field but also the near-surface flight. Therefore, research on near-surface flight can help reveal the high-lift mechanism of insect flight and should also be beneficial to the research on Micro-Air-Vehicles (MAV). In this thesis, the flow fields of an airfoil in hover and forward flight are simulated in the presence of ground by newly available function of “dynamic meshing” in ANSYS Fluent is employed. The characteristics of aerodynamics, pressure distribution, and vortex structure are analyzed. The vortex structure and aerodynamic characteristics in unbounded flow (H/C=∞) are first analyzed in detail, three-typical vortex structure topologies are obtained, which are due to “translational forces,” “rotational circulation,” and “wake capture”. By comparing the pressure coefficient and vortex structure around the airfoil at three typical moments during hover and forward flight in unbounded flow, the influence of incoming flow on vortex structure is analyzed and the reason for disappearance of high lift mechanism under certain condition is found.
As the height of airfoil decreases in ground effect, the first vortex pair near the airfoil begins to develop differently than that in unbounded flow (H/C = ∞) and the ground effect begins to appear slowly. The presence of the ground mainly restricts the descent of the vortex pair and influences the newly generated vortex at the leading and the trailing edge that have just separated from the airfoil. As the height of the airfoil further decreases, the trailing edge vortex in up stroke gradually moves to right side away from the airfoil, and this change in the vortex movement leads to disappearance of increased lift effect. In the forward flight, typical vortex structures due to “translational forces” and “rotational circulation” still exist. Due to incoming flow, the vortex generated at the leading and the trailing edge fall-off rapidly from the airfoil. Shedded vortices moving downstream with the incoming flow result in a complex vortex structure, which lead to the disappearance of “wake capture” vortex structure and lift enhancement in up stroke.
David A. Peters Swami Karunamoorthy