近壁面柱体涡激振动的迟滞效应

柱体涡激振动是典型的流固耦合问题,其响应规律标识码在升速流动和远离壁面条件下获得的. 而自然环境流动通常不断经历升速和降速过程,近壁面柱体的涡激振动可呈现与远离标识码体不同的响应特征. 本研究结合大型波流水槽,设计了具有微结构阻尼的柱体涡激振动装置. 基于量纲分析,开展系列水槽标识码验,通过同步测量柱体涡激振动位移时程和绕流流场变化,研究了升降流速作用下柱体涡激振动触发和停振的临界速度(即上临标识码临界速度)变化规律,探究了近壁面柱体涡激振动迟滞效应. 采用自下向上激光扫射的 PIV 流场测量系统,对比分析了固定柱体标识码振动柱体的绕流特征. 实验观测表明,近壁面柱体涡激振动触发的临界速度呈现随壁面间距比减小而逐渐减小的变化趋势;但标识码速条件下的涡激振动停振所对应的下临界速度却明显小于升速时的涡激振动触发所对应的上临界速度. 采用上临界与下临界约标识码差值可定量表征涡激振动迟滞程度,研究发现该值随着柱体间距比减小呈线性增大趋势. 涡激振动迟滞现象通常伴随振幅阶跃标识码阶跃值则随着间距比减小而非线性减小. 

HYSTERESIS IN VORTEX-INDUCED VIBRATIONS OF A NEAR-WALL CYLINDER1)

The vortex-induced vibration (VIV) of a cylinder is a typical fluid-solid coupling problem. Previous investigations on VIV responses were mainly made under increasing-velocity flow and wall-free conditions. Nevertheless, the natural flow always features with alternately increasing or decreasing velocities, so that the VIV response of a near-wall cylinder holds different characteristics from that of a wall-free cylinder. In this study, a VIV device for a cylinder with low structural damping was designed and constructed in conjunction with a flume. Based on dimensional analyses, a series of flume model tests were carried out to investigate the critical velocities for the initiation and the cease of VIV (i.e., the upper critical and lower critical reduced velocities) of a near-wall cylinder under the action of increasing-velocity and decreasing-velocity flows, respectively. To examine wall-proximity effects on the VIV hysteresis, synchronous measurements were made for the time-variation of vibration displacement and the corresponding flow fields around the cylinder. Meanwhile, a specially designed PIV system with bottom-up laser scanning was employed to capture the flow field characteristics. Experimental observations indicate that the critical velocity for the initiation of VIV of a near-wall cylinder decreases with the decrease of gap-to-diameter ratio. The lower-critical reduced velocity for the cease of VIV under decreasing-velocity conditions is however much smaller than the upper-critical value for the initiation of VIV under increasing-velocity conditions. The deviation of the upper-critical reduced velocity from the lower-critical one is used for quantitative characterization of the hysteresis in VIVs, which increases approximately linearly with the decrease of gap-to-diameter ratio. Moreover, it was found that such VIV hysteresis is always accompanied with the jump of vibration amplitude, whose value decreases nonlinearly with the decrease of gap-to-diameter ratio.