In the present study, flow field around rigid flat plate wings executing main flapping motion has been studied using phase-locked two-dimensional particle image velocimetry measurements. Experiments have been conducted in water as a fluid medium for an asymmetric upper–lower stroke single degree of freedom main flapping motion. Two different aspect ratio (1.5 and 1.0) rectangular wings at 1.5 and 2.0 Hz flapping frequency in hovering flight mode (advance ratio, J = 0), zero wing pitch angle, and chord-based Reynolds number of the order of 104 have been studied. Velocity field and vorticity field with λ2 criterion information have been obtained for the complete stroke in great detail to reveal the minute aspects of flow dynamics. The flow features during the downstroke and upstroke have been observed to be consistent for all four cases investigated. The predominant characteristic of the flow during downstroke and upstroke has been referred to as vortex filamentation and fragmentation phenomena. Quantities such as circulation, rate of strain, rate of rotation, and enstrophy have been studied to identity the effect of minor change in aspect ratio and flapping frequency. It is found that for higher aspect ratio wing hyperbolic behavior is predominant except for end of downstroke and beginning of upstroke where elliptic behavior is observed. For lower aspect ratio, wing elliptic behavior is predominant except for end of upstroke and beginning of downstroke where hyperbolic behavior is seen. The hyperbolic behavior became stronger at higher frequency. From enstrophy distribution it is evident that higher frequencies play a more dominant role than aspect ratio in determining the budget.

中文翻译：

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扑动运动中的涡旋丝状和破碎现象以及纵横比和频率对全局应变、旋转和熵的影响

在本研究中，使用锁相二维粒子图像测速测量研究了执行主要扑翼运动的刚性平板机翼周围的流场。实验已经在作为流体介质的水中进行，用于不对称的上下行程单自由度主扑动运动。两种不同展弦比（1.5 和 1.0）的矩形机翼在 1.5 和 2.0 Hz 扑动频率下悬停飞行模式（前进比，J = 0）、零机翼俯仰角和基于弦的雷诺数为 104 量级已被研究. 已经详细获得了完整行程的速度场和涡度场以及 λ2 准则信息，以揭示流动动力学的细微方面。已经观察到在向下冲程和向上冲程期间的流动特征对于所调查的所有四种情况是一致的。向下冲程和向上冲程期间流动的主要特征被称为涡旋丝状和碎裂现象。已经研究了诸如循环、应变率、旋转率和内耗之类的数量，以识别纵横比和扑翼频率的微小变化的影响。发现对于更高展弦比的机翼双曲线行为占主导地位，除了观察到椭圆行为的下冲程结束和上冲程开始。对于较低的展弦比，机翼椭圆行为占主导地位，除了上冲程结束和下冲程开始时可以看到双曲线行为。双曲线行为在较高频率下变得更强。从熵分布可以明显看出，在确定预算时，较高的频率比纵横比起着更重要的作用。