Direct force and time-resolved two-dimensional particle image velocimetry measurements were performed on a jellyfish-like ornithopter model, which consists of two anti-phase flapping wings in a side-by-side arrangement. The focus is to study the effect of the time asymmetric pitching motion on the propulsive performance of this kind of ornithopter in a hovering state. It was shown that the fast downstroke and slow upstroke pattern is superior to symmetric back and forth pitching. Namely, more thrust and less fluctuations in the side force can be achieved. In order to provide explanations for this observation, various analyzing techniques, including vortex identification and tracking, spectral analysis, velocity triple decomposition, and reduced-order representation, were taken for a systematical characterization of the flow field in the wake. The spatiotemporal evolution of leading-edge vortices shedding from the wingtip during the downstroke and upstroke stages, as well as their mutual interaction, was found to be one of the key factors to account for the role of time asymmetric pitching on the alternation of thrust generation. Moreover, the delay of the transition of the wake to a turbulent state was observed in the scenario of fast downstroke. This is expected to be beneficial for the improvement of the hovering stability of the ornithopter.

中文翻译：

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具有不对称俯仰运动的水母状飞鸟的推进性能和流场特性

在类似水母的飞行器模型上执行直接力和时间分辨的二维粒子图像测速仪测量，该模型由两个并排的反相位拍打翼组成。重点是研究时间不对称俯仰运动对这种处于悬停状态的直升机的推进性能的影响。结果表明，快下冲程和慢上冲程模式优于对称的来回俯仰。即，可以实现更大的推力和较小的侧向力波动。为了对此观测提供解释，采用了各种分析技术，包括涡流识别和跟踪，频谱分析，速度三重分解和降阶表示，以系统地表征尾流中的流场。发现在下冲程和上冲程阶段从翼尖脱落的前沿涡流的时空演变以及它们之间的相互作用是解释时间不对称俯仰在推力产生交替中的作用的关键因素之一。 。此外，在快速下冲程的情况下，观察到了尾流向湍流状态过渡的延迟。预期这对于改进飞行器的悬停稳定性是有益的。在快速下冲程的情况下，观察到了尾流向湍流状态转变的延迟。预期这对于改进飞行器的悬停稳定性是有益的。在快速下冲程的情况下，观察到了尾流向湍流状态转变的延迟。预期这对于改进飞行器的悬停稳定性是有益的。