管道弯头导波传播特性与散射机理研究

The pipelines in vessel are complicated and comprise lots of elbows, which hinders the application of guided wave inspection in vessel’s pipeline system. As to settle the problem of guided wave inspection in pipeline with elbows, two underlying issues, that is the propagation characteristics of guided waves in elbows and the guided wave scattering of elbows in pipeline, are studied. Firstly, based on the semi-analytical finite element method, the dispersion equation of curved waveguide is solved, the dispersion curves for elbow and wave structure in cross section are derived, and influences of elbow’s geometry parameters and mode coupling phenomenon on the propagation characteristics of guided wave are further studied. These studies will reveal the wave motion in elbows, and guide the selection of excitation frequency and wave mode in practical testing. Secondly, guided wave scattering of elbow is studied with the Normal Mode Expansion method, the orthogonality of normal modes in free elbow is firstly proved, and then based on this principle, the incident mode is regarded as an initial excitation of the elbow, and the respondence is calculated with the Normal Mode Expansion method, which also derived the reflected and transmitted energy ratio of those scattered waves. Finally, with the knowledge obtained in the previously two steps, the complicated detected signals for elbows can be interpreted, and the flaw signal can be extracted, which makes the guided wave inspection of pipeline with elbows feasible. The researches of this project will further improve the guided wave detection theory for complicated pipeline system, and drive the application of guided wave inspection of pipelines in vessel.

船舶管道系统复杂，特别是弯头众多，这给船舶管道系统的导波检测带来了困难。针对管道弯头导波检测难问题，围绕弯头导波传播特性分析与弯头导波散射机理研究两个关键问题展开研究。首先，基于半解析有限元方法，求解弯曲波导导波频散方程，计算弯头导波频散曲线与波结构，深入分析弯头尺寸参数以及模态耦合等特殊现象对导波传播特性的影响，系统揭示导波在弯头中的运动规律，为弯头导波检测的激发频率和检测模态选取提供依据；其次，基于简正模态展开方法研究弯头导波散射机理，在理论推导证明自由状态下弯管各模态导波正交的基础上，将入射导波看成弯头的初始激励，利用简正模态展开方法分析其激励响应，从而得出导波在弯头处的散射情况；最后，基于弯头导波传播特性分析和弯头导波散射机理研究，完成对复杂弯头导波检测信号的解读，破解弯头导波检测难题。本项目的研究将进一步完善复杂管道系统的导波检测理论，推动导波检测在船舶管道检测中的应用。

船舶管道系统复杂，特别是弯头众多，这给船舶管道系统的导波检测带来了困难。针对管道弯头导波检测难问题，围绕弯头导波传播特性分析与弯头导波散射机理研究两个关键问题展开研究。首先，基于半解析有限元方法，求解了弯曲波导导波频散方程，计算了弯头导波频散曲线与波结构，深入分析了弯头尺寸参数对导波传播特性的影响，系统揭示了导波在弯头中的运动规律，为弯头导波检测的激发频率和检测模态选取提供依据，进一步研究了弯头频散曲线中的模态耦合现象，根据特征值对波数的二阶倒数值来判别模态耦合是否发生，并分析了波结构在模态耦合处的演变情况；其次，基于简正模态展开方法研究了弯头导波散射机理，理论推导了自由状态下弯管各模态导波的正交关系式，将入射导波看成弯头的初始激励，利用简正模态展开方法分析其激励响应，推导了入射导波在弯头处的散射情况，研究结果发现，由于直管与弯管的本构关系式不同，算得的入射导波模态的透射系数大于1，这一结论明显违背了能量守恒定律的情况，这表明弯头导波模态无法表示直管的导波模态，研究揭示了弯头散射的复杂性，简化的线性力学模型无法用于计算导波弯头散射。最后，设计了弯头导波检测实验，实验研究了弯头尺寸对导波传播特性的影响，研究结果验证了理论计算的正确性，实验分析了弯头散射情况， 实验观测到了明显的反射和模态变换现象。本项目的研究进一步完善复杂管道系统的导波检测理论，推动导波检测在船舶管道检测中的应用。