| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 94 === On the Natural Frequencies and Mode Shapes of Dragonfly Wings
Abstract
A base excitation modal testing technique is adopted to measure the natural frequencies and mode shapes of dragonfly wings severed from the torsos. The severed wings are glued onto the base of a shaker, which is capable of inducing translational motion in the lateral direction of the wing plane. Photonic probes are used to measure the displacement history of the shaker base and the painted spots of the wing simultaneously. The excitation frequency of the shaker is set to sweep from 100 to 500 Hz. A spectrum analyzer is responsible for calculating the frequency response functions, from which we can extract the natural frequencies and the associated mode shapes of the wing structure. Our experimental results show that the fundamental natural frequency of dragonfly wings is in the order of 170 Hz when it is clamped at the wing base. The average flapping frequency 27 Hz of dragonflies is about 16% of the fundamental frequency. Therefore, the inertial force of the wing is negligible compared to the elastic force when the dragonfly flaps its wings. In other words, the wing deformation is solely due to external aerodynamic force and can be considered as quasi-static. The corresponding mode shapes contain both bending and twisting deformations.
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