Studies on the flow-induced vibration (FIV) of tandem unequal-diameter flexible cylinders in a broad gap ratio range are scarce. To reveal the effect of the gap ratio on and characterize the evolution features of the cylinder FIV with hydrodynamic interference, this paper experimentally investigates the FIV of two tandem flexible cylinders with unequal diameters at different gap ratios. Both cross-flow and in-line vibrations of the cylinders are permitted. Five gap ratios (i.e., T/d = 4, 6, 8, 10, and 12) are tested with the diameter ratio (d/D) set to be 0.5, where T is the center-to-center distance, d and D are the diameters of the upstream cylinder (UC) and downstream cylinder (DC), respectively. The effective cylinder length is 2.7 m, of which 1.2 m is submerged in water, and the aspect ratios of the UC and DC are 84.375 and 168.750, respectively. Based on the strains measured by the fiber bragg grating (FBG) sensors and the modal decomposition theory, detailed analyses of the displacement and frequency response are conducted. The FIV response of the UC resembles the vortex-induced vibration (VIV), whereas the DC undergoes wake-induced vibration (WIV). Increasing the gap ratio decreases the displacement amplitude of the DC but hardly affects the dominant frequency response. This study identifies three response regions (i.e., the VIV region, the transition region, and the WIV region) for the FIV response of the DC. The dual-cylinder interaction and dual-cylinder competition induce a coupled multi-physical vibration morphology of the cylinder. The “displacement-frequency co-variation” phenomenon of the DC and the “double-frequency lock-in” phenomenon of the UC are observed in the WIV region.

对宽间隙比范围内串联不等径柔性圆柱体的流致振动 (FIV) 的研究很少。为了揭示间隙比对具有流体动力干扰的圆柱FIV的影响并表征其演化特征，本文通过实验研究了两个不同直径的串联柔性圆柱在不同间隙比下的FIV。气缸的横流振动和直列振动都是允许的。测试五种间隙比（即T / d  = 4、6、8、10和12），直径比（d / D）设置为0.5，其中T为中心距，d和丁分别是上游圆柱体 (UC) 和下游圆柱体 (DC) 的直径。有效圆柱长度为2.7 m，其中1.2 m浸没在水中，UC和DC的纵横比分别为84.375和168.750。基于光纤布拉格光栅（FBG）传感器测量的应变和模态分解理论，对位移和频率响应进行了详细分析。UC 的 FIV 响应类似于涡激振动 (VIV)，而 DC 经历尾流激振 (WIV)。增加间隙比会降低 DC 的位移幅度，但几乎不会影响主频率响应。本研究确定了 DC 的 FIV 响应的三个响应区域（即 VIV 区域、过渡区域和 WIV 区域）。双缸相互作用和双缸竞争引起圆柱的耦合多物理振动形态。在 WIV 区域观察到 DC 的“位移-频率共变”现象和 UC 的“双频锁定”现象。