海底管道的横向振动可能引发其疲劳破坏，严重威胁管道的安全运行。作为 一个典型的流固耦合问题，海底管道这类圆柱体结构的横向振动得到了工程界和 学术界的广泛关注。以往研究主要关注圆柱体在单向流作用下的振动响应，而自 然海洋环境中波浪与海流的耦合作用使得柱体尾迹模式及振动触发机理变得更 加复杂。本文聚焦于波流耦合作用下的柱体横向振动问题，采用一套低结构阻尼 系数柱体横向振动模拟装置开展物理模型试验，并辅以理论分析，系统研究了波流耦合作用下柱体的尾流特性、振动触发模式、临界触发速度及幅频响应特性， 并考虑了近壁面效应和迟滞效应，旨在为海洋油气资源开采中的近海管道工程设 计和安全运行保障提供科学依据。

       基于量纲分析，开展水槽物理模型试验，探究了亚临界雷诺数范围内近壁面 圆柱体的尾流特性。在单向流作用下，当间隙比e/D ≥ 1.50时，壁面对柱体旋涡 脱落的影响可忽略，其斯特劳哈尔数（St）的值与远离壁面的圆柱体基本一致。 当0.40 ≤ e/D < 1.50 时，下侧旋涡脱落出现轻微向上的偏移；而当e/D < 0.40时， 尾流脉动频谱呈现出多峰特征，这主要是由圆柱体下部剪切层与壁面边界层之间 的强烈相互作用引起的。引入了剪切参数（K）来表征近壁面效应对St的影响， 基于边界层理论推导得到了K与e/D的显式表达，并进行了试验验证；以柱体中 心高度处的水平速度定义St，建立了St随K或e/D变化的经验公式。当K < 0.04 时，St值基本保持恒定；而当K > 0.04时，St值随K值增大而增大。在单独波浪 作用下，当KC < 3.8时，柱体尾流脉动频谱呈现单峰特征，柱体振动频率与波浪 频率一致。随着间隙比逐渐减小，尾流脉动频谱呈现多峰特征。在波流耦合作用 下，远壁面圆柱体的尾流流场大致可以分为四个区域：波浪主导区域，亚谐波锁 定区域，谐波锁定区域，以及单向流主导区域。其中亚谐波锁定区域能够出现于 非常小的间隙比条件下，如e/D = 0.10；而谐波锁定区域在远壁面和近壁面条件 下表现出较大差异。

       在对固定管道绕流特性研究的基础上，进一步研究了远壁面（e/D = 3.0）条 件下柱体的横向振动特性。本文定义了一种简化的波流耦合来流条件，称为准单 向流。准单向流是一种类似于单向流的来流条件，但其诱导的柱体振动响应又可 以反映波流耦合作用下柱体振动的基本特征。在准单向流条件下，柱体振动的触 发过程可分为五个典型阶段。阶段①：波浪主导的强迫振动，类似于在单独波浪 作用下的柱体振动；阶段②：波流亚谐波锁定，振动出现亚谐波锁定现象；阶段 ③：波流过渡，柱体振动由波浪主导转为水流主导；阶段④：水流主导的预同 步，类似于在单向流作用下的振动，出现间歇振动；阶段⑤：水流主导的共振，与单向流作用下的共振阶段基本一致。在除准单向流以外的其他波流耦合作用下， 柱体振动过程也包括以上五个阶段，但阶段特征受到频率比、流速比和KC数的 影响。KC数对振动的影响主要局限于阶段①和阶段②，在水流主导的区域影响 有限。基于对波流耦合作用下柱体尾流特性和振动响应过程的分析，发现了三种 振动触发模式：模式I（共振模式），模式II（谐波锁定模式）和模式III（亚谐波 锁定模式）。其中模式I与共振现象相关，可诱发阶段①、②、③或④中出现振 幅阶跃；模式II与谐波锁定现象相关，在阶段③出现振幅间歇增大；模式III与 亚谐波锁定现象相关，在阶段②出现振幅线性增加。此外，基于对波流耦合作用 下柱体振动幅频响应特性的分析，建立了波流耦合作用下的无量纲临界触发速度 随KC数变化的经验公式。

