| Summary: | 博士 === 國防大學中正理工學院 === 國防科學研究所 === 94 === This dissertation aims to study the aerodynamic characteristics in flapping motions. Three kinds of wings have been investigated, which consist of the elliptic, NACA0012 and flexible flapping NACA0014 foils. All parameters about the various dynamic behaviors are calculated by Navier-Stokes governing equations. The computational domain is constructed with a combination of conformal hybrid mesh and dynamic mesh techniques. In addition, C program is applied to develop computer code available in representing the continuous deformation of the flapping airfoil.
Firstly, an elliptic foil with an aspect ratio of 1/8 is used for the test model that is regarded as the wing of dragonfly. The trajectory of flapping motion could be expressed as a formula that indicates the combination of translational and rotational motions. And then, a superior dynamic mesh technique can be applied in the computation. This study is mainly emphasized on understanding the relationship between lift and thrust forces evaluated by the pre-described flapping motions. Besides, the elliptic wing is evidenced that the aerodynamic lift force can be obviously enhanced at low aspect ratios. The simulated result shows that the best aspect ratio is 1/12 in all test runs.
Furthermore, The effects of inclined angle and amplitudes of strokes on the aerodynamic performance are most concerned for a hummingbird in hovering flight in this study. The simulated result of the eight of figure-of-eight flapping motion presents an exponential relationship between the flapping mass-specific power of hovering flapping and inclined angle in horizontal flapping motion. The present results reveal that the lift force will be enhanced about 15 % with every 0.1 increasing of the ratio vertical to horizontal amplitude, while the trend of mass-specific-power exhibits a linear rising type. Besides, a flapping motion with the ratio vertical to horizontal amplitude over 0.2 will cause drastic variation of lift and drag forces along with time. This variation will affect the flight stability during normal hovering.
Modeling the “flexible” flapping wing is the most contributed work in this dissertation. By vector geometry theory, a complicated computer code is developed to control the dynamic meshes, and by which the deformation of the flexible flapping airfoil could be practiced. The results indicate that the optimum propulsive efficiency could be achieved at the condition of higher flapping frequencies while Reynolds number (Re) is low. The highest propulsive efficiency is corresponded to Strouhal number (St) of 0.255, which coincides with the investigations by other researchers. The maximum value of thrust power coefficient is occurred at the flexure extent of 0.1. Drag-indicative wake structures would be formed when the flexure extent is larger than 0.5. And the propulsive efficiency is up to 30.73% at a flexure extent of 0.3.
Based on the results of single flexible flapping, the effect of the deformation for a biplane counter-flapping airfoil could be understood. In addition, a curvature radius method is applied to avoid non-rational extensions occurred during the re-allocation of dynamic meshes. The maximum propulsion for the biplane counter-flapping airfoil could be found at the flexure extent of 0.1, which is 2.52 times to that for the single flexible airfoil. The great contribution to propulsive capability is made by flexible biplane counter-flapping. At the flexure extent of 0.25, the propulsive efficiency is increased to 31.16%.
In this work, it is expected these results could be extend to 3D modeling. Then, some useful suggestions to the design of micro air vehicles (MAV) would be further provided.
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