冰激振动中的锁频共振分析

STUDY ON FREQUENCY LOCK-IN RESONANCE IN ICE-INDUCED VIBRATION

冰激振动(ice-induced vibration, ⅡV)中的锁频共振严重威胁结构安全,恶化人员工作环境,然而对其机理的认识仍然不清.本文基于作者和合作者以前建立的一个冰间歇破坏型ⅡV模型(黄国君和刘鹏飞, 2009)对柔性结构的锁频共振机理进行了理论研究.应用该模型预报了发生在一个冰速区间内的锁频共振现象,并研究了结构和冰特性参数:结构阻尼和刚度以及冰的压缩刚度和冰破坏的破坏区长度、韧脆转换速度和随机性对ⅡV及锁频共振的影响,在此基础上探索了锁频共振机理.研究表明:在锁频共振冰速区间内,结构响应和冰力主频都锁定在结构固有频率,然而不同冰速下的频谱结构和振动形态各异,从常规单频共振到多频共振、从等幅振动到振幅周期性变化的拍振动,呈现出丰富的动力学特征;结构和冰特性参数可改变锁频共振冰速区间的长度和位置以及结构振幅,冰破坏的随机性和应变率效应发挥着一种竞争作用;锁频共振来源于冰破坏的应变率效应,其力学机制是频率调制和对结构-冰动能传递的非对称性正反馈效应放大的双重作用,本文分析揭示的这一新的锁频共振机理属于耦合振动,与传统的负阻尼自激振动机制有着本质区别.本文分析结果及对锁频共振机理的认识有助于相关实验研究和冰区结构设计以及ⅡV减振技术的研发.

Frequency lock-in resonance in ice-induced vibration(ⅡV) threatens severely the safety of the structures and worsens the working environment for operators. Its underlying mechanism is unclear yet. This paper presents a theoretical study on the mechanism of the frequency lock-in resonance for compliant structures. The study is based on an intermittent ice-crushing type of ⅡV model developed previously by the author and co-worker(Huang and Liu, 2009). The frequency lock-in resonance is predicted over an ice velocity span. Then the parametric analysis is performed on ⅡV and frequency lock-in resonance for some influential factors, including structural damping and stiffness, ice stiffness, ice-crushing zone length, ductile-brittle transitional ice-velocity and randomness in the ice-crushing strength and ice-crushing zone length.From these theoretical studies, the mechanism of the frequency lock-in resonance is investigated. It is shown that although both the predominant ice and structural response frequencies are locked to the structural natural frequency when frequency lock-in resonance takes place, the time history profiles of ice force and structural response and their frequency spectra are different corresponding to the different ice velocity. Not only the conventional mono-frequency resonance with the uniform amplitude but also the multi-frequency beat resonance with the periodically changing amplitudes are predicated.For the frequency lock-in resonance, structural and ice properties affect the length and location of the ice velocity span as well as the response amplitude, and the randomness and strain rate effect in ice-crushing are the two competing factors.It is unveiled that the strain rate effect of the ice-crushing strength is responsible for the frequency lock-in resonance, by frequency modulation and by promoting the uneven kinetic energy transfer between ice and structures, a positive feedback mechanism. The present novel mechanism is a coupled vibration that is essentially different from the conventional one,i.e., the negative damping in self vibration predicted from the continuous ice-crushing type of ⅡV models. The present result is instructive to the further systematic experimental study on the frequency loc-in resonance and to devising some effective techniques for the mitigation of intensive ⅡV.