       考虑到海底管道通常位于近床面，并且自然环境中的流动通常会经历升速与 降速过程，本文也探讨了圆柱体横向振动的近壁面效应和迟滞效应。在单向流条 件下，e/D = 1.0时的柱体振动触发过程并未受到近壁面效应的显著影响，振动幅 频响应与e/D = 3.0时基本一致；然而，当e/D减小到0.50时，振动触发过程呈 现显著的近壁面效应，柱体振动被提前触发。在单独波浪作用下，对比e/D = 3.0 时柱体振动响应过程，e/D = 1.0与0.50的振动锁定现象将在更小的折减速度处 出现，即柱体振动也被提前触发。在准单向流条件下，随着间隙比的减小，阶段 ②的范围逐渐扩大，阶段③、④的范围则逐渐缩小，柱体振动同样被提前触发。 与远壁面条件下的柱体振动特性类似，在单向流、单独波浪以及准单向流条件下， 近壁面柱体振动触发机制也遵循模式I。而在其他波流耦合作用下（准单向流除 外），柱体振动由模式I和模式III 触发，由模式II触发的振动响应可能不会出 现。针对升降流速作用下柱体振动迟滞效应的研究表明，在单向流与准单向流条 件下，远壁面柱体振动基本不出现迟滞现象，即加速与降速阶段的幅值及频率响 应基本一致；但对于其他波流耦合作用下的近壁面柱体振动响应，迟滞现象则较 为明显。

    Transverse vibration of submarine pipelines can induce fatigue failures, posing significant risks to their safe operation. The vibration of circular cylinders such as submarine pipelines is a typical fluid-structure interaction problem, which has attracted considerable attention from both the engineers and researchers. Previous investigations predominantly focused on the vibration of circular cylinders under the unidirectional flow. Nevertheless, the complex interaction of waves and current in natural offshore environments can alter the wake patterns as well as the triggering mechanisms of the vibration. This paper is aimed at the issue of transverse vibration of a cylinder under combined waves and current. A series of physical modeling was conducted based on a device with low structural-damping parameters, complemented by theoretical analysis. The wake characteristics, the vibration triggering modes, the critical reduced velocity, and the amplitude-frequency response characteristics of the cylinder under combined waves and current were systematically investigated. In addition, the effects of wall proximity and hysteresis were also considered. The purpose of this study is to provide a scientific basis for the design and safe operation of offshore pipeline in the exploitation of marine oil and gas resources.

    Based on dimensional analysis and physical modeling in a wave flume, the wake characteristics of the near-wall circular cylinder within the subcritical Reynolds number range are explored. Under unidirectional flow, the influence of the wall on vortex shedding from the cylinder can be neglected when the gap ratio e/D ≥ 1.50, with the Strouhal number (St) values consistent with those of a wall-free cylinder. For 0.40 ≤ e/D < 1.50, vortex shedding on the lower side shows a slight upward shift. However, for e/D < 0.40, the wake fluctuation spectrum exhibits multi-peak characteristics, primarily due to the intense interaction between the lower shear layer of the cylinder and the wall boundary layer. The shear parameter (K) is introduced to characterize the effect of wall-proximity on St, with an explicit expression for K as a function of e/D derived based on boundary layer theory and experimentally validated. St is defined based on the horizontal velocity at the center height of the cylinder, and an empirical formula for St as a function of K or e/D is then established. When K < 0.04, the values of St remain essentially constant; however, as K > 0.04, the values of St increase with increasing K. Under wave-only flow conditions, the wake fluctuation displays a single peak spectrum when KC < 3.8; as the gap ratio decreases, the wake transitions to a multi-peak spectrum. Under combined waves and current, the wake flow field of a wall free cylinder can be categorically divided into four distinct regions: wave-dominated, subharmonic lock-on, harmonic lock-on, and current-dominated regions. The subharmonic lock-on region may appear under extreme small gap ratio conditions, such as e/D = 0.10; while significant differences in the harmonic lock-on region are observed between wall-free and near-wall conditions.

