Aspect ratio-dependent hysteresis response of a heavy inverted flag
Aspect ratio-dependent hysteresis response of a heavy inverted flag
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DOI：
10.1017/jfm.2022.339
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发表时间：
2022-05
影响因子：
3.7
通讯作者：
Oluwafemi Ojo;Yu-cheng Wang;A. Erturk;K. Shoele
中科院分区：
文献类型：
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作者：
Oluwafemi Ojo;Yu-cheng Wang;A. Erturk;K. Shoele
摘要 研究了不同纵横比（$AR$）的重型倒置旗帜的双稳态颤动响应，以确定涡流结构如何影响旗帜的间歇振动响应。均匀流中的重倒置标志可能会表现出多种响应模式；其中包括在扩展速度范围内发生的三种主要模式：静止、大规模周期性振荡和单侧偏转模式。对于所有 $AR$，在这些模式之间的过渡处观察到显着的迟滞双稳态，这与具有固定前缘和自由后缘的传统旗形振动显着不同，在传统旗形振动中，在 $AR$ 下限 ($AR<1$) 下没有观察到迟滞。这种差异与旗帜周围漩涡的不同作用有关。在风洞中对弹簧钢制成的旗帜进行了实验，其中流速逐渐增加然后降低，以获得重型倒置旗帜的不同振荡模式。实验结果用于验证同一问题的数值模型。研究发现，流速的增加和减少存在不同的临界速度，并且所有 $AR$ 都存在持续的滞后现象，由起始阈值和前缘和侧缘涡流的增长控制。通过制定模态力分配方法来量化双稳态振荡区域中涡流的影响。结果表明，$AR$ 可以显着改变与柔性板的静态和动态涡流相互作用，从而改变旗标的滞后行为和双稳态响应。
Abstract The bistable fluttering response of heavy inverted flags with different aspect ratios ($AR$) is investigated to determine how the vortical structures affect the intermittent vibration response of the flag. A heavy inverted flag in a uniform flow may exhibit several response modes; amongst them are three major modes that occur over an extended velocity range: stationary, large-scale periodic oscillation and one-sided deflected modes. Significant hysteretic bistability is observed at the transition between these modes for all $AR$, which is notably different from the conventional flag vibration with a fixed leading edge and free trailing edge where no hysteresis is observed at the lower $AR$ limit ($AR<1$). The difference is associated with the distinct roles of vortices around the flag. Experiments with flags made of spring steel are conducted in a wind tunnel, where the flow speed is steadily increased and later decreased to obtain different oscillatory modes of the heavy inverted flags. The experimental results are used to validate the numerical model of the same problem. It is found that different critical velocities exist for increasing and decreasing flow velocities, and there is a sustained hysteresis for all $AR$ controlled by the initiation threshold and growth of the leading-edge and side-edge vortices. The effect of the vortices in the bistable oscillation regime is quantified by formulating a modal force partitioning approach. It is shown that $AR$ can significantly alter the static and dynamic vortex interaction with the flexible plate, thereby changing the flag's hysteresis behaviour and bistable response.