正弦振荡来流下柔性立管涡激振动发展过程

在风浪流的作用下，海洋浮式结构物将带动悬链线立管在水中作周期性往复运动，从而在立管运动方向上产生相对振荡来流，这种振荡来流将激励立管悬垂段发生“间歇性” 的涡激振动. 在海洋工程水池中对一个4m 长的立管微段进行模型试验研究，以探索相对振荡来流作用下立管涡激振动产生的机理及其发展的物理过程. 试验通过振荡装置带动模型作正弦运动来模拟不同最大约化速度U_Rmax、不同KC（Keulegan-Carpenternumber）的相对振荡来流，利用光纤应变片测量立管涡激振动响应. 结合模态分析方法处理试验数据得到位移响应时历，继而提出相对振荡来流下柔性立管涡激振动发展的3 个阶段:建立阶段、锁定阶段以及衰减阶段. 并进一步总结了最大约化速度U_Rmax，KC 对涡激振动发展过程的影响规律. 最终获得不同最大约化速度U_Rmax下，涡激振动各发展阶段随KC 所占时间分布比例图. 

VIV DEVELOPING PROCESS OF A FLEXIBLE CYLINDER UNDER OSCILLATORY FLOW

Offshore floating structures would bring the catenary risers moving in the water periodically under sea loads, then to generate relatively oscillatory flow between the riser and the water particles. Such oscillatory flow would easily trigger the "intermittent VIV" at sag-bend of the catenary riser. Experimental investigations on the behaviors of a 4m long straight flexible cylinder in the relatively oscillatory flow were carried out in this paper by forcing it oscillating with different combinations of both maximum reduced velocity U_Rmax and Keulegan-Carpenter number (KC) in still water. Fiber brag grating (FBG) strain sensors were used to measure the VIV responses of the model cylinder. Meanwhile, modal analysis was adopted to process the experimental data, and three VIV developing process in oscillatory flow including "building-up", "locking-in" and "dying-out" were firstly proposed. Furthermore, effects on VIV in oscillatory flow from both reduced velocity U_Rmax and KC were discussed and summarized. Finally, normalized time ratio contour plots of three VIV developing phases in different maximum reduced velocities were obtained.