Vortex-induced vibrations (VIV) can cause fatigue damage of marine pipeline and mooring lines.The current VIV studies focus on a canonical problem that a circular cylinder freely to oscillates within a flow perpendicular to the body axis.In practical applications,cylinder structures are often inclined with respect to the direction of the oncoming flow.Aiming at this complex situation,the independence principle (IP) that the VIV of an inclined circular cylinder is assumed to be equivalent to the VIV of a vertical circular cylinder excited by the projection component of oncoming flow velocity in the direction perpendicnlar to the cylinder's axis is proposed.However,more attentions need to be paid to the validity of the independence principle.Here,laboratory tests were conducted for vortex-induced vibrations (VIV) of an inclined flexible cylinder with a ratio of length to diameter of 350 and an incline angle of 45° in order to check the validity of the independence principle (IP).Strain gages were adopted to measure the cylinder's dynamic response,a modal analysis approach was used to analyze the measured data.The test results of a vertical cylinder and the inclined cylinder were compared.It was shown that the higher order modes are easy to be excited for the inclined cylinder in the controlled mode transition region;the strain and displacement at measured points of both cylinders are obviously different and the higher order modes' features of VIV responses are observed.All the results indicated that the use of IP may not be correct to describe the VIV of the flexible cylinder inclined at 45 °.%涡激振动是诱发海洋管道与浮式平台系泊缆线等细长柔性圆柱结构疲劳损伤的重要因素,已有研究大多关注结构轴向与来流垂直的情况,实际的海洋工程中,圆柱结构轴向与来流并不完全垂直,存在一定倾斜角度.针对这种复杂的情况,倾斜圆柱涡激振动的不相关原则被提出,即假定倾斜圆柱涡激振动与来流速度在结构轴向的垂直方向投影分量引起的垂直圆柱情况等价.然而,较大倾角条件下不相关原则的正确与否仍未知.为了论证大倾角倾斜柔性圆柱涡激振动不相关原则,开展了长径比为350,倾角为45°的倾斜柔性圆柱涡激振动实验,采用应变传感器测量结构应变信息,运用模态分析法对实验数据进行处理.通过大倾角倾斜圆柱与垂直圆柱的实验结果对比,发现在控制模态转化区域,倾斜圆柱更容易被激发高阶模态,同时倾斜圆柱与垂直圆柱在测点处的应变和位移差异较大,并均出现高频特征.最终得出了较大倾角状态下倾斜柔性圆柱涡激振动不相关原则不成立的结论.
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