Vortex-Induced Vibrations (VIVs) of a near-wall cylinder involve complex interactions between the structure and the boundary layers in the shear flow. Based on dimensional analyses, the wall-proximity effects on the triggering of transverse VIVs of a circular cylinder are physically investigated with flume observations. An accessorial low structural-damping VIV simulation device was developed and an upward-illumination Particle Image Velocimetry (PIV) system was employed in the physical modeling. Although the regular Kármán-like vortex-shedding could be suppressed for small gap-to-diameter ratios (e.g., e/D < 0.40) as reported in previous studies, the present power spectral analyses on the velocity fluctuations in the lee-wake indicate that the characteristic frequency can still be well identified, which could further trigger the vibrations of a low mass-damping cylinder. PIV measurements show that, as the cylinder approaches the bottom, the asymmetry of swirling-strengths in the lee-wake gets more remarkable, and the values of Strouhal number increase correspondingly. Four typical stages can be identified in the process of the vibration being triggered under the wall-free condition, including (a) the fully-stationary, (b) the intermittent-vibrating, (c) the sharp-jump, and (d) the upper-branch vibration stages. But, for e/D $\le$ 0.20, the intermittent-vibrating stage disappeared. During the sharp-jump stage, the peak jump-amplitude decreases dramatically as the cylinder approaches the bottom, which has negative correlation with the vibration frequency. The effects of the swirling-strength attenuation and the downward collisions on the vibration responses are discussed. The relationship between critical reduced velocity ${\mathit{V\; r}}_{\mathrm{cr}}$ and e/D is finally established by fitting the experimental results. It is indicated that the wall-proximity effects could be significant and should be taken into account while evaluating the critical velocity for triggering transverse vibrations of the near-wall cylinder in the engineering practices.

近壁圆柱体的涡激振动 (VIV) 涉及剪切流中结构与边界层之间的复杂相互作用。基于尺寸分析，通过水槽观察物理研究了壁面附近对圆柱体横向 VIV 触发的影响。开发了一个辅助的低结构阻尼 VIV 仿真设备，并在物理建模中采用了向上照明粒子图像测速 (PIV) 系统。尽管对于小间隙直径比（例如e / D< 0.40）如先前研究报告的那样，目前对背风尾流速度波动的功率谱分析表明特征频率仍然可以很好地识别，这可以进一步触发低质量阻尼圆柱体的振动。PIV 测量表明，随着圆柱体接近底部，背风尾涡旋强度的不对称性变得更加显着，并且 Strouhal 数的值相应增加。在无壁条件下触发振动的过程中可以确定四个典型阶段，包括（a）完全静止，（b）间歇振动，（c）急剧跳跃和（d）上分支振动阶段。但是，对于e / D $\le$0.20，间歇振动阶段消失。在急跃阶段，随着圆柱体接近底部，峰值跃变幅度急剧下降，与振动频率呈负相关。讨论了旋流强度衰减和向下碰撞对振动响应的影响。临界减速度之间的关系${\mathit{Vr}}_{\mathrm{铬}}$并通过拟合实验结果最终建立e / D。结果表明，近壁效应可能是显着的，在工程实践中评估触发近壁圆柱横向振动的临界速度时应予以考虑。