This experimental study investigates the fluid–structure–surface interactions of a flexibly mounted rigid plate in axial flow, focusing on flow-induced vibration (FIV) response and vortex dynamics of the system within a reduced velocity range of $U^*=0.29\unicode{x2013}8.73$ , corresponding to a Reynolds number range of $Re=518\unicode{x2013}15\,331$ . The plate, with one and two degrees of freedom (DoFs) for pitching and plunging oscillations, is examined at various submerged heights near the free surface. Results show that the plate exhibits divergence instability at low reduced velocities in both 1DoF and 2DoF systems. As the flow velocity surpasses a critical reduced velocity, periodic limit-cycle oscillations (LCOs) occur, increasing in amplitude until a second critical reduced velocity is reached. Beyond this point, LCOs are suppressed, and the plate experiences an increased static divergence angle with further flow velocity increase. The proximity to the free surface significantly influences the FIV response, with decreasing submerged heights leading to reduced LCO amplitudes and a shift of instabilities to higher reduced velocities. Vortex dynamics are analysed using time-resolved volumetric particle tracking velocimetry and hydrogen bubble flow visualisation. The analysis reveals disruptions in the symmetric flow field near the free surface, causing elongation and fragmentation of vortices in the wake of the plate, as well as vortex coupling. Proper orthogonal decomposition (POD) identifies dominant coherent structures, including leading-edge and trailing-edge vortices, captured in the first and second paired modes. On the other hand, higher POD modes capture the interaction of vortices in the wake and near the free surface.

中文翻译：

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靠近自由表面的灵活安装的俯仰和切入平板的流体-结构-表面相互作用

本实验研究研究了轴向流中柔性安装刚性板的流体-结构-表面相互作用，重点关注系统在降低的速度范围内的流致振动 (FIV) 响应和涡流动力学 $U^*=0.29\unicode{x2013}8.73$ ，对应于雷诺数范围 $Re=518\unicode{x2013}15\,331$ 。该板具有一个和两个自由度 (DoF)，用于俯仰和俯冲振荡，在自由表面附近的不同浸没高度处进行检查。结果表明，该板在 1DoF 和 2DoF 系统中在低速降低时表现出发散不稳定性。当流速超过临界折合速度时，会发生周期性极限循环振荡 (LCO)，其振幅不断增加，直到达到第二个临界折合速度。超过这一点，LCO 会受到抑制，并且随着流速的进一步增加，板的静态发散角也会增加。接近自由表面会显着影响 FIV 响应，降低浸没高度会导致 LCO 振幅减小，并且不稳定性会转向更高的降低速度。使用时间分辨体积粒子跟踪测速和氢气泡流可视化来分析涡流动力学。分析揭示了自由表面附近对称流场的破坏，导致板尾迹中涡流的伸长和破碎以及涡流耦合。适当的正交分解（POD）识别主要相干结构，包括在第一和第二配对模式中捕获的前缘和后缘涡流。另一方面，较高的 POD 模式捕获尾流中和自由表面附近的涡流相互作用。