多频涡激振动状态下柔性立管的时变水动力特性识别

本文提出了遗忘因子最小二乘法识别柔性立管发生多频涡激振动下的时变涡激力系数。该算法在最小二乘法基础上，引入遗忘因子，其给予更接近当前时刻的数据更大的权重。这一修正提高了该算法对时变参数的敏感度，使其能够识别系统的时变参数。本文首先使用质量-弹簧-阻尼器模型验证了遗忘因子最小二乘法可准确识别系统的时变参数。随后使用该方法识别了柔性立管发生多频涡激振动时横流向的时变涡激力系数。结果显示柔性立管发生多频涡激振动时，其涡激力系数会周期性变化，其时间平均值亦不同于基频下的涡激力系数，这是基频与高频耦合作用的结果。根据识别的时变涡激力系数进行反演重构得到的涡激力与真实涡激力相吻合，验证了该方法识别多频耦合下的时变涡激力系数可以准确重构涡激力载荷。
Abstract：Here, the forgetting factor least squares (FF-LS) method was proposed for identification of time-varying vortex-induced force coefficients of a flexible riser under multi-frequency vortex-induced vibrations (VIVs).This method introduced a forgetting factor to give a greater weight to the data closer to the present time instant.This modification improved the sensitivity of the algorithm to time varying parameters to make it be able to recognize a system’s time-varying parameters.Here, a mass-spring-damping model was used to verify the FF-LS algorithm’s ability to identify a system’s time-varying parameters accurately.Then, the FF-LS algorithm was used to identify time-varying vortex-induced force coefficients in cross-flow (CF) direction of a flexible riser under multi-frequency VIVs.The results showed that under multi-frequency VIVs, vortex-induced force coefficients of the flexible riser change periodically, their time mean values are different from the riser’s vortex-induced force coefficients under the fundamental frequency VIVs, this is due to coupling effect between fundamental frequency VIV and higher frequencies VIV; the vortex-induced force the reconstructed using identified time-varying vortex-induced force coefficients agrees well with the actual vortex-induced force to verify the time-varying vortex-induced force coefficients under multi-frequency coupling identified by the FF-LS algorithm being able to reconstruct vortex-induced force correctly.