    Based on the study of flow characteristics around a fixed cylinder, this paper further investigates the transverse vibration characteristics of a wall-free cylinder (i.e., e/D = 3.0). This paper defines a simplified combined waves and current inflow condition called quasi-unidirectional flow. This is a type of inflow condition similar to unidirectional flow, but the induced vibration response can also reflect the fundamental characteristics of vibration under combined waves and current. In the quasi unidirectional flow condition, the triggering process of cylinder vibration can be divided into five typical stages. Stage ① involves wave-dominated forced vibration, similar to cylinder vibration under wave-only flow; Stage ② is characterized by wave current subharmonic lock-on, where vibration experiences subharmonic lock-on phenomena; Stage ③ represents a wave-current transition, where cylinder vibration shifts from being wave-dominated to current-dominated; Stage ④ is identified as current-dominated pre-synchronization, similar to vibration under unidirectional flow, with the occurrence of intermittent vibration; Stage ⑤ is current-dominated synchronization, essentially consistent with the resonance stage under unidirectional flow. Apart from quasi-unidirectional flow, the vibration process of the cylinder under other combined waves and current conditions also encompasses these five stages, but the characteristics of each stage are influenced by the frequency ratio, flow velocity ratio, and the KC number. The influence of the KC number on vibration is mainly confined to stages ① and ②, with limited influence in the current-dominated regions. Based on the analysis of wake characteristics and vibration response processes under combined waves and current, three vibration triggering modes are identified: Mode I (resonance mode), related to resonance phenomena, can induce amplitude jumps in stages ①, ②, ③, or ④; Mode II (harmonic locking mode), related to harmonic lock on phenomena, manifests as intermittent amplitude increases in Stage ③; and Mode III (subharmonic locking mode), associated with subharmonic lock-on phenomena, presents as linear amplitude increases in stage ②. Furthermore, based on the analysis of amplitude-frequency response characteristics of transverse vibration, an empirical formula for the critical reduced velocity under combined waves and current as a function of the KC number is established.

    Considering that the submarine pipelines are typically located near the seabed and that the flow in natural offshore environments often undergoes acceleration and deceleration processes, this paper also explores the effects of wall-proximity and hysteresis on vibration. Under unidirectional flow, the vibration triggering process of the cylinder at e/D = 1.0 does not exhibit significant wall-proximity effects, with its amplitude-frequency response essentially matching that at e/D = 3.0; however, when e/D = 0.50, significant wall-proximity effect emerge, causing the vibration to be triggered in advance. Under wave-only flow, compared to the vibration response process of the cylinder at e/D = 3.0, the lock-on phenomenon at e/D = 1.0 and 0.50 occurs at lower reduced velocities, meaning that the vibration is also triggered early. In the quasi-unidirectional flow condition, as the gap ratio decreases, the range of Stage ② (subharmonic lock-on) gradually increases, while the ranges of Stages ③ (wave current transition) and ④ (current-dominated pre-synchronization) are gradually reduced, resulting in an earlier trigger of cylinder vibration. Similar to the characteristics of vibration response of the wall-free cylinder, the triggering mechanism of near-wall cylinder vibration under unidirectional flow, wave-only flow, and quasi unidirectional flow conditions also follows Mode I. In case of other combined waves and current conditions (except for quasi-unidirectional flow), cylinder vibration is triggered by Modes I and III, and the vibration responses triggered by Mode II may not occur. Research on the hysteresis effect of transverse vibration of a circular cylinder in an increasing-decreasing velocity cycle shows that under unidirectional flow and quasi- perturbed flow conditions, no hysteresis effect of a wall-free cylinder can be identified, i.e., the amplitude and frequency responses during the acceleration and deceleration stages are almost consistent; however, for near-wall cylinder vibration under other combined waves and current conditions, hysteresis is more pronounced.