Flying animals can inspire practical approaches to a more advanced way of flying. Dragonflies demonstrate a special flapping pattern in which their wings perform torsional movement while flapping, which is different from that of birds. This flapping pattern is referred to as nonsynchronous flapping in this article. We present a hypothesis that nonsynchronous flapping provides a driving force for enhancing the haemolymph circulation inside dragonfly wings. To support this hypothesis, a controlled experiment was designed and conducted with living dragonflies. By observing the liquid motion inside the vein within free flapping wings and restricted wings of living dragonflies, this hypothesis was supported. A mathematical model of the flapping wing was built and numerically studied to further support the function of the nonsynchronous flapping pattern in driving the circulation. With these studies, a theoretical explanation for the mechanism of enhancing the haemolymph circulation by nonsynchronous flapping was provided.
In order to obtain an isolator with low resonance amplitude as well as good isolation performance at high frequencies, this paper explores the usage of nonlinear stiffness elements to improve the transmissibility efficiency of a sufficient linear damped vibration isolator featured with the Zener model. More specifically, we intend to improve its original poor highfrequency isolation performance and meanwhile maintain or even reduce its already low resonance amplitude by adding a nonlinear secondary spring into the isolator. Its isolation performances are evaluated under two input scenarios namely force transmissibility under force input and displacement transmissibility under base excitations, respectively. Thereafter, both analytical and numerical study is performed to compare the high-frequency transmissibility as well as resonance condition of the nonlinear isolator with its corresponding linear one. Results show that the introduction of nonlinear secondary spring in the Zener model can achieve an ideal improvement, i.e., reducing the transmissibility at high frequencies and meanwhile suppressing the resonance amplitude. It is also shown that both force and displacement transmissibility of the non-
This paper develops an adjustable high-static-low-dynamic (AHSLD) vibration isolator with a widely variable stiffness. By adjusting deformations of its horizontal springs, the natural frequency of the isolator can be substantially changed starting from a quasi-zero value. In this paper, the nonlinear static and dynamic analyses of the AHSLD isolator are presented. Effects of horizontal adjustments on the variation range of the stiffness and nonlinear dynamic characteristics are investigated. Good performance of the stiffness variation is validated by free-vibration tests. The wide-range variable stiffness from 0.33 N/mm to 23.2 N/mm is achieved in tests, which changes the natural frequency of the isolator from an ultra-low value of 0.72 Hz to 5.99 Hz. Besides, its nonlinear dynamic characteristics are also experimentally identified by applying the Hilbert transform. Both analytical and experimental results demonstrate the weakly hardening nonlinearity in the tested AHSLD isolator, which will not degrade its performance in practical applications.
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