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

海上石油行业需要知道如何设计从海面的石油钻井平台到海床的张拉管(或“立管”)，垂直距离可能远远超过1000米。其中一个问题是，在这个长度上，洋流可能会导致立管像吉他弦一样振动。振动可能导致金属疲劳，也可能导致阵列中相邻的立管相互碰撞。疲劳故障和碰撞都可能带来灾难性的后果。知道隔水管的间距以避免碰撞是一个相当重要的问题，因为隔水管的间距对任何深水海上安装都有巨大的成本影响。隔水管的振动是由两到三种不同的机构产生的，这些机构产生不同的结构模式的激励，而响应由多种模式组成，很像构成吉他弦振动模式的模式。导致立管振动的最重要的机制是旋涡脱落和尾迹驰动。第一个与交替旋转方向的旋涡的周期性脱落有关。尾迹驰振是指一个物体在另一个物体下游的运动，由尾迹中流动的不均匀产生。仅由涡流脱落引起的振动的最大幅度不会超过一个直径，但在立管中，这种情况可能发生在高阶模式下，由此产生的大弯曲应力可能会显著降低疲劳寿命。另一方面，尾迹驰骋会在更低的频率下引起许多直径的漂移，并倾向于激发立管的最低振荡模式。几乎所有关于这两个过程的已知信息都来自于在更简单的条件下研究过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，该弹性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，在实践中，涡流诱导的振动和尾流驰振与立管的两个不同的自然频率共振。此外，这两种流体机制相互作用。涡激振动对阻力有很大影响，从而对尾流中立管的不稳定性有很大影响。尾迹驰振立管的运动会影响其相对入射流速，从而决定涡激振动的频率和幅度。在这个项目中，我们计划建立一个实验，首次使我们能够系统地研究同时受到这两个过程激励的圆柱体的响应。为此，下游气缸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：与入射电流平行和横向。这个实验有几个冒险的特点，所以我们应该注意确保，在系统被限制在单一自然频率的条件下，我们可以重现早期的测量结果。在随后的试验中，我们将调查一系列的情况，其中圆柱体在一个频率上经历涡激振动，同时在另一个频率上发生尾迹驰骋。研究结果将帮助我们了解它们之间的相互作用，并将为世界各地的几个小组提供独特的基准数据，这些小组正在开发软件，以预测立管对这些流动诱导力的反应，并评估疲劳损伤和碰撞的可能性。

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.