Abstract
Experiments with elastic flaps applied on a common airfoil profile were performed to investigate positive effects on lift and drag coefficients. An NACA0020 profile was mounted on a force balance and placed in a wind tunnel. Elastic flaps were attached in rows at different positions on the upper profile surface. The Reynolds number of the flow based on the chord length of the profile is about 2 × 10 5 . The angle of attack is varied to identify the pre- and post-stall effects of the flaps. Polar diagrams are presented for different flap configurations to compare the effects of the flaps. The results showed that flaps generally increase the drag coefficient due to the additional skin friction and pressure drag. Furthermore, a significant increase of lift in the stall region was observed. The highest efficiency was obtained for the configuration with flaps at the leading and trailing edges of the profile. In this case, the critical angle was delayed and lift was increased in pre- and post-stall regions. This flap configuration was used in a gust simulation in the wind tunnel to model unsteady incoming flow at a critical angle of attack. This investigation showed that the flow separation at the critical angle was prevented. Additionally, smoke–wire experiments were performed for the stall region in order to visualize the flow around the airfoil. The averaged flow field results showed that the leading-edge flaps lean the flow more towards the airfoil surface and reduce the size of the separated region. This reduction improves the airfoil performance in the deep stall region.
Highlights
Passive feathers on the wings of birds provide one possibility to control flow separation in critical flight conditions, such as strong gusts or landing maneuvers [1]
Because this study focuses on aluminum foil, there is a demand to investigate other elastic materials, since aluminum foil has the disadvantage of plastic deformation, which can be caused by strong gusts
The aim of our study is to present the trends of the lift and drag coefficients, which are still guaranteed by the uncorrected data
Summary
Passive feathers on the wings of birds provide one possibility to control flow separation in critical flight conditions, such as strong gusts or landing maneuvers [1]. In order to slow down, the bird increases the angle of attack of its wings This could potentially lead to flow detachment starting from the trailing edge of the wing, which is known as stall. In this situation, self-adaptive feathers rise and prevent further detachment towards the leading edge of the wing. Self-adaptive feathers rise and prevent further detachment towards the leading edge of the wing This interaction between feathers and flow allows for better control of lift at high angles of attack. Examples for usage include small wind turbines, where strong gusts could lead to sudden drops in lift and induce strong mechanical loads
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