THE MULTI-MODE RESPONSE OF A CYLINDER UNDERGOING SIMULTANEOUS VORTEX-INDUCED AND WAKE-INDUCED VIBRATIONS

海上石油工业需要知道如何设计从海面的石油钻井平台到海床的张力管道（或“立管”），垂直距离可能超过1000米。其中一个问题是，在这个长度上，洋流会使立管像吉他弦一样振动。振动会导致金属疲劳，也会导致阵列中相邻的隔水管相互碰撞。疲劳失效和碰撞都可能造成潜在的灾难性后果。为了避免碰撞，隔水管之间的间距是一个相当重要的问题，因为它们的间距对任何深水海上安装都有巨大的成本影响。立管的振动是由两种或三种不同的机制产生的，这些机制产生不同结构模式的激励，而响应是由模式的组合组成的，很像那些构成吉他弦振动模式的机制。引起隔水管振动的最重要的机制是旋涡脱落和尾流奔腾。其中第一种是与交替旋转方向的漩涡周期性脱落有关。尾迹驰动是指一个物体在另一个物体的下游运动，是由尾迹中流动的不均匀性产生的。仅由旋涡脱落引起的振动的最大振幅不会超过一个直径，但在立管中，这可能发生在高模式下，由此产生的大弯曲应力会大大降低疲劳寿命。另一方面，尾流的驰动可以在更低的频率下引起许多直径的偏移，并倾向于激发立管的最低振荡模式。几乎所有对这两个过程的了解都来自于在简单条件下研究其中一个过程的实验。以前采用的一种方法是研究安装在弹性系统上的刚性圆柱体的运动，该系统固定其单一固有频率（代表立管的多个固有频率之一）。这样做的问题是，在实际操作中，涡激振动和尾流疾驰与隔水管的两个不同的固有频率共振。此外，这两种流体机制相互作用。涡流引起的振动对阻力有很大影响，因此对一个立管在另一个立管后面的不稳定性也有很大影响。隔水管的运动影响其相对入射速度，而相对入射速度又决定了涡激振动的频率和振幅。在这个项目中，我们计划建立一个实验，这将使我们第一次能够系统地研究由这两个过程同时激发的圆柱体的响应。为了做到这一点，下游圆柱体必须安装在一个复合弹性系统上，该系统在每个方向上都有两个固有频率：与入射电流一致，与入射电流横向。这个实验有几个冒险的特点，因此我们必须小心确保，在系统被限制在单一固有频率的条件下，我们可以重现早期的测量结果。在随后的试验中，我们将研究一系列情况，在这些情况下，圆柱体在一个频率上经受涡激振动，同时尾流在另一个频率上疾驶。这些结果将帮助我们了解它们之间的相互作用，并将为世界各地正在开发软件的几个团队提供独特的基准数据，以预测立管对这些流动诱导力的响应，并评估疲劳损伤和碰撞概率。

The offshore oil industry needs to know how to design tensioned pipes (or 'risers') that go from the oil rig at the sea surface down to the sea bed, a vertical distance that may be much more than 1000m. One of the problems is that over this length ocean currents can cause the risers to vibrate like guitar strings. Vibrations can lead to metal fatigue and can also cause adjacent risers in an array to clash into each other. Both fatigue failures and clashing can have potentially disastrous consequences. Knowing how far apart the risers should be in order to avoid clashing is an issue of considerable importance, since their spacing has huge cost implications for any offshore installation in deep water. Vibrations of risers are generated by two or three different mechanisms which produce excitation of different structural modes, and the response consists of a combination of modes, rather like those that make up the pattern of vibrations of a guitar string. The most important mechanisms that cause risers to vibrate are vortex shedding and wake galloping. The first of these is associated with the periodic shedding of vortices of alternating directions of rotation. Wake galloping refers to the motion of one body downstream of another, generated by the non-uniformity of the flow in the wake. The maximum amplitude of vibrations caused by vortex shedding alone is not much more than one diameter, but in a riser this can occur in high modes and the resulting large bending stresses can drastically reduce fatigue life. Wake galloping on the other hand can cause excursions of many diameters at much lower frequencies, and tends to excite the riser's lowest modes of oscillation. Almost all of what is known about these two processes comes from experiments in which one or other has been studied under simpler conditions. An approach that has been followed before is to study the motion of a stiff cylinder mounted on an elastic system which fixes its single natural frequency (representing one of the multiple natural frequencies of a riser). The problem with this is that in practice vortex-induced vibrations and wake galloping resonate with two distinct natural frequencies of a riser. Moreover, these two fluid mechanisms interact. Vortex-induced vibrations have a major effect on drag, and thus on the instability of one riser in the wake of another. The motion of a riser undergoing wake galloping affects its relative incident flow speed, which in turn determines the frequency and amplitude of vortex-induced vibrations.In this project we plan to build an experiment that will for the first time allow us systematically to study the response of a cylinder which is excited by these two processes simultaneously. To do this, the downstream cylinder has to be mounted on a compound elastic system that has two natural frequencies in each direction: in-line with, and transverse to the incident current. The experiment has several adventurous features and so we shall take care to ensure that, in conditions in which the system is restricted to a single natural frequency, we can reproduce earlier measurements. In subsequent tests we shall investigate a range of cases where the cylinder is undergoing vortex-induced vibration at one frequency at the same time as wake galloping at another. The results will help us to understand the interaction between them, and will provide unique benchmarking data for several groups around the world who are developing software to predict the response of risers to these flow-induced forces and assess fatigue damage and the probability of clashing.