细长柔性立管涡激振动的数值模拟研究

NUMERICAL INVESTIGATION ON THE VORTEX-INDUCED VIBRATION OF A FLEXIBLE SLENDER RISER

应用基于浸入边界法(IBM)的三维水动力并行计算程序Cg LES_IBM,并结合隐式结构动力计算程序X-code,研究了低雷诺数条件下细长柔性立管的涡激振动问题。研究发现:立管的振动体现出明显的驻波振动特征。立管在顺流向主要激发奇数阶模态,而横流向处于激发状态模态的奇偶性取决于涡激振动卓越频率和立管固有频率之间的关系。涡激振动的频谱呈现为单谱模式,所有的振动能量都集中在一个窄频段上,波节处能量较弱,波腹处能量较强。波节处,时均能量传递系数为负值,涡激振动处于抑制状态,相邻波节之间,时均能量传递系数为正值,并沿展向呈现出"马鞍形"分布,涡激振动处于激发状态。柔性立管涡激振动的泻涡模式呈现出三维特性,近尾流区为2S模式,但由于倾斜泻涡造成尾涡沿展向移动,尾涡模式随后变为2P0模式,随着较弱漩涡的耗散,在远尾流区,尾涡模式又回到2S模式。

The vortex-induced vibration of a flexible slender riser in cross-flow with a low Reynolds number was numerically investigated. The methodology adopted was a combination of Cg LES_IBM – an in-house 3D parallel CFD code based on the immersed boundary method（IBM） and X-code – an in-house implicit structural dynamic code. It was found that the vortex-induced vibration（VIV） responses show clear standing-wave characteristics. The in-line vibration is dominated by odd-number modes while the odd-even property of the cross-flow vibration is determined by which natural frequency is close to the vibration frequency. The power spectral density（PSD） of VIV shows a single-spectrum mode – all vibrating energy concentrates in a narrow frequency band. Lower PSD is observed at the node, while higher PSD anti-node. At the node, time-averaged energy transfer coefficient Cfv is negative which means that energy is transferred from vibration system to outer flow, and thus VIV is suppressed. Between two subsequent nodes, Cfv is positive and shows a saddle-shaped curve along the span-wise direction. Energy is transferred in the opposite direction and VIV is excited. Obvious three-dimensionality is detected in the downstream near-wake in which the 2S mode dominated. Due to the span-wise transporting of the vortices caused by oblique vortex-shedding, the near-wake pattern changes to a 2P0 mode. However, the near-wake pattern is dominated by the 2S mode again when the weaker vortex in the 2P0 mode dissipates.