The application of a flapping wing mechanism offers a vast range of development possibilities for unmanned aerial vehicles (UAVs) and autonomous underwater vehicles (AUVs). The influence of wake transitions on flapping wing mechanism’s capabilities is not fully understood particularly at low Reynolds numbers. The numerical investigation of a symmetric airfoil performing sinusoidal heaving oscillations is performed to explore the wake transitions. The influence of heaving parameters on wake transitions when exposed to a constant velocity flow is investigated. The existence of reverse von Karman vortex street, deflected wake and chaotic wake is observed. The wake deflection is found to switch its direction before transforming into a chaotic wake. The coherent structures and its evolution with the flow are presented using proper orthogonal decomposition (POD). The underlying structures and their interactions for different wake situations are identified. Correlations for the nondimensional maximum velocity in the wake in terms of frequency and amplitude is proposed. The wake dynamics is found to depend significantly on the leading edge vortices. The time-varying velocity fluctuations in the flow field are presented and discussed in detail. The velocity fluctuation contours are used to identify the regions of momentum transfer. The transient nature of the flow field is studied using the phase plot. A transition route from the periodic to chaotic regime though a quasi-periodic regime is established using time series analysis. The wake transitions are observed to be more sensitive towards heaving frequency than the heaving amplitude.

中文翻译：

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定制起伏翼型的推进性能和尾流过渡

扑翼机构的应用为无人机（UAV）和自主水下航行器（AUV）提供了广泛的发展可能性。尾流过渡对扑翼机构能力的影响尚未完全了解，特别是在低雷诺数时。对执行正弦起伏振荡的对称翼型进行数值研究以探索尾流过渡。研究了当暴露于恒速流时，起伏参数对尾流过渡的影响。观察到反向冯卡门涡街、偏转尾流和混沌尾流的存在。发现尾流偏转在转变为混沌尾流之前会改变其方向。使用适当的正交分解（POD）呈现相干结构及其随流的演变。确定了不同尾流情况的基础结构及其相互作用。提出了尾流中无量纲最大速度在频率和幅度方面的相关性。发现尾流动力学显着依赖于前缘涡流。详细介绍并讨论了流场中随时间变化的速度波动。速度波动等值线用于识别动量传递区域。使用相图研究流场的瞬态特性。使用时间序列分析建立了从周期性到混沌状态的过渡路径，虽然是准周期性状